Multichannel Seismic Profiles Research Articles

Tectonic features associated with the overriding of an accretionary wedge on top of a rifted continental margin: An example from Taiwan

Off southwest Taiwan, a west-advancing orogenic wedge has obliquely impinged on the northern continental slope of the South China Sea (SCS) margin. We analyzed a dense grid of multi-channel seismic profiles to reveal the tectonic features in this oblique collision setting. In the upper SCS slope and adjacent to the accretionary wedge, the rifted continental margin is characterized by a deep-seated, oceanward-dipping, listric and active normal fault with rotated hangingwall strata beneath the slope. To the west of this slope segment, the mid-Oligocene to Recent post-breakup sediments show prograding and aggrading shelf-margin clinoforms that cover a series of small, possibly Paleogene, rift basins. In the submarine accretionary wedge, a series of west-vergent, NNW-striking fold-and-thrust structures characterizes the lower wedge, which can be further divided into frontal and rear segments with distinct structural features. The frontal segment is characterized by four west-vergent blind thrusts with gently folded limbs. In contrast, the rear, arcward segment is characterized by west-vergent, emergent and imbricate thrusts with tilted beds truncated at the hangingwalls. Strata within slope basins are tilted arcward, with dips that increase with depth, indicating continued relative uplift along thrust planes during sedimentation. Pulsed thrust activity is further evidenced by an array of arcward-dipping unconformable surfaces with westward onlapping strata in the basins. Longitudinal sedimentary tapering of pre-orogenic sediments correlates strongly with curvature of the submarine frontal accretionary belt, suggesting that pre-orogenic sediment thickness is the major control on the geometry of frontal structures. The preexisting SCS slope that lies obliquely in front of the advancing accretionary wedge has impeded the advancing of frontal folds resulting in a successive termination of folds against and along strike of the SCS slope. The existence of the SCS slope also leads the strike of impinging folds with NNW-trend to turn more sharply to a NE-strike, parallel to strike of the SCS slope. Our analysis shows that the pre-orogenic mechanical/crustal heterogeneities and seafloor morphology exert strong controls on the thrust-belt development in the incipient Taiwan arc-continent collision zone.

East Greenland Ridge in the North Atlantic Ocean: An integrated geophysical study of a continental sliver in a boundary transform fault setting

The combined Greenland‐Senja Fracture Zones (GSFZ) represent a first‐order plate tectonic feature in the North Atlantic Ocean. The GSFZ defines an abrupt change in the character of magnetic anomalies with well‐defined seafloor spreading anomalies in the Greenland and Norwegian basins to the south but ambiguous and weak magnetic anomalies in the Boreas Basin to the north. Substantial uncertainty exists concerning the plate tectonic evolution of the latter area, including the role of the East Greenland Ridge, which is situated along the Greenland Fracture Zone. In 2002, a combined ocean‐bottom seismometer and multichannel seismic (MCS) survey acquired two intersecting wide‐angle reflection and coincident MCS profiles across and along the East Greenland Ridge. We present the results of integrated reflection seismic interpretation, first‐arrival tomography, 2D kinematic raytracing, full‐wave amplitude modeling, and gravity modeling of the intersecting profiles. The results show that (1) the Greenland Basin is characterized by a normal oceanic crustal velocity structure, (2) the velocity structure of the East Greenland Ridge is of overall continental type, and (3) a major faulted basin province above highly extended continental crust exists to the NE of the ridge. The results further suggest that a zone of extremely thin and faulted continental crust above partially serpentinized mantle peridotite defines the NW edge of the East Greenland Ridge and the transition to the NE Greenland margin.

Structural and tectonic evolution of western Cuba fold and thrust belt

We present balanced and restored cross sections across the western Cuba fold and thrust belt to illustrate its structure and tectonic evolution, based on the interpretation of multichannel seismic profiles, field data and balancing techniques. The NNW–SSE trending Soroa cross section intersects, from south to north, (1) the Los Palacios Basin, (2) the Sierra del Rosario antiformal stack, (3) the Bahía Honda thrust unit, (4) the frontal ∼3‐km‐thick imbricated thrust sheets, and (5) the foredeep basin. At depth, a duplex that links the Sierra del Rosario unit and the frontal imbricates is defined using the thickness of the cover thrust sheets of the imbricated North American margin. The minimum calculated shortening is of 130 km in a SSE or south direction to fit large‐scale plate tectonic reconstructions. However, a more allochthonous alternative using thinner cover sequences is also discussed, which restores the Bahía Honda unit 220 km to the south of its present position. Syntectonic deposits in combination with a forward kinematic model provide a good constraint for the tectonic evolution of the orogen. The internally deformed Bahía Honda unit overthrusted the North America margin cover successions during Danian and Selandian times (∼65.5–60 Ma), before the stacking of these cover rocks during Selandian and Thanetian times (∼60–56 Ma). The growth of a large antiformal stack beneath the Sierra del Rosario occurred at early middle Eocene (∼56–45 Ma), generating about 4 km of structural relief between the paleoisland of Cuba and the synorogenic seafloor before the final infill of the foredeep.

Pacific Plate subduction beneath the central Mariana and Izu‐Bonin fore arcs: New insights from an old margin

Multichannel seismic (MCS) profiles and bathymetric data from the central Mariana and Izu‐Bonin subduction systems image the subducting Pacific Plate from the outer trench slope to beneath serpentinite seamounts on the outer fore arc. Subducting oceanic crust varies along the Mariana margin from 5.3 to 7 km thick and is covered by 0.5–2 km thick sediments and numerous seamounts. Oceanic crustal thickness east of the Izu‐Bonin Trench is ∼6 km. Faulting resulting from flexure of the incoming Pacific Plate begins up to 100 km east of the trench axis, near the 6 km depth contour. The plate is cut by normal faults that reactivate inherited tectonic fabric where that fabric strikes <25° to the trench. Where the strike is >25°, incoming crust breaks along new faults with a trench‐parallel strike. The Mariana Trench axis is commonly a graben that accommodates an abrupt change (within <25 km) of plate dip from <4° (commonly ≤2°) on the incoming plate to >8° beneath the outer fore arc. We infer that the plate fails there rather than simply bends under the applied loads. Along portions of the Mariana margin, subducting seamounts displace the trench axis westward and uplift the toe of the slope. Surprisingly, west of the toe, there is no geophysical evidence of disturbance of the upper plate in response to seamount subduction, nor of significant subduction erosion or sediment underplating. MCS profiles across the base of the Mariana inner trench slope provide evidence for both complete subduction and small‐scale accretion of Pacific Plate sediments; however, we found no evidence for long‐term sediment accretion. The subducting plate dips 9–12° beneath serpentinite seamounts on the Izu‐Bonin and Mariana fore arcs. Along the Mariana margin, the majority of these seamounts are located ∼50–70 km west of the trench where the mantle wedge is 3–7 km thick between 8–10 km thick fore‐arc crust and the top of the subducting plate. The apparent lack of significant deformation of the Mariana fore arc crust by subducting seamounts may be the result of a weak serpentinized mantle wedge and/or progressive fracturing as the subducting plate increases in dip as it passes through the trench graben.

A Cenozoic tectono-sedimentary model of the Tainan Basin, the South China Sea: evidence from a multi-channel seismic profile

The Tainan Basin is one of the set of Cenozoic extensional basins along northern margin of the South China Sea that experienced extension and subsequently thermal subsidence. The Tainan Basin is close to the Taiwan Arc-Trench System and straddles a transition zone between oceanic and continental crust. A new regional multi-channel seismic profile (973-01) across the region of NE South China Sea is introduced in this paper. In seismic stratigraphy and structural geology, a model of Cenozoic tectono-sedimentation of the Tainan Basin is established. The results show that three stages can be suggested in Tainan Basin; In Stage A (Oligocene (?)-Lower Miocene) the stratigraphy shows restricted rifting, indicating crustal extension. Terrestrial sedi- ments mostly filled the faulted sags of the North Depression on the continental shelf. Structural highs, including the Central Uplift, blocked material transportation to the South Depression in abyssal basin. In Stage B the Tainan Basin (Middle-Upper Miocene) exhibits a broad subsidence resulting from the post-rifting thermal cooling. The faulted-sags in North Depression had been filled up. Terrestrial materials were transported over the structural highs and deposited directly in the South Depression through sub- marine gullies or canyons. This sedimentation resulted in a crucial change in the slope to a modern shape. In Stage C (Latest Miocene-Recent) a phase change from extension to compression took place due to the orogeny caused by the overthrusting of the Luzon volcanic arc. Many inverse structures, such as thrusts, fault bend folds, and a regional unconformity were formed. Forland basin began developing.

Glacigenic features and Tertiary stratigraphy of the Magellan Strait (Southern Chile)

The seismostratigraphic and structural analysis of the whole length of the Magellan Strait, from the Atlantic to the Pacific entrance is for first time illustrated on the basis of multichannel seismic (MCS) profiles. The Strait crosses a geologically complex region that includes different morphotectonic provinces, and has been subdivided into three distinct segments, eastern (Atlantic), central, and western (Pacific), being each segment characterized by peculiar sedimentary and tectonic architectures. The MCS profiles shed light on the subsurface of the region in particular on the Quaternary and Tertiary features. In the foreland basin province, the MCS profiles imaged an almost undeformed structural and stratigraphic frame with very thick Cretaceous to Tertiary package. Seismic evidence of deformation of the foreland units occurs in the fold-and-thrust belt province. Along the Cordillera province, the Magallanes-Fagnano transform fault exerts an important morpho-tectonic control that strongly conditions its bathymetric profile. The seismic profiles also highlighted a number of depositional features linked to the up to 150 m thick sedimentary record of the glacial cycles. Whereas the eastern segment (outer foreland province) is devoid of significant glacial-related deposits, the central segment (inner foreland and fold-and thrust belt provinces) shows evidence of repeated advances and retreats of the Magellan glacier. An important moraine ridge complex, probably corresponding to the glacial advance “D”of Clapperton et al., has been seismically imaged in the central segment, as well as an older, large bank of ice distal sediments that have been interpreted as proglacial lake deposits, which show evident signs of repeated glacial erosions. Ice-contact features in the form of frontal moraine complexes made up of dipping foreset strata are present in the fjord-like, western segment of the Strait (Cordillera province), along with their related ice-proximal and ice-distal facies. Eventually, the occurrence of preglacial sediments tectonized by the Magallanes-Fagnano transform fault has been reported in the same segment. This fact, which is supported by small outcrops reported in the updated geologic map, if substantiated by further investigations (i.e. advanced seismic reprocessing, sea bottom samplings), would prove the relatively young age (Late Miocene?) of the Magallanes-Fagnano transform fault.

From unthinned continent to ocean: The deep structure of the West Iberia passive continental margin at 38°N

The crustal and lithospheric mantle structure at the south segment of the west Iberian margin was investigated along a 370 km long seismic transect. The transect goes from unthinned continental crust onshore to oceanic crust, crossing the ocean–continent transition (OCT) zone. The wide-angle data set includes recordings from 6 OBSs and 2 inland seismic stations. Kinematic and dynamic modeling provided a 2D velocity model that proved to be consistent with the modeled free-air anomaly data. The interpretation of coincident multi-channel near-vertical and wide-angle reflection data sets allowed the identification of four main crustal domains: (i) continental (east of 9.4°W); (ii) continental thinning (9.4°W–9.7°W); (iii) transitional (9.7°W–∼ 10.5°W); and (iv) oceanic (west of ∼ 10.5°W). In the continental domain the complete crustal section of slightly thinned continental crust is present. The upper (UCC, 5.1–6.0 km/s) and the lower continental crust (LCC, 6.9–7.2 km/s) are seismically reflective and have intermediate to low P-wave velocity gradients. The middle continental crust (MCC, 6.35–6.45 km/s) is generally unreflective with low velocity gradient. The main thinning of the continental crust occurs in the thinning domain by attenuation of the UCC and the LCC. Major thinning of the MCC starts to the west of the LCC pinchout point, where it rests directly upon the mantle. In the thinning domain the Moho slope is at least 13° and the continental crust thickness decreases seaward from 22 to 11 km over a ∼ 35 km distance, stretched by a factor of 1.5 to 3. In the oceanic domain a two-layer high-gradient igneous crust (5.3–6.0 km/s; 6.5–7.4 km/s) was modeled. The intra-crustal interface correlates with prominent mid-basement, 10–15 km long reflections in the multi-channel seismic profile. Strong secondary reflected PmP phases require a first order discontinuity at the Moho. The sedimentary cover can be as thick as 5 km and the igneous crustal thickness varies from 4 to 11 km in the west, where the profile reaches the Madeira-Tore Rise. In the transitional domain the crust has a complex structure that varies both horizontally and vertically. Beneath the continental slope it includes exhumed continental crust (6.15–6.45 km/s). Strong diffractions were modeled to originate at the lower interface of this layer. The western segment of this transitional domain is highly reflective at all levels, probably due to dykes and sills, according to the high apparent susceptibility and density modeled at this location. Sub-Moho mantle velocity is found to be 8.0 km/s, but velocities smaller than 8.0 km/s confined to short segments are not excluded by the data. Strong P-wave wide-angle reflections are modeled to originate at depth of 20 km within the lithospheric mantle, under the eastern segment of the oceanic domain, or even deeper at the transitional domain, suggesting a layered structure for the lithospheric mantle. Both interface depths and velocities of the continental section are in good agreement to the conjugate Newfoundland margin. A ∼ 40 km wide OCT having a geophysical signature distinct from the OCT to the north favors a two pulse continental breakup.

Assessing the significance of along‐strike variations of middle to late Miocene prograding clinoformal sequence geometries beneath the New Jersey continental shelf

ABSTRACTSeismic mapping of high‐resolution multichannel seismic profiles along the New Jersey margin illustrates how characteristics of middle–late Miocene clinoformal sequence boundaries (SBs) change markedly along strike in presumed response to local depositional and erosional processes. Most SBs converge from SW to NE, in part as a result of the influence of underlying basin morphology on accommodation space, but also in response to differential subsidence and presumed along‐strike variations in sediment supply from the adjacent margin. The curvature of clinoform breaks, historically viewed as marking palaeo‐shelf edges, is variable and such breaks are rarely sharp. Gently curved palaeo‐shelf/slope transitions cannot be assigned precise palaeobathymetric significance and probably instead reflect post‐depositional sediment reworking. The amount of erosional truncation landward of clinoform breaks varies significantly. Documented along‐strike variability in SB morphology occurs, even though middle–late Miocene palaeo‐shelf edges are nearly linear in plan view. Therefore, such linearity cannot be a product of uniform sedimentary processes and/or accumulation along strike, but instead reflects elongation of depocentres of originally variable cross‐sectional geometry, possibly with the assistance of along‐strike currents. The observed lateral geometric heterogeneity of Neogene sequences can exert profound and unwanted influences on the outcome of scientific drilling intended to calibrate seismic stratigraphic interpretations in the absence of sufficient three‐dimensional (3D) seismic control.

Relationships between magmatism and tectonics in a continental rift: The Pantelleria Island region (Sicily Channel, Italy)

Field geological data of the Pantelleria Island, a large Late Quaternary volcano located in the Sicily Channel rift zone, integrated with offshore geophysical information, are used to derive the structural setting of the Island and the surrounding region, and to analyse the relationships between tectonics and magmatism. Field work shows that the principal faults exposed on the Island fall into two systems trending NNE–SSW and NW–SE. Mapped faults from offshore multichannel seismic profiles show similar trends, and some of them represent the offshore extension of the Pantelleria Island structures. The NW–SE faults bound the Pantelleria Graben, one of the three main depressions formed since the Late Miocene–Early Pliocene within the African continental platform, which compose the Sicily Channel rift zone. A 3-D Moho depth geometry, derived from inversion of Bouguer gravity data, shows a significant uplift of the discontinuity up to 16–17 km beneath the westernmost part of the Pantelleria Graben and beneath the Pantelleria Island; it lows rapidly to 24–25 km away from the graben north-eastward and south-westward. The Moho uplift could explain the presence of a shallow magma chamber in the southern part of the Island, where processes of magmatic differentiation are documented. Geological and geophysical data suggest that the northwestern part of the Sicily Channel is presently dominated by a roughly E–W directed extensional regime. Crustal cracking feeding the Quaternary volcanism could be also related to this extensional field that would be further responsible for the development of the N–S trending volcanic belt that extends in the Sicily Channel from Lampedusa Island to the Graham Bank. This mode of deformation is confirmed also by geodetic data. This implies that in the northwestern part of the Sicily Channel, the E–W extension replaced the NE–SW crustal stretching that originated the NW-trending tectonic depressions constituting the rift zone.

The Cosmonaut Sea Wedge

A set of multi-channel seismic profiles (∼15,000 km) is used to study the depositional evolution of the Cosmonaut Sea margin of East Antarctica. We recognize a regional sediment wedge, below the upper parts of the continental rise, herein termed the Cosmonaut Sea Wedge. The wedge is situated stratigraphically below the inferred glaciomarine section and extends for at least 1,200 km along the continental margin with a width that ranges from 80 to about 250 km. The morphology of the wedge and its associated depositional features indicate a complex depositional history, where the deep marine depositional sites were influenced by both downslope and alongslope processes. This interaction resulted in the formation of several proximal depocentres, which at their distal northern end are flanked by elongated mounded drifts and contourite sheets. The internal stratification of the mounded drift deposits indicates that westward flowing bottom currents reworked the marginal deposits. The action of these currents together with sea-level changes is considered to have controlled the growth of the wedge. We interpret the Cosmonaut Sea Wedge as a composite feature comprising several bottom current reworked fan systems. The wide spectrum of depositional geometries in the stratigraphic column reflects dramatic variations in sediment supply from the continental margin as well as varying interaction between downslope and alongslope processes.

Nonlinear variations of the physical properties along the southern Ecuador subduction channel: Results from depth-migrated seismic data

We use two high-quality pre-stack depth-migrated multichannel seismic profiles acquired to quantify physical properties variations of underthrust sediments along the first ~ 30 km of subduction off the erosional southern Ecuadorian margin. Seismic data show three zones along the subduction channel (referred to as Zones I, II and III) characterized by distinct velocity and velocity-derived physical properties, which are in agreement with values estimated from experimental results of deformation in granular media. These three zones result from transformational changes of underthrust sediments governed by fundamentally different physical processes that control their mechanical behavior at increasing confining pressures. Based on our observations and its comparison with experimental results, we argue that the transformations undergone by underthrust sediments as they dip into the subduction zone are the following: within Zone I, progressively increasing velocity (and decreasing velocity-derived porosity) indicates continuous sediment compaction, which must be accompanied by effective fluid drainage along the décollement and/or across the accretionary wedge. The underthrust material is here unconsolidated from a mechanical point of view. Laboratory experiments indicate that the dominant processes at this range of pressures are grain rolling, particle rotation and frictional slip at grain contacts. Within Zone II, velocity (and porosity) remains constant for ~ 16 km (SIS-72) and ~ 12 km (SIS-18). This suggests undrained conditions resulting in growing fluid overpressure at the subduction channel. Grain deformation is similar to Zone I. Within Zone III, velocity increases and porosity falls rapidly, indicating sediment compaction and subsequent release of over-pressured fluids, where grain deformation is likely to be elastic. This might be the dominant process until the grains attain their crushing strength, resulting in granular cataclasis and, eventually, in the collapse of the system. We suggest that over-pressured fluid release may induce hydrofracturation and it is likely to increase inter-plate coupling down from Zone III.

Wide-angle seismic experiment crossing the southern Izu-Ogasawara Arc and multi-channel seismic profiling of the Ogasawara Plateau –KR07-03 cruise–

Wide-angle seismic experiment crossing the southern Izu-Ogasawara Arc and multi-channel seismic profiling of the Ogasawara Plateau –KR07-03 cruise–

The structure of the Atlantic–Mediterranean transition zone from the Alboran Sea to the Horseshoe Abyssal Plain (Iberia–Africa plate boundary)

The diffusive plate boundary between Iberia and Africa has been analyzed by means of already published and new 28 multichannel seismic profiles covering most of the Atlantic margins of SW Spain and NW Morocco. This region includes the prolongation of the Betic–Rifean tectonic units along the Gibraltar Arc. One of the most characteristic units in the study area is the Miocene seismically chaotic unit that has been divided in a tectonic and a gravitational domains in accordance to previous interpretation by Torelli et al. [Torelli, L., Sartori, R., Zitellini, N., 1997. The giant chaotic body in the Atlantic Ocean off Gibraltar: new results from a deep seismic reflection survey. Marine and Petroleum Geology 14, 125–138.]. In this work we redefine the limits of both domains and rename them as the Gulf of Cadiz Imbricate Wedge and the Horseshoe Gravitational Unit. We interpret the Gulf of Cadiz Imbricate Wedge as a west-migrating thick thrust system that build up in a relatively short period of time. This unit constitutes the continuation of the Miocene chaotic units observed onshore to the front of the External Units in the Betics and the Rif (the Guadalquivir Allochthonous Unit and the Rides Prerifaines). The Horseshoe Gravitational Unit, covering an approximate area of 18 000 km 2 with a volume of around 23 000 km 3, is interpreted as giant submarine debris flows sourced from the up-building of the Gulf of Cadiz Imbricate Wedge, surrounding basement highs and the continental Iberian Slope. The fast westward migration of the Flysch units, the External Units and the Gulf of Cadiz Imbricate Wedge, from the Early Miocene to the Late Tortonian corresponds to a different tectonic process than the protracted but slow NNW to NW convergence of Africa. Our interpretation favours the westward slab retreat theory to form the arcuate shape of the Atlantic side of the Iberia–Africa plate boundary. The subduction mechanism as well as potential roll-over was only active from late Early Miocene to Late Miocene spanning about 12 Myr.

Gas-related morphologies and diapirism in the Gulf of Cádiz

Recently developed high-resolution profiling (multibeam, sonar) and surface sampling were used to map seafloor morphology of the Gulf of Cadiz middle continental slope. Multichannel seismic profiling has made it possible to elucidate the geologic origin of these features as well as the main triggering mechanisms of gas-related morphologies, principally mud volcanoes, carbonate mud mounds, pockmarks and slides. Throughout the entire Gulf of Cadiz, from the continental slope to the shelf and even on land, a close correlation between morphology and gas mobility and associated diapirism can be observed. The middle slope area is strongly deformed by several diapiric ridges, named Guadalquivir, Cadiz and Donana. Most of the diapirs identified in this study are related to the Allochthonous Unit of the Gulf of Cadiz, a chaotic body emplaced during the Tortonian, containing salt and shale nappes affected by later gravitational extension collapse and reactivated compression thrusts. It can be proposed that diapirism and related tectonics provided gas migration pathways.

Crustal wedging triggering recent deformation in the Andean thrust front between 31°S and 33°S: Sierras Pampeanas‐Precordillera interaction

We document a new model of crustal structure of the Andean front in Argentina where numerous historic earthquakes destroyed the cities of Mendoza in 1861 (Ms= ∼7) and San Juan in 1944 (Mw= 7.0). The Cerro Salinas anticline is formed above the west directed Cerro Salinas thrust. An east facing monocline with an amplitude of about 2 km folds the Cerro Salinas thrust and overlying Neogene succession. This monocline is formed above a blind crustal thrust in the basement. Its dip of 14° west is inferred from fold geometry. This thick‐skinned east directed blind thrust and the thin‐skinned west directed Cerro Salinas thrust define a tectonic wedge; the wedge tip occurs at a depth of 5.4 km. Growth of the monocline after ∼8.5 Ma is revealed on multichannel seismic (MSC) profile 31017 (Repsol‐YPF). Rates of Cerro Salinas thrust displacement are of the order of 1 mm/yr, whereas vertical uplift of ∼0.45 mm/yr results from the combined displacement on the Cerro Salinas thrust and growth of east facing monocline. The lateral extent of the east directed crustal blind ramp corresponds with the along‐strike extent of the Eastern Precordillera. When combined with the low displacement rate, a long earthquake recurrence interval is implied. Smaller magnitude earthquakes, however, indicate that segments of the blind thrust ramps ruptured in historic events. If all the segments of the blind thrust ruptured together the fault area is ∼7000 km2and could produce aMw∼ 7.7 earthquake. The crustal wedge model provides new constraints on the origin and potential size of earthquakes that threaten the densely populated region.

A cross section of the eastern Betic Cordillera (SE Spain) according field data and a seismic reflection profile

The BT3 multichannel seismic profile was acquired by the C.G.G. (Compagnie General de Géophysique) in 1977 for hydrocarbon exploration in the eastern Betic Cordillera. REXIMseis Ltd scanned and vectorized a paper copy and then performed post-stack processing, including coherence filtering and deconvolution. The receiver functions of a broad-band seismic station located near the village of Vélez Rubio, at the SE end of the profile, were analysed by Julia et al. [Julia, J., Mancilla, F., Morales, J., 2005. Seismic signature of intracrustal magmatic intrusions in the Eastern Betics (Internal Zone), SE Iberia, Geophysical Research Letters 32, L16304, doi:10.1029/2005GL023274.] to determine the structure of the underlying crust. We have used these Vp data to convert the profile to depth. The profile has a mean SE–NW trend, with a SE-Section 44 km in length followed by a NW-Section 20 km in length. The record includes the first 4 s (twtt), which corresponds to 11 km. Two main areas can be seen in the profile. At the SE-end, a band of high-amplitude discontinuous reflectors dips towards the north. The band is 100 to 200 ms thick, increasing even more northwards. This band reaches the surface at the top of the Maláguide Complex (the upper complex of the Internal Zones). Above these reflectors, an area with chaotic seismic facies and no reflectors corresponds to the outcrops of the olistostromes and turbidites of the Solana Formation, and it is in turn overlain by discontinuous reflectors of the Subbetic rocks. At the NW-end of the profile, a set of high-amplitude continuous reflectors with SE dips point to the location of the Prebetic. Below this section, oblique reflectors of intermediate amplitude indicate the Variscan basement. Over the Prebetic, we have marked the basal thrusts of the Intermediate Units and the Subbetic. Using this seismic data, as well as field observations, we propose a geological cross-section of the upper crust of the eastern Betic Cordillera and a model of the most recent evolution of the orogen. In this model, the Internal Zones and the Subbetic have been welded together from the Middle Burdigalian to the present day and acted as an orogenic wedge that deformed the Intermediate Units and the Prebetic.

Combined use of the GGSFT data base and on board marine collected data to model the Moho beneath the Powell Basin, Antarctica

The Powell Basin is a small oceanic basin located at the NE end of the Antarctic Peninsula developed during the Early Miocene and mostly surrounded by the continental crusts of the South Orkney Microcontinent, South Scotia Ridge and Antarctic Peninsula margins. Gravity data from the SCAN 97 cruise obtained with the R/V Hesperides and data from the Global Gravity Grid and Sea Floor Topography (GGSFT) database (Sandwell and Smith, 1997) are used to determine the 3D geometry of the crustal-mantle interface (CMI) by numerical inversion methods. Water layer contribution and sedimentary effects were eliminated from the Free Air anomaly to obtain the total anomaly. Sedimentary effects were obtained from the analysis of existing and new SCAN 97 multichannel seismic profiles (MCS). The regional anomaly was obtained after spectral and filtering processes. The smooth 3D geometry of the crustal mantle interface obtained after inversion of the regional anomaly shows an increase in the thickness of the crust towards the continental margins and a NW-SE oriented axis of symmetry coinciding with the position of an older oceanic spreading axis. This interface shows a moderate uplift towards the western part and depicts two main uplifts to the northern and eastern sectors.

Morphology and genesis of slow-spreading ridges-seabed scattering and seismic imaging within the oceanic crust

A grid of 32 across-axis and five axis-parallel multichannel seismic (MCS) reflection profiles were acquired at an axial volcanic ridge (AVR) segment at 57° 45'N, 32° 35'W on the slow-spreading Reykjanes Ridge, Mid-Atlantic Ridge, to determine the along-axis variation and geometry of the axial magmatic system and to investigate the relationship between magma chamber structure, the along-axis continuity and segmentation of melt supply to the crust, the development of faulting and the thickness of oceanic layer 2A. Seismic reflection profiles acquired at mid-ocean ridges are prone to being swamped by high amplitude seabed scattered noise which can either mask or be mistaken for intracrustal reflection events. In this paper, we present the results of two approaches to this problem which simulate seabed scatter and which can either be used to remove or simply predict events within processed MCS profiles. The 37 MCS profiles show clear intracrustal seismic events which are related to the structure of oceanic layer 2, to the axial magmatic system and to the faults which dismember each AVR as it ages through its tectono-magmatic life cycle and which form the median valley walls. The layer 2A event can be mapped around the entirety of the survey area between 0.1 and 0.5 s two-way traveltime below the seabed, being thickest at AVR centres, and thinning both off-axis and along-axis towards AVR tips. Both AVR-parallel and ridge-parallel trends are observed, with the pattern of on-axis layer 2A thickness variation preserved beneath relict AVRs which are rafted off-axis largely intact. Each active AVR is underlain by a mid-crustal melt lens reflection extending almost along its entire length. Similar reflection events are observed beneath the offset basins between adjacent AVRs. These are interpreted as new AVRs at the start of their life cycle, developing centrally within the median valley. The east–west spacings of relict AVRs and offset basins is ∼5–7 km, corresponding to a life span of the order of 0.5–0.7 Myr, during which AVRs appear to undergo multiple 20–60 Kyr tectono-magmatic cycles.

New data on seismostratigraphy and sedimentation on the western slope of the middle Caspian

The sedimentary sequence of the Caspian Sea is generally well studied by geological and geophysical methods [1], particularly multichannel seismic profiling, which was mainly aimed at study of the deep structure of the sedimentary cover with perspectives for the discovery of hydrocarbon traps (see, e.g., [2]). However, data available for the upper part of the Pliocene‐ Quaternary sedimentary sequence are insufficiently detailed, because the majority of regional continuous seismic profiling (CSP) works were carried out in the 1970s‐1980s using low-resolution equipment [3]. New detailed data on the structure of these sediments in the Middle Caspian area near the Dagestan coast from the upper part of the western slope to the adjacent deep part of the Derbent basin were obtained by a team of scientists from the Institute of Oceanology and Moscow State University during Cruise 19 of the R/V Rift in autumn 2004‐winter 2005 (Fig. 1). The expedition was conducted in the framework of the Federal Targeted Program “The World Ocean” (Subprogram 8 “Complex Study of Processes, Characteristics, and Resources of the Caspian Sea”) simultaneously with engineeringgeological prospecting of the Yalama‐Samur structure for the Lukoil Company. A Geont-shelf high-resolution single-channel seismoacoustic profiler equipped with a spark transmitter (sparker with capacity of 600 J), which provided vertical resolution up to 2 m at a depth of 300‐400 mbsf, was used to study the fine structure of the upper sedimentary sequence. Geological works in the cruise included sampling of sediments with gravity piston corers and study of their physicochemical properties at the Yalama‐Samur test site (Figs. 1, 2). The study area is located in the steepest area of the western slope of the Derbent basin characterized by the highest dip angles and maximal relief differentiation in the entire Middle Caspian [4]. The CSP data show that the slope is composed of two distinct sedimentary complexes with different seismic record patterns. They are separated by a near-horizontal intense reflector (R-2), which extends in the eastern part at a depth of approximately 400 m and gently rises toward the coast (Fig. 2). Comparison with previous materials, including drilling

2-D and 3-D modelling of wide-angle seismic data: an example from the Vøring volcanic passive margin

This study presents the modelling of 2-D and 3-D wide-angle seismic data acquired on the complex, volcanic passive margin of the Voring Plateau, off Norway. Three wide-angle seismic profiles were shot and recorded simultaneously by 21 Ocean Bottom Seismometers, yielding a comprehensive 3-D data set, in addition to the three in-line profiles. Coincident multi-channel seismic profiles are used to better constrain the modelling, but the Mesozoic and deeper structures are poorly imaged due to the presence of flood basalts and sills. Velocity modelling reveals an unexpectedly large 30 km basement high hidden below the flood basalt. When interpreted as a 2-D structure, this basement high produces a modelled gravity anomaly in disagreement with the observed gravity. However, both the gravity and the seismic data suggest that the structure varies in all three directions. The modelling of the entire 3-D set of travel times leads to a coherent velocity structure that confirms the basement high; it also shows that the abrupt transition to the slower Cretaceous basin coincides in position and orientation with the fault system forming the Ran Ridge. The positive gravity anomaly over the Ran Ridge originates from the focussed and coincident elevation of the high velocity lower crust and pre-Cretaceous basement. Although the Moho is not constrained by the seismic data, the gravity modelled from the 3-D velocity model shows a better fit along the profiles. This study illustrates the interest of a 3-D acquisition of wide-angle seismic over complex structures and the benefit of the subsequent integrated interpretation of the seismic and gravity data.