Structure of the Lesser Antilles subduction forearc and backstop from 3D seismic tomography

In 2007 the Sismantilles II experiment was conducted to constrain structure and seismicity in the central Lesser Antilles subduction zone. The seismic refraction data recorded by a network of 27 OBSs over an area of 65 km×95 km provide new insights on the crustal structure of the forearc offshore Martinique and Dominica islands. The tomographic inversion of first arrival travel times provides a 3D P-wave velocity model down to 15 km. Basement velocity gradients depict that the forearc is made up of two distinct units: A high velocity gradient domain named the inner forearc in comparison to a lower velocity gradient domain located further trenchward named the outer forearc. Whereas the inner forearc appears as a rigid block uplifted and possibly tilted as a whole to the south, short wavelength deformations of the outer forearc basement are observed, beneath a 3 to 6 km thick sedimentary pile, in relation with the subduction of the Tiburon Ridge and associated sea floor reliefs. North, offshore Dominica Island, the outer forearc is 70 km wide. It extends as far as 180 km to the east of the volcanic front where it acts as a backstop on which the accretionary wedge developed. Its width decreases strongly to the south to terminate offshore Martinique where the inner forearc acts as the backstop. The inner forearc is likely the extension at depth of the Mesozoic magmatic crust outcropping to the north in La Désirade Island and along the scarp of the Karukera Spur. The outer forearc could be either the eastern prolongation of the inner forearc, but the crust was thinned and fractured during the past tectonic history of the area or by recent subduction processes, or an oceanic terrane more recently accreted to the island arc.Peer Reviewe

Seismic refraction imaging of the southern Corinth Rift shoulder at Derveni

International audienceTwo land seismic spreads installed on the shoulders of the Corinth Rift during the July 2001 R/V Maurice Ewing seismic reflection campaign provide records of waves that probe the shallow structure of the southern flank of the graben. The direct waves converted at the southern coast propagate in the shallow block faulted carbonates beneath the Derveni array at a velocity of 4.3 km s−1. The first-arrivals at the Galaxidi array indicate a basement velocity of 6 km s−1. The first-arrival times at the Derveni array are fitted for waves refracted along a north-dipping interface between the carbonate layer and the basement. The average dip of the interface beneath the southern shoulder and the basin part of the rift is 15°. A migration of the traces down to the emergence point of the refracted waves beneath the Derveni array shows a short wavelength undulation of the interface superposed to the regional 15° dip. The arrival times and the amplitudes of the waves are consistent with block faulting above a shallow detachment structure beneath the southern Corinth Rift shoulder at Derveni

Seismicity Distribution Near a Subducting Seamount in the Central Ecuadorian Subduction Zone, Space-Time Relation to a Slow-Slip Event

International audienceA temporary onshore‐offshore seismic network deployed during the 2‐year period of the Observación SISmológica en ECuador project provides a detailed and well‐focused image of the seismicity for magnitudes as low as 2.1 at the Central Ecuadorian subduction zone. During this 2‐year experiment, the shallow and locked subduction patch shows little evidence of background seismicity that instead occurred downdip of the coupled patch at ~20‐km depth. In this region, seismicity is possibly controlled by the crustal faults bounding the sedimentary basin of Manabí and the rheology of the upper plate. The dip angle of the interplate contact zone, defined by a smooth interpolation through the hypocenters of thrust events, is consistent with a progressive increase from 6° to 25° from the trench to 20‐km depth. Offshore, a seismic swarm, concomitant with a slow‐slip event rupturing the highly coupled subduction megathrust, highlights the reactivation of secondary active faults within the thickened crust of the subducting Carnegie Ridge at the leading edge of a large oceanic seamount

Deep structure of the continental margin and basin off Greater Kabylia, Algeria – New insights from wide-angle seismic data modeling and multichannel seismic interpretation

International audienceDuring the Algerian-French SPIRAL survey aimed at investigating the deep structure of the Algerian margin and basin, two coincident wide-angle and reflection seismic profiles were acquired in central Algeria, offshore Greater Kabylia, together with gravimetric, bathymetric and magnetic data. This ~260 km-long offshore-onshore profile spans the Balearic basin, the central Algerian margin and the Greater Kabylia block up to the southward limit of the internal zones onshore. Results are obtained from modeling and interpretation of the combined data sets.The Algerian basin offshore Greater Kabylia is floored by a thin oceanic crust (~4 km) with P-wave velocities ranging between 5.2 and 6.8 km/s. In the northern Hannibal High region, the atypical 3-layer crustal structure is interpreted as volcanic products stacked over a thin crust similar to that bordering the margin and related to Miocene post-accretion volcanism. These results support a two-step back-arc opening of the west-Algerian basin, comprising oceanic crust accretion during the first southward stage, and a magmatic and probably tectonic reworking of this young oceanic basement during the second, westward, opening phase. The structure of the central Algerian margin is that of a narrow (~70 km), magma-poor rifted margin, with a wider zone of distal thinned continental crust than on the other margin segments. There is no evidence for mantle exhumation in the sharp ocean-continent transition, but transcurrent movements during the second opening phase may have changed its initial geometry. The Plio-Quaternary inversion of the margin related to ongoing convergence between Africa and Eurasia is expressed by a blind thrust system under the margin rising toward the surface at the slope toe, and by an isostatic disequilibrium resulting from opposite flexures of two plates decoupled at the continental slope. This disequilibrium is likely responsible for the peculiar asymmetrical shape of the crustal neck that may thus be a characteristic feature of inverted rifted margins