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Among the seismic surveys carried out in the framework of the EU -THALES WAS RIGHT project in the Lesser Antilles subduction zone, the SISMANTILLES II cruise of N/O ATALANTE (IFREMER, PI M. Laigle) collected 3 375 km of multi-channel reflection seismics with its 4.5 km long, 360 channels streamer.This survey focuses on the updip portion of the contact zone between the forearc and oceanic crusts, a proxy of the updip limit of the sismogenic portion of the subduction megathrust fault. The geometry of the survey has been designed based on the results of a preliminary SISMANTILLES cruise with N/O NADIR (IFREMER). It consists in a grid of profiles comprising 7 north-south strike-lines (300 km long and spaced by 15 km) crossed by 12 dip-lines (150 km long and spaced by 25 km), with an Ocean Bottom Seismometer network (OBS) deployed on the nodes of this MCS grid.We present the 12 dip-line transects spaced at about 25 km from each other and sampling a 280 km long segment of the subduction, from offshore Martinique Island in the south up to offshore Antigua Island in the north. They have all been processed on board with CGG-Veritas Geovecteur and Geocluster softwares up to post-stack time-migration with constant water velocity. Some profiles have been reprocessed at IFM-GEOMAR (Kiel, Germany) in the frame of a EU-TMR project with pre-stack depth migration (PSDM) processing after deconvolution and multiple attenuation and will be presented instead.The 12 dip-line transects reveal the trenchward-dipping forearc basement, the transition between the forearc sedimentary domain and the accretionary prism, as well as the arcward-dipping decollement and oceanic crust. The forearc basement can be followed beneath the 4 westernmost crossing strike-lines, reaching distances of 160-190 km from the volcanic arc, and up to 5 s twt beneath the sea-bottom reflection. In the northern half, together with the previous survey, 4 dip-lines reached out over the deformation front of the accretionary wedge over the incoming Atlantic lithosphere of the North American plate. The downgoing decollement and oceanic crust are imaged from the deformation front over a distance of approximately 75-80 km, and the signal can be followed down to the sea-bottom multiple, 6-7 s twt beneath the sea-bottom reflection west to the easternmost crossing strike-line (∼ 12-15 km depth).A first-order result is the tremendous along-strike variations in the forearc domain of its basement topography and basin thickness, as well as in the frontal part of the accretionary domain of the decollement and oceanic crust topography. A second first-order result is that these dip-lines reveal images that illustrate different stages of the upper-plate deformation induced by the oblique subduction of the two WNW-ESE aseismic ridges (topographical highs): the Barracuda ridge in the northern part, previously identified by the first survey to prolongate beyond the deformation front beneath the frontal accretionary wedge, and now also the Tiburon ridge in the southern part. Here, the P...

Subducting slabs carry water into the mantle and are a major gateway in the global geochemical water cycle. Fluid transport and release can be constrained with seismological data. Here we use joint active-source/local-earthquake seismic tomography to derive unprecedented constraints on multi-stage fluid release from subducting slow-spread oceanic lithosphere. We image the low P-wave velocity crustal layer on the slab top and show that it disappears beneath 60–100 km depth, marking the depth of dehydration metamorphism and eclogitization. Clustering of seismicity at 120–160 km depth suggests that the slab’s mantle dehydrates beneath the volcanic arc, and may be the main source of fluids triggering arc magma generation. Lateral variations in seismic properties on the slab surface suggest that serpentinized peridotite exhumed in tectonized slow-spread crust near fracture zones may increase water transport to sub-arc depths. This results in heterogeneous water release and directly impacts earthquakes generation and mantle wedge dynamics.

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The structure and nature of the crust underlying the Santos Basin-São Paulo Plateau System (SSPS), in the SE Brazilian margin, are discussed based on five wide-angle seismic profiles acquired during the Santos Basin (SanBa) experiment in 2011. Velocity models allow us to precisely divide the SSPS in six domains from unthinned continental crust (Domain CC) to normal oceanic crust (Domain OC). A seventh domain (Domain D), a triangular shape region in the SE of the SSPS, is discussed by Klingelhoefer et al. (2014). Beneath the continental shelf, a~100 km wide necking zone (Domain N) is imaged where the continental crust thins abruptly from~40 km to less than 15 km. Toward the ocean, most of the SSPS (Domains A and C) shows velocity ranges, velocity gradients, and a Moho interface characteristic of the thinned continental crust. The central domain (Domain B) has, however, a very heterogeneous structure. While its southwestern part still exhibits extremely thinned (7 km) continental crust, its northeastern part depicts a 2-4 km thick upper layer (6.0-6.5 km/s) overlying an anomalous velocity layer (7.0-7.8 km/s) and no evidence of a Moho interface. This structure is interpreted as atypical oceanic crust, exhumed lower crust, or upper continental crust intruded by mafic material, overlying either altered mantle in the first two cases or intruded lower continental crust in the last case. The deep structure and v-shaped segmentation of the SSPS confirm that an initial episode of rifting occurred there obliquely to the general opening direction of the South Atlantic Central Segment.

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The Natal Valley, offshore Mozambique, is a key area for understanding the evolution of East Gondwana. Within the scope of the integrated multidisciplinary PAMELA project, we present new wide-angle seismic data and interpretations, which considerably alter Geoscience paradigms. These data reveal the presence of a 30-km-thick crust that we argue to be of continental nature. This falsifies all the most recent palaeoreconstructions of the Gondwana. This 30-km-thick continental crust 1,000 m below sea level implies a complex history with probable intrusions of mantle-derived melts in the lower crust, connected to several occurrences of magmatism, which seems to evidence the crucial role of the lower continental crust in passive margin genesis. S U PP O RTI N G I N FO R M ATI O NAdditional supporting information may be found online in the Supporting Information section. Figure S1. Example of the OBS data MZ1OBS30 on profile MZ1. Data S1. Multi-channel seismic (MCS) data.

International audienceThe structure of the Moroccan and Nova Scotia conjugate rifted margins is of key importance for understanding the Mesozoic break-up and evolution of the northern central Atlantic Ocean basin. Seven combined multichannel reflection (MCS) and wide-angle seismic (OBS) data profiles were acquired along the Atlantic Moroccan margin between the latitudes of 31.5° and 33° N during the MIRROR seismic survey in 2011, in order to image the transition from continental to oceanic crust, to study the variation in crustal structure, and to characterize the crust under the West African Coast Magnetic Anomaly (WACMA).The data were modeled using a forward modeling approach. The final models image crustal thinning from 36 km thickness below the continent to approximately 8 km in the oceanic domain. A 100 km wide zone characterized by rough basement topography and high seismic velocities up to 7.4 km/s in the lower crust is observed westward of the West African Coast Magnetic Anomaly. No basin underlain by continental crust has been imaged in this region, as has been identified north of our study area. Comparison to the conjugate Nova Scotian margin shows a similar continental crustal thickness and layer geometry, and the existence of exhumed and serpentinized upper mantle material on the Canadian side only. The oceanic crustal thickness is lower on the Canadian margin