Résumé
The Lesser Antilles is a subduction zone where devastating earthquakes occur with a long recurrence interval. Here, oceanic lithosphere from the Mid-Atlantic spreading center subducts at a slow convergence velocity of 20 mm/yr underneath the Caribbean Plate. Offshore Antigua and Barbuda previous work proposed the existence of a patch of amagmatically accreted oceanic crust bearing a high percentage of serpentinite and therefore a high fluid content. During the ANTITHESIS cruise a seismic profile was acquired in this zone of relative seismic quiescence. The combined wide-angle and reflection seismic profile was recorded using 25 ocean-bottom seismometers, a 126 l (7699 cu. in.) tuned airgun array and a 2.8 km long seismic streamer. The resulting velocity model images the forearc and oceanic crust to a depth of 30 km. The oceanic crust is only about 5 km thick and was best modelled as one single layer with a constant velocity gradient. Gravity modelling indicates that the oceanic crustal densities are lower than magmatic rock densities, thus in good agreement with the presence of relatively light serpentinised mantle material incorporated in the crust. The fluids leaving this highly hydrated subducting slab at different depths might be responsible for the subdued seismicity of the study region. The forearc is about 25 km thick and has velocities slightly higher than continental crust. Along the forearc the crustal thickness is highly variable between 15 and 30 km.