Résumé
The amount of water carried downward into subduction zones and released at depth has direct consequences on the seismicity and the fore-arc tectonic deformation style. The Lesser Antilles subduction zone represents an endmember subduction with old Atlantic oceanic crust (80-100 Ma) being subducted beneath the Caribbean Plate with a low convergence rate of about 20 mm/a. Devastating earthquakes have occurred in the Central Lesser Antilles (Martinique-Montserrat) and in the Greater Antilles, from Porto-Rico to Haiti. Contrastingly, in between the Northern Lesser Antilles (Montserrat-Virgin Island) subduction has produced very few earthquakes with Mw>5. Seismic and heat flow data recorded during ANTITHESIS and SISMANTILLES cruises suggest N-S variations in the nature of the incoming crust. Wide-angle seismic velocity models reveal that the subducting Atlantic plate in the Northern Lesser Antilles is about 5-7 km thick and the Caribbean crust at the arc is nearly equal 18-km-thick. A comparison to seismic data from the Central Lesser Antilles images an arc crust of comparable thickness and velocities. However the structure of the downgoing oceanic plate is significantly different. The basement likely consists in classical gabbro and basalt rocks offshore of Central Antilles and in exhumed and serpentinized mantle rocks offshore of Northern Antilles. Mechanical properties and fluid content of this material differ possibly impacting the tectonic deformation, the amount and composition of channelized fluids as well as the seismogenic behaviour. The heat-flow in the trench and outer fore-arc is 30% lower in the Lesser Antilles region than in the Central Antilles region likely related to efficient hydrothermal cooling through the pervasive fluids pathways that cut through the serpentinized mantle rocks. Modelling both scenarii results in colder temperatures along the interplate contact at the Northern Lesser Antilles, compared with the central Lesser Antilles segment, and a nearly equal 40% reduction of the distance between of the 60 degrees C and 450 degrees C isotherms, commonly associated with the the seismogenic behavior along the interplate contact. Analysis of the seismic and bathymetry data allows mapping of fluid migration paths in both regions and reveals the existence of wedge shaped sedimentary masses and faults.