The 300-km-long north-central segment of the Lesser Antilles subduction zone, including Martinique and
Guadeloupe islands has been the target of a specific approach to the seismic structure and activity by a cluster
of active and passive offshore–onshore seismic experiments. The top of the subducting plate can be followed
under the wide accretionary wedge by multichannel reflection seismics. This reveals the hidden updip limit
of the contact of the upper plate crustal backstop onto the slab. Two OBS refraction seismic profiles from the volcanic
arc throughout the forearc domain constrain a 26-km-large crustal thickness all along. In the common assumption
that the upper plate Moho contact on the slab is a proxy of its downdip limit these new observations
imply a three times larger width of the potential interplate seismogenic zone under the marine domain of the
Caribbean plate with respect to a regular intra-oceanic subduction zone. Towards larger depth under the mantle
corner, the top of the slab imaged fromthe conversions of teleseismic body-waves and the locations of earthquakes
appearswith kinks which increase the dip to 10–20° under the forearc domain, and then to 60° from 70 km depth.
At 145 km depth under the volcanic arc just north of Martinique, the 2007 M 7.4 earthquake, largest for half a
century in the region, allows to document a deep slab deformation consistent with segmentation into slab
panels. In relation with this occurrence, an increased seismic activity over the whole depth range provides
a new focussed image thanks to the OBS and land deployments. A double-planed dipping slab seismicity is
thus now resolved, as originally discovered in Tohoku (NE Japan) and since in other subduction zones.
Two other types of seismic activity uniquely observed in Tohoku, are now resolved here: “supraslab” earthquakes
with normal-faulting focal mechanisms reliably located in the mantle corner and “deep flat-thrust”
earthquakes at 45 km depth on the interplate fault under the Caribbean plate forearc mantle.
None such types of seismicity should occur under the paradigm of a regular peridotitic mantle of the upper
plate which is expected to be serpentinized by the fluids provided from the dehydrating slab beneath. This
process is commonly considered as limiting the downward extent of the interplate coupling. Interpretations
are not readily available either for the large crustal thickness of this shallow water marine upper plate, except
when remarking its likeness to oceanic plateaus formed above hotspots.
The Caribbean Oceanic Plateau of the upper plate has been formed earlier by the material advection from a
mantle plume. It could then be underlain by a correspondingly modified, heterogeneous mantle, which
may include pyroxenitic material among peridotites. Such heterogeneity in the mantle corner of the present subduction zone may account for the notable peculiarities in seismic structure and activity and impose
regions of stick-slip behavior on the interplate among stable-gliding areas.