At subduction zones, downgoing topographic features exert first-order structural and hydrologic effect on the plate boundary and the upper plate. Such process has been rarely documented by clear observations, especially at great depths, and it remains elusive how the altered structural and physical characteristics of the upper plate control seismogenic behavior and tectonic evolution of margins. Here, we present a reprocessed multichannel seismic (MCS) profile together with bathymetry and earthquake data in the central Lesser Antilles. A reflector imaged at 15-18 km depth ahead of the Tiburon ridge delimits the base of inner forearc crust with pervasive reflective anomalies. It is interpreted to represent a shallow fluid-rich decollement warped over the rough topography, where the underlying materials consist largely of oceanic sediments identical to those accreted at the Barbados prism and basement fragments from basal erosion. Our results suggest that fluids are expelled upward from the band of subducted sediments, leading to a NW-SE elongated zone of hydrofractured and weakened crust above a serpentinized mantle corner coinciding with a prominent aseismic corridor. The high interplate seismic activity offshore Martinique at ~30–65 km depths may correspond to deeply subducted indurated sediments that act as a strong asperity on the plate interface.