Oceanic crust formed at slow-spreading ridges is currently subducted in only a few places on Earth and the tectonic and seismogenic imprint of the slow-spreading process is poorly understood. Here we present seismic and bathymetric data from the Northeastern Lesser Antilles Subduction Zone where thick sediments enable seismic imaging to greater depths than in the ocean basins. This dataset highlights a pervasive tectonic fabric characterized by closely spaced sequences of convex-up Ridgeward-Dipping Reflectors, which extend down to about 15 km depth with a 15-to-40° angle. We interpret these reflectors as discrete shear planes formed during the early stages of exhumation of magma-poor mantle rocks at an inside corner of a Mid-Atlantic Ridge fracture zone. Closer to the trench, plate bending could have reactivated this tectonic fabric and enabled deep fluid circulation and serpentinization of the basement rocks. This weak serpentinized basement likely explains the very low interplate seismic activity associated with the Barbuda-Anegada margin segment above.