Sea Beam bathymetric and water gun seismic reflection data off the west coast of Costa Rica image the base of the trench slope, its underlying structure, and the behavior of a décollement surface in the presence of relief on the subducting plate. A low-angle, landward-dipping reflection marks the master décollement, above which only about 80 m of a 350 m oceanic sedimentary section is offscraped and accreted to the lower trench slope. A small horst block at least 9 km long, 1.5 km wide, and 300 m high is being obliquely subducted beneath the landward trench slope. On five seismic lines the décollement rises over the horst and then descends into the next incoming graben as the fault zone maintains a specific stratigraphic level. The undulating geometry of the décollement implies that the subducting grabens are not sites for sediment entrapment and subduction and that the horsts are not causing tectonic erosion of the landward trench wall. Substantial dewatering within the subducting sediments is documented within about 4 km of the deformation front. The dewatering may favor development of bedding-parallel zones of low shear strength within the subducting sediments and the master décollement may seek one of these zones.