Prestack depth-migrated seismic lines provide accurate images of the convergent Ecuadorian margin, where the southern flank of the Carnegie Ridge subducts. The margin is fronted by a 5–7-km-wide compressional sediment prism. Beneath the slope, margin thinning is accomplished by gradual thinning of the lower part of the basement, accompanied by pervasive seaward-dipping normal faulting, indicating basal subduction erosion at a low-friction plate interface. Along the plate boundary, a thin subduction channel locally thickens to form low-velocity, ∼600-m-thick lenses of underthrusted fluid-rich sediment supplied by slope mass wasting. The contrast between the fluid-rich sediment and the surrounding thinner and drier sections of the subduction channel creates a three- dimensional patchiness across the plate boundary, implying variations in mechanical interplate coupling. The subduction channel patchiness modulates subduction erosion processes by alternately favoring margin basement weakening and material removal. Basement weakening would occur (1) at the base of the upper plate, where enhanced reflectivity indicates basement damage by overpressured fluids from the subduction channel, and (2) at the basement apex, where basement breakup is caused by superposition of compressional and extensional fault systems resulting from time-variable interplate mechanical coupling. The detachment of rock debris in the higher coupled sections and the subsequent dragging of the fragments into the subduction channel cause tectonic erosion. Deeper in the subduction, the subduction channel patchiness may influence processes like earthquake nucleation and rupture propagation and material recycling in the mantle.