＇Subduction erosion＇ has been invoked to explain material missing from some continents along convergent margins(1). It has been suggested that this form of tectonic erosion removes continental material at the front of the margin or along the underside of the upper (continental) plate(2-4). Frontal erosion is interpreted from disrupted topography at the base of a slope and is most evident in the wake of subducting seamounts(5,6). In contrast, structures resulting from erosion at the base of a continental plate are seldom recognized in seismic reflection images because such images typically have poor resolution at distances greater than similar to 5 km from the trench axis. Basal erosion from seamounts and ridges has been inferred(7,8), but few large subducted bodies-let alone the eroded base of the upper plate-are imaged convincingly. From seismic images we identify here two mechanisms of basal erosion: erosion by seamount tunnelling and removal of large rock lenses of a distending upper plate. Seismic cross-sections from Costa Rica to Nicaragua indicate that erosion may extend along much of the Middle America convergent margin.