The role of a backstop in subduction zones has been the subject of numerous laboratory and numerical modeling studies; however, few field observations exist revealing how backstops control deformation in subduction zones and accretionary wedge construction. A seismic reflection and refraction survey acquired in 1998 with the R/V Maurice Ewing reveals the geometry of the forearc igneous crust, accretionary wedge, and forearc basin structure of the northern Guadeloupe area of the Lesser Antilles forearc. An accreted block of buoyant crust, accreted in the late Miocene, forms the toe of the overriding arc crust and forms the backstop. We imaged the top of this surface, beneath the forearc basin, to its seaward edge where it meets the subducting oceanic crust. The toe of the backstop was thrust upward and forms a steep buttress in contact with the lower half of the accretionary wedge. The steep buttress produces a narrow inner deformation zone with minimal backthrusting of the accretionary complex landward over the backstop, and a narrow <10 km transition between accreted and forearc basin sediment. Seismic reflections from the subducting crust and the decollement appear beneath the entire accretionary wedge and below the backstop toe. Separating the decollement and the subducting crust is an interval, usually between 500 and 750 m, of underthrust sediment carried underneath the accretionary wedge and subducted 15 km landward and beneath the toe of the backstop. We speculate that the upturned geometry of the toe of the backstop and a weak fluid-rich decollement may facilitate sediment subduction beneath the backstop and potentially into the mantle.