We use multichannel seismic reflection (MCS) and gravity data from the southern Izu-Bonin convergent plate margin to test for tectonic erosion. The Hahajima Seamount is a serpentinite seamount on the Izu-Bonin forearc. This serpentinite body likely originated from the mantle wedge beneath the Izu-Bonin arc, but it subsequently detached. MCS data and gravity modeling demonstrate that the Ogasawara Plateau on the incoming Pacific plate has been partly subducted beneath the Hahajima Seamount. Our analysis indicates that the Hahajima Seamount is a “rootless” serpentinite seamount, and the “root” of the serpentinite body was eroded during subduction of the western edge of the Ogasawara Plateau. Structural features of the Hahajima Seamount, the Ogasawara Plateau, and the Philippine Sea plate suggest that tectonic erosion, particularly basal erosion, has been occurring throughout this area.