U-Pb dating of detrital zircons from the Lower Cretaceous Sanbagawa and the recently recognized Upper Cretaceous Shimanto high-pressure (HP) metamorphic rocks in southwestern Japan has revealed the presence of abundant Proterozoic (ca. 1500–2000 Ma) detrital grains. In contrast, coeval non- to weakly metamorphosed accretionary complex (AC) and forearc basin sediments in southwestern Japan lack these older signatures. The only possible source of the Proterozoic detrital grains is the Jurassic AC in southwestern Japan, which structurally overlies the Cretaceous HP units. The Proterozoic grains were incorporated into the protoliths of HP-ACs, without polluting coeval forearc basin to trench sediments, likely by tectonic erosion in the forearc domain. Along the Cretaceous Wadati-Benioff plane, the tectonic erosion peeled off the sole part of the pre-existing forearc crust and mixed it with the subducting trench sediments prior to the peak HP metamorphism. In the Cretaceous subduction-related margin around Japan, the tectonic erosion likely occurred twice.