Deformation in a subduction zone and the related transition from smectite to illite within the aseismic-seismic transition zone (2–4 km below the seafloor) were analyzed by studying an onland accretionary complex that was previously buried to a depth of just 2–4 km. The early to middle Miocene Hota accretionary complex of central Japan is an excellent example of an accretionary complex that records shallow underthrusting at the updip end of the seismogenic zone. Two types of subduction-related deformation are preserved in the complex: phacoidal deformation (D1) characterized by rhombus-shaped fragments of mudstone with a random fabric and a thin rim of clay minerals with a preferred orientation, similar to the deformation features of the primary décollement zone at the toe of modern accretionary prisms (as revealed by ocean drilling); and block-in-matrix deformation (D2) characterized by an asymmetric S-C foliation with shear bands and an intense shape-preferred orientation of clay minerals, similar to the deformation features of tectonic mélange in ancient, mature décollement zones. D2 is marked by a large reduction in the amount of smectite and a corresponding increase in illite. During D2, the shear zone increased in strength due to the disappearance of weak smectite, which has a low friction coefficient, and due to an increase in the cohesion of sediments associated with a reduction in porosity and the development of a preferred orientation of clay minerals. Such strain hardening represents a fundamental mechanical/chemical change in the properties of sediments immediately before entering the seismogenic zone.