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...