Transition of accretionary wedge structures around the up-dip limit of the seismogenic subduction zone

The Nankai accretionary prism is divided into three segments: outer and inner wedges and their transition zone. These wedges reflect different aspects of wedge taper, internal deformation, and basal plate boundary fault. The outer wedge is characterized by narrow critical taper, internal deformation by in-sequence-fold-and-thrust and aseismic décollement. The inner wedge represents a stable narrow taper, weakly deformed internal structure and seismogenic plate boundary fault along its base. The transition zone between the two wedges shows large critical taper with steep surface slope, internal structure of out-of-sequence thrust, and step-down of décollement onto the sediment–oceanic basement interface. The trench slope break and oceanward margin of forearc basin is located around the landward edge of this transition zone. These common aspects might be related to the lithification of both accreted and underthrust sediments and the resultant switch of the plate boundary fault. Deformation and lithification process recorded in exhumed on-land mélange of accretionary complexes suggest that the step-down of the plate boundary décollement occurs around the up-dip limit of seismogenic subduction zone.