Fluid inclusions in crack-seal veins are analyzed in accreted mélange now on land. The vein I inclusions are observed in the necked parts of sandstone blocks in mélange, and the vein II inclusions developed in shale below the thrust fault, which cuts the mélanges. The pressure and temperature estimations from the inclusions within vein I show that the cracks were sealed at ∼125–195 °C under ∼92–144 MPa of fluid pressure. Vein II formed in cracks that might have opened in a damage zone caused by the thrust fault as it broke through oceanic crust and into the mélange; these veins contain fluid inclusions trapped at ∼135–245 °C under ∼107–149 MPa of fluid pressure. The depth (∼4–6 km below the seafloor) and temperature estimates are consistent with the conditions where an aseismic décollement within sedimentary rocks steps down into the oceanic basement, so that a thin section of oceanic crust underplates in the hanging wall. Such a step-down site is the updip limit of the seismogenic zone in the modern Nankai Trough.