Widely distributed thrust and strike-slip faults within subducting oceanic crust in the Nankai Trough off the Kii Peninsula, Japan

• Thrust and strike-slip faults are widely distributed within oceanic crust.
• The thrust displacements control geometry of crust surface.
• The thrusts cut the Moho and may contribute to mantle serpentinization.
• The thrusts within crust are seismically active and originate near the trough.
• High friction at the plate interface could generate the intraoceanic thrusts.

We identified widely distributed thrust and strike-slip faults within subducting oceanic crust in the Nankai Trough, southeast of the Kii Peninsula, Japan, on the basis of 2D and 3D seismic reflection data. The seafloor seaward of the trough axis is deformed by displacement on these intraoceanic reverse faults, producing topographic highs (part of Kashinosaki Knoll). Because the thrust faults extend to the Moho and offset the Moho reflection, they may be related to serpentinization of the mantle due to seawater invasion. These faults are seismically active, given that their geometries are consistent with the focal mechanisms of intraplate earthquakes and microearthquakes. The thrust faults appear to extend landward to a high-density dome within the accretionary prism off the Kii Peninsula. Because the dome and the associated thick accretionary prism are expected to generate high friction at the plate interface due to their large vertical load, the intraoceanic thrusts are likely to have grown with ongoing subduction. Furthermore, because the geometry of the fault system we identified off the Kii Peninsula has characteristics similar to faults at Zenisu Ridge east of our study area, the thrusts observed in the study area may be considered to be the westward continuation of those at Zenisu Ridge. Since the Euler rotation pole of relative motion between the Philippine Sea plate and Zenisu Ridge is consistent with the high-density dome off the Kii Peninsula, we interpret the high-density dome as well as Kashinosaki Knoll as a westward termination of the Zenisu compression zone.