Sedimentology, structural geology, and tectonics of the Shikoku subduction zone, southwestern Japan

The Shikoku subduction zone is developed along the Nankai Trough where the Philippine plate is underthrust beneath the Asian plate. The landward wall of the Nankai Trough consists of horizontal parallel ridges and basins that trend northeastward. A Deep Sea Drilling Project site (Leg 31, site 298) on the landward flank of the deepest ridge penetrated 525 m of beds in normal stratigraphic position and 86 m of overturned beds (all of Quaternary age), indicating an overturned anticline. The tight, overturned anticline, which trends parallel to the Nankai Trough, has an inter-limb angle of 9°, an axial surface inclined 9° to 14° landward, and a convergently fanning axial plane fracture cleavage. A coarsening-upward turbidite sequence defines a trench facies and demonstrates direct accretion of deposits from this environment.

The convergence rate in the Shikoku subduction zone is estimated to be from 1 to 2 cm/yr, with a strain rate of about 10⁻¹³/sec. Tectonic consolidation has reduced the volume of the subducted and accreted rocks at least one-third. Olistrostromes form as a direct consequence of fold evolution in the submarine environment and can be immediately underthrust, thereby developing a structural fabric.