Submarine Geology around the Japanese Islands Part 3. The Nankai Trough and its Vicinity

Abstract

The Nankai Trough is regarded as a trench from the structural point of view, in spite of its shallowness and flatness. The topography of the sea floor of the Nankai Trough and its vicinity shows typical features consisting of the continental shelf, the upper slope (shelf slope), the deep-sea terrace, the lower slope (landward wall of the trough), the trough floor, and the ocean basin. The upper slope is comparatively monotonic. The deep-sea terrace is a large structural basin lying at the foot of the upper slope, behind the outer ridge rising on the continental slope. The lower slope from the outer margin of the deep-sea terrace to the trough floor consists of accretions which form ridges and troughs. This is divided into smaller scale units by the structure of faults and folds. Sediments on the sea bed of the Nankai Trough have been deformed in conformity with the formation of the hollow of the trough. The Nankai Trough and its related topographies, namely the upper slope, the outer ridge shifting landward on the whole, and the ridge-and-trough zone, were formed by the subduction of the Philippine Sea plate. Topographic trends of the Nankai Trough area are deviated largely at the boundary, which can be estimated by distribution of hypocenters, between sub-slabs in the subducting Philippine Sea plate. The age of formation of the major topographies of the Nankai Trough and its vicinity falls within the post-Pliocene.