Pliocene shortening direction in Nankai Trough off Kumano, Southwest Japan, Sites IODP C0001 and C0002, Expedition 315: anisotropy of magnetic susceptibility analysis for paleostress

Abstract

Integrated Ocean Drilling Program Expedition 315 recovered cores from accretionary units, which are overlain by cover sequences, at a site on the hanging wall of the mega‐splay fault (C0001), and at a seaward margin of a forearc basin (C0002) in Nankai Trough off Kumano, Japan. In order to investigate the amount and the style of deformation and shortening direction of the accretionary prism units, anisotropy of magnetic susceptibility (AMS) of cored samples were measured. AMS of the late Pliocene to late Miocene accretionary prism at C0001 reveals a deformed magnetic ellipsoid (prolate type), and a restored direction of the AMS orientation indicates northwest–southeast shortening which is the same as the present‐day stress field measured by borehole breakout. The older Miocene accretionary prism cored beneath the forearc basin at C0002 does not show a typical prolate type but more oblate feature in parallel to plunged bedding planes. This type can be interpreted as an intermediate type between prolate and oblate types, by indicatives of AMS parameters, which was formed under a bedding vertical loading and a relatively weakly lateral compaction. The restored AMS orientation of the accretionary prism beneath the forearc basin also indicates the northwest‐southeast shortening which is different from the present‐day principal horizontal stress orientations at C0002. It is supposed that the shortening direction had been recorded in Pliocene time probably when the sequence was sited in the outer wedge. The northwest–southeast directions of AMS in the Pliocene time agree with the past subducting direction of the Philippine Sea Plate since the Pliocene.