Miocene to Pleistocene stress field transitions, around the Miyako-jima Island, South Ryukyu, Japan

摘要

Three paleostress transitions are measured in Miyako-jima Island. NW-SE and NE-SW extensions are recorded in the Simajiri Group. Strike-slip type stress state where σ_1 directs to N-S and σ_3 directs to E-W, is recorded in the Ryukyu Group. Transition of paleostress is considered as follows. Before the sedimentation of the Shimajiri Group, NW-SE extension occurred in the Miyako-jima because the Miyako-jima was extended toward the Ryukyu Trench by the trench retreat during the subduction of the Philippine Sea plate. During the sedimentation of the Shimajiri Group, the arc-parallel stretching occurred in the Miyako-jima because the Okinawa Trough started to push the Miyako-jima toward the Ryukyu Trench. Then NE-SW extension occurred in the area. During the sedimentation of the Ryukyu Group, strike-slip type stress state occurred by the sinistral strike-slip movement along the central rift in the Okinawa Trough.