Geologic structure and paleostress analysis in the Northern part of Koshikijima Islands, West Kyushu, Japan

Upper Cretaceous to Paleogene sedimentary successions and igneous intrusions in the Koshikijima Islands to the west of Kyushu, Japan are keys to understand Cenozoic tectonics in the junction of the southwest Japan and Ryukyu arcs. On the basis of the detailed geological mapping, fault classification, and fault slip analysis in the islands, we distinguished two deformation events in this area since the Paleogene. The older event occurred sometime from the late Paleogene to the middle Miocene. During this event, the NW−SE trending normal faults and dikes with same trend were created in a NE−SW extensional stress field. This fault activity divided the sedimentary pile into northeastward tilting blocks. The younger event after the middle Miocene is represented by NNE−SSW trending oblique-slip normal faults and dikes with the same trend in a WNW−ESE extensional stress field. This fault activity subdivided a part of the sedimentary pile into northwestward tilting blocks.