富士川河口断層帯，星山丘陵周辺の1Ma以降のテクトニクス

抄録

We have examined the surface geology and tectonic landforms of the Hoshiyama Hills and its surrounding areas in the Fujikawa-kako Fault Zone (FKFZ), central Japan. The FKFZ is generally regarded as a collision zone between the Honshu Arc, mainly composed of the pre-Neogene accretionary and the Neogene collisional complexes of South Fossa Magna region, and the Neogene Izu-Bonin Volcanic Arc on the Philippine Sea Plate. This zone is believed to be the most active and dangerous area of Japan in association with violent earthquakes. One of the keys to understand the activities of this zone since the Pleistocene is to reveal the geologic structures of the Hills, as well as the characteristics of the Omiya and Iriyamase Faults along the northeastern and southeastern margin of this Hills, respectively. Our surface geological survey reveals that the upper Lower-lower Middle Pleistocene Ihara Group, main constituent of the basement of the Hoshiyama Hills, has complicated structures including several-hundreds meters scale steeply-dipping beds without distinctive preferred orientations. The structures also include chevron-shaped anticlines and flat synclines, suggesting that the E-W horizontal shortening due to fault-related foldings was the main cause of their formations. The flexure-landform associated with the Omiya Fault clearly suggests that the Fault, previously believed to be a high-angled normal fault dipping toward NE, is a reverse fault dipping toward SW. However, the landform around the Iriyamase Fault, also believed to be a NW dipping reverse fault, show no evidence of its existence. Finally, we have summarized the tectonic and volcanic events in and around the FKFZ since about 1 Ma. These results suggest that the Quaternary tectonics and seismic activities of the FKFZ should be necessary to re-evaluate based not only on the surface geological and landform data but also on the subsurface geological structures now being poorly known.