Process of River Capture and Neotectonics in the Ikachi Basin, Southeastern Yamaguchi Prefecture, Southwest Japan

Abstract

 Based on geomorphologic and geologic investigations, small-scale fluvial terraces along Rivers Yugawa and Shiwarigawa in Ikachi Basin, southeastern Yamaguchi Prefecture, southwestern Japan are identified. Such terraces are classified into six surfaces defined as Ikachi I to VI surfaces in an order of the altitude and the relative height above the present river bed. Distribution and inclination of the terraces, and differences in terrace deposits overlaying their bedrock of Ikachi II and III surfaces, which are developed in an approximately 5km reach along the upstream to midstream of River Yugawa, suggest that the direction of a paleocurrent was in the opposite to that of the present stream before the formation of Ikachi II surface. By an aerial photo-interpretation and field survey, the geomorphic features showing a fault displacement was identified, and a fault outcrop which cutting terrace deposits was discovered in Ikachi Basin, and the existence of the NE-SW-trending active fault (Hizumi Fault: a new designation) was clarified. Change in the paleocurrent might be controlled by the river capture which was caused by the riverbed lowering with an incision of River Yugawa. It was suggested that the river capture resulted mainly from regional tectonics which caused the bedrock uplift and the active fault movement.