Changes to River System and Faulting in the Ikachi Basin and Surrounding Area in the Southwestern Part of the Chugoku Mountains, Southwest Japan

Abstract

 This paper examines changes to the river system and faulting in the Ikachi Basin and surrounding area in the southwestern part of the Chugoku Mountains since the Middle Pleistocene, based of an investigation of the fluvial terrace and tectonic landforms. Fluvial terrace surfaces in the study area are classified into five levels: H, M1, M2, L1, and L2, in descending order. The M1 terrace surface is widely observed in the Ikachi Basin, and there is a narrow band of Sanbe-Kisuki tephra on the top layer of the terrace deposit, suggesting that the surface was formed around 110-115 ka. Aira-Tn tephra is observed in the L2 terrace deposit, indicating that it was formed around 30 ka. The distribution of terrace and deposit indicates the existence of the Paleo-Shiwari River, which differed from the river system existing today. The Paleo-Shiwari River flowed northwestward from the southeastern margin of the Ikachi Basin, and from near Hizumi, westward through the basin. There is a possibility that the upper reaches of the Paleo-Shiwari River reached Yashiro Island. The Paleo-Shiwari River lost its upper reaches as a result of river capture around the current Obatake-Seto in Middle Pleistocene. Furthermore, as a result of continued large-scale uplifting in the downstream area of the Paleo-Shiwari River basin, accompanied by activities of the Hizumi and Oguni faults since the Middle Pleistocene, the height of the riverbed of the Paleo-Shiwari River increased and its riverbed slope became gentle. At the same time, continued large-scale subsidence with faulting from the downstream basin of the Yuu River to Aki-Nada led to a gradual steepening of the riverbed of the Yuu River, and the valley head of the Yuu River along the fracture zone expanded due to erosion. Subsequently, the Paleo-Shiwari River was captured by the Yuu River at the Hizumi depression around 110-115 ka during the formation period of the M1 surface. It is concluded that river capture between the Yuu River and the Shiwari River occurred due to the influence of crustal movements.