Tectonic Evolution of the Biwa-ko Seigan Active Fault System in the Kinki Region, Central Japan

摘要

Large-scale active reverse faults, the Biwa-ko Seigan Fault System (the Hira-Hiei fault and the Katata fault) trending in the NNE-SSW direction are distributed at the western shore of Lake Biwa. The Hira-Hiei fault is a main geological boundary between pre-Cenozoic basement rocks and Kobiwa-ko Group composed of late Pliocene-Pleistocene sediments.The Katata fault is located at the eastern side of the Hira-Hiei fault. The fault is an active fault separating the Katata hills from the alluvial plain of Lake Biwa. An unsymmetrical anticline exists at the front of the hills, and tilted layers are observed toward the mountain in the hills. These geological setting is considered to the thrust front migration. A low angle thrust in the Quaternary sediments caused horizontal shortening as flexure 200 m wide is recognized in the seismic profile. The faulting of the Hira-Hiei fault makes a wedge accumulating layered sedimentary structure on the plain side. After thickening of the wedge sedimentary structure, the shortening deformation occurs to the low angle thrust using the bedding plane or detachment in the basement rocks. From this time, the geological main boundary fault stops moving. The low angle frontal thrust advances several km in front of the main boundary fault. This tectonic evolution in the Katata hills are concordant to the model of the fault-propagation fold.