Active fault Outcrops Concerning the Saigatao Tectonic Line in Central-Southern Part of Yamaguchi Prefecture, Southwest Japan

Abstract

The NE-SW-trending Saigatao Tectonic Line with the length of 20 km long in central-southern part of Yamaguchi Prefecture, Southwest Japan, has been known as a geological boundary fault between Suo Metamorphic Rocks of the Triassic to Jurassic period and Ota Group in the Permian period. Tectonic reliefs and active fault outcrops concerning the tectonic line have not yet been described. Fault outcrop Loc. 1(Nagaono, Ube City)and Loc. 2(Mana, Mine City) near the Saigatao Tectonic Line are newly identified by topographic and geological investigations. In the fault outcrop Loc. 1, the fault is recognized between mudstone of Ota Group and pelitic schist of Suo Metamorphic Rocks and this fault is corresponding to the Saigatao Tectonic Line, but does not give Lower river terrace deposits the displacement and deformation. In the fault outcrop Loc. 2, the fault in the sandstone and mudstone belonging to Ota Group is recognized. This outcrop is located in the 40m westward to the Saigatao Tectonic Line and the fault concerning the Saigatao Tectonic Line, because the strikes of the fault in the outcrop and the Saigatao Tectonic Line have the same strike NE-SW. The boundary between the fractured zone and the host rock displaces the Middle river terrace deposits(after 85,000-90,000～130,000 y.b.p.). Therefore, it is possible that, at least, the part of the Saigatao Tectonic Line is an active fault and the latest movement age of the active part is after the Late Pleistocene.