A fault outcrop on an inferred active fault developed at the foot of Mt. Wakurayama in eastern Matsue City, Shimane Prefecture, southwest Japan

Abstract

　 An inferred active fault has been reported at the foot of Mt. Wakurayama in the eastern part of Matsue City, Shimane Prefecture, southwest Japan. No fault outcrop has been identified along the surface trace of the inferred fault. The basement rock at Mt. Wakurayama is mainly the Wakurayama Dacite, which yields whole-rock K-Ar ages of 6.3-5.0 to 0.9-0.7 Ma.

　 We conducted a field survey to search for fault outcrops and determine the age of basement rocks along the trace. An exposed fault was found. The fault strikes ENE, dips steeply to the north, and is accompanied by a damage zone that contains shear joints and fault breccia. Structures in the fault breccia and striations on the fault plane indicate a dextral sense of shear. The geometry and sense of displacement of the fault are consistent with those of the inferred active fault; therefore, we infer that the lineament resulted from displacement on the fault. The fault cuts dacite that yields a U-Pb zircon age of 0.83±0.03 Ma, which indicates that the fault was active in and/or after the middle Pleistocene. The net-slip rate of the fault, as estimated from the fault outcrop, is roughly 0.25 mm/yr.