横手盆地東縁断層帯の後期鮮新統～更新統のフィッション・トラック年代

Abstract

We reconstruct the evolution of the active fault zone at Yokote Basin in northeast Japan based on new fission-track ages. The active fault zone consists of Obonai, Shiraiwa, and Senya faults. Fission-track dating was carried out from the late Pliocene to Pleistocene strata, developing along the active fault zone consisting of these faults. The number of samples was five. (1) Fission-track age of 1.5 ± 0.1 Ma (OB-03) was obtained from welded tuff in the Tazawa Formation around the Obonai fault. (2) Fission-track age of 1.85 ± 0.13 Ma (YG-01) was obtained from tuff in the Tazawa Formation around the south of the Obonai fault. (3) Fission-track age of 1.6 ± 0.3 Ma (FT-01) was obtained from the Kurisawa Formation around the Shiraiwa fault. (4, 5) Fission-track ages of 0.93 ± 0.14 Ma (FT-02) and 2.7 ± 0.4 Ma (FT-03) were obtained from tuffs in the Senya Formation around the Senya fault. YG-01 and FT-03 are likely to be young because of the possibility of reworked zircon crystals. As a result of making balanced and restored cross-sections across the active fault zone, the western boundary fault of the Mahiru mountains moves in the active fault zone after the frontal fault movements. Obonai and Shiraiwa faults were moved by the western boundary fault of the Mahiru mountains when the Senya fault began movement of the frontal fault.