Crush zone structure in a compressional step:

an example of the western part of the Atotsugawa Fault, Hida City, Gifu Prefecture of Japan

Abstract

Crush zone structures are heterogeneous due to occurrence of fault steps. This paper provides a geological description of crush zones in the western part of the active Atotsugawa Fault, Hida City, Gifu Prefecture of Japan, to understand the crush zone structure in a compressional step of a strike-slip fault. By means of aerial photograph interpretation and regional distribution of crush zones, we recognize that a compressional step occurs in the Sangawara area, whereas in the Itani area a long strand of the Atotsugawa Fault with ENE−WSW trend is extended. Detailed geological observation suggests that shear planes with NNE−SSW and NW−SE strikes, oblique to the trend of the Ato-tsugawa Fault at high angles, are characteristically developed in the Sangawara area, whereas shear planes with ENE−WSW strike dominate in the Itani area. The shear planes with NNE−SSW and NW−SE strikes in the Sangawara area can be a part of the composite planar fabric such as R₂ surfaces which are dominantly developed within compressional steps. Based on the clay mineral composition including smectite and kaolinite, high density of sealed microcracks, and the K-Ar ages of fault gouge and andesite dike, fault breccia and gouge in the Sangawara area are mainly formed by fracturing at a shallow depth (under c.a. 100°C) after the dike intrusion during Oligocene to early Miocene time.