Crush zone structure in a compressional step; An Example of the western part of the Atotsugawa Fault, Hida City, Gifu Prefecture of Japan

Niwa, Masakazu   ; Shimada, Koji   ; Kurosawa, Hideki; Miwa, Atsushi*

This paper provides a geological description of crush zones in the western part of the active Atotsugawa Fault, Gifu Prefecture of Japan, to understand crush zone structures in steps of strike-slip faults. By means of aerial photograph interpretation, we recognized that the occurrence of a step is suggestive in the Sangawara area, and that a long strand of the Atotsugawa Fault with ENE-WSW trend is traced in the Itani Area. On the Basis of the detailed geological observation, attitude and sense of shear planes, and thickness and clay mineral composition of fault gouge are different between crush zones of the Sangawara and Itani areas. In the Sangawara area, shear planes with NNE-SSW and NW-SE strikes, high-angle oblique to the trend of the Atotsugawa Fault, are characteristically developed. These shear planes can be a part of the composite planar fabric such as R2 surfaces which are dominantly developed within compressional steps.