Pulverized fault rocks and damage asymmetry along the Arima-Takatsuki Tectonic Line, Japan

Abstract

We present field and laboratory data on pulverized rocks at the Hakusui-kyo outcrop of the Arima-Takatsuki Tectonic Line (ATTL), which is a dextral strike slip fault with ~ 17 km displacement juxtaposing granite to the south against rhyolite to the north. The majority of slip at the surface is localized to a clay-rich gouge fault core 8–10 cm in width, surrounded by a coarsening outwards fault breccias up to 3 m wide. Fault damage is highly asymmetric with respect to the slipping zone. The granite south of the fault has a pulverized damage zone up to 200 m wide, while the rhyolite to the north has only about 3 m wide non-pulverized fault breccia. The degree of pulverization in the granite decreases approximately logarithmically with normal distance from the slip zone. The highly fractured pulverized rocks exhibit several distinct textural characteristics. In thin section, grains appear to be highly comminuted but the original grain shapes and margins are recognizable. Microfractures tend to be tensile in no preferred orientation. Grain fragments display little to no rotation and lack evidence of in-situ shear. Consequently, at macroscale the rocks appear to preserve original granitic textures, despite being highly fractured and friable. The observed pulverization and rock damage asymmetry are most consistent with generation mechanism involving ruptures on a bimaterial interface with statistically preferred propagation direction, leading to damage primarily on the side with higher seismic velocity at depth. This is supported by laboratory measurements of P-wave ultrasonic velocities on intact samples which indicate that the granites have consistently higher velocity than the rhyolite with increasing confining pressure.