Origin of multiple principal slip zones in a fault gouge zone within granitoids

Fault zones in crystalline rocks generally possess a relatively narrow (<50-cm-thick) fault core (i.e., a fault gouge layer/zone). However, the Geumwang Fault, a major strike-slip fault developed in granitic rocks in Korea, has a wider (∼24-m-thick) fault core with several gouge layers. Here we conduct detailed field and microanalytical investigations on the fault core of the Geumwang Fault. Results showed that some of the gouges are primary gouges in which seismic slip was localized as the principal slip zones (PSZs), whereas others are secondary gouges that formed by the injection of fluidized gouge material into fractures. Within the PSZs, the presence of amorphous carbon and graphite implies that frictional heat generated by coseismic slip triggered siderite decomposition. It was likely the consequence of gouge fluidization induced by thermal pressurization under fluid drained conditions. In addition, the gouge fluidization seems to neutralize the foliation of the PSZ. This process, together with weakening of the damage zone by long-term alteration, may facilitate subsequent seismic slip occurred along foliated breccia or the fault core boundary rather than the former neutralized PSZs, thereby widening the Geumwang Fault. We propose that coseismic fluidization of PSZ by thermal pressurization can act as a potential mechanism to widen the fault core and generate multiple gouge layers in crystalline rocks.