S–C cataclasite in granitic rock

Abstract

Foliated cataclasites in granitic rocks along the Rokko–Awaji fault zone, Japan, display S–C fabrics similar to those of mylonites. The S–C cataclasites have S-surfaces defined by shape-preferred orientation of biotite fragments and aggregates of quartz and feldspar fragments, and C-surfaces defined by microshears. All quartz and feldspar fragments show mainly brittle deformational microstructures. The biotite fragments, however, have some fabric characteristics like those observed in S–C mylonites, such as biotite `fish', cleavage-steps, bending, and folding. These differences in microfabric between biotite, quartz and feldspar suggest that there are marked differences in their deformational behavior under the same deformational conditions. Biotite deformed by a combination of brittle–plastic shearing processes, and quartz and feldspar were brittlely damaged. One of the most significant microstructural differences between S–C cataclasites and S–C mylonites is the absence of dynamically recrystallized grains in the S–C cataclasites. The microstructural characteristics and the geological evidence suggest that the S–C cataclasites in the Rokko–Awaji fault zone formed at temperatures between 150 and 250°C, corresponding to depths of 5–8 km, at a continental geothermal gradient of 30°C/km, before being uplifted and exposed. This study of cataclastically deformed rocks shows that the fault zone rheology of the upper 5–10 km of crust is greatly influenced by deformational processes in S–C cataclasites.