Formative process of fracture in granite on the basis of geological history after granitic magma emplacement; A Case study at the Toki Granite, central Japan

Sasao, Eiji   ; Ishibashi, Masayuki; Yuguchi, Takashi

Fractures in the crystalline rock (i.e. granite) can act as groundwater pathways. Thus, understanding of the fracture characteristics is an important subject for the disposal of high-level nuclear waste. Geological setting and formation process of the granite influence the fracture characteristics. Thus, this study discuss the relation between "formation of fracture and infilling mineral" and geological history from emplacement to present in the Toki granite, central Japan. The Toki granite cooled rapidly during 20 m.y. after the emplacement to about 250C, and then the granite underwent exhumation with slowly cooling during 50-30 m.y. until the subaerial exposure. Brittle fracture occurred below about 300-400C in a cooling granite, and the fracture fillings also occurred below such temperature. The fracture fillings show hydrothermal chlorite and calcite and meteoric water-derived calcite. Chloritization temperature (200-300C) is an indicator of formation temperature of the hydrothermal infillings. The meteoric water-derived calcite occurred in a period between the sedimentation of the Mizunami Group (20 Ma) and present.