Estimation of the paleostress field from the 3-D orientation distribution of microcracks and their geothermal conditions in the Toki Granite, central Japan

Takagi, Hideo*; Miwa, Shigenori*; Yokomizo, Yoshiyuki*; Nishijima, Kei*; Enjoji, Mamoru*; Mizuno, Takashi   ; Amano, Kenji  

Based on the 3-D orientations of intracrystalline healed, sealed and open extension microcracks in quartz grains in the Late Cretaceous Toki Granite, we discuss the paleostress field and the history of microcracking combining the microthermometry of fluid inclusions in healed microcracks and sealing material identification in sealed microcracks. Twenty one oriented samples are collected mainly from the DH-15 core (240-1000 mabh) drilled by Japan Atomic Energy Agency (JAEA) and additionally from five outcrops in the Tono region. 3-D orientations of healed microcracks indicate the orientation of N-S to NW-SE in almost all sites, whereas those of sealed and open microcracks indicate the dominant orientation of E-W. Two or three orthogonal sets of microcracks are common in both healed and sealed microcracks. The formation of these sets can be attributed to the switch of principal stress axis due to stress release just after the crack formation. Healed microcracks probably reflect more regional paleostress field because of consistency of the orientations than the case of sealed and open microcracks that show rather scattering orientations. N-S to NW-SE trending healed microcracks are formed around 60 Ma on the basis of K-Ar biotite ages of the Toki Granite and formation temperature (c. 300-400C) of fluid inclusions estimated from microthermometry in the case of intrusion depth (3.5 km=100 MPa) of the Toki Granite. Thus the orientation trended NW-SE after the restoration of clockwise rotation of SW Japan at c. 15 Ma. Following the healed microcrack formation, E-W trending high-angle sealed microcracks filled with carbonate mineral are formed. Open microcracks presumably formed at near-surface at the last stage of sealed microcrack formation after c. 20 Ma before the Mizunami Group deposited unconformably on the granite.