3-D GEOMETRIC ANALYSES OF INTRAGRANULAR AND TRANSGRANULAR, HEALED AND SEALED MICROCRACKS IN GRANITE: EXAMPLES NEAR THE NOJIMA FAULT, SW JAPAN

3-D crack analysis is significant for evaluating the path of fluid in rocks and for estimating paleostress field. 3-D analyses of microcracks in the Late Cretaceous granitic rocks were carried out to obtain the paleostress field and aerial relationship of crack density with the active fault. The oriented granitic samples were taken near the NNE striking Nojima fault, which has displaced during 1995 Kobe earthquake. Polished slabs and thin sections cut to three right-angled surfaces (horizontal, vertical parallel to E-W, and to N-S) were used for 3-D geometric analyses. U-stage was used for healed and sealed microcracks in quartz grains, and CL images were used for healed microcracks in feldspar grains. The orientation of intragranular healed cracks in quartz give N-S strike with vertical dip from all samples, and thus sigma-3 must be E-W. However, an E-W strike with vertical dip is also obtained from several samples. Such co-existence of two perpendicular peaks can be explained that the sigma-1 direction switched to sigma-3 just after the release of stress during fault movement. The orientation of transgranular healed cracks in feldspars determined using CL images concentrates NNW strike with vertical dip, which approximates the orientation of intragranular healed microcracks in quartz. The orientation of intragranular and transgranular sealed microcracks concentrates two; ENE strikes with vertical dip, and horizontal. Existence of horizontal microcracks can be explained the switch of paleostress axis between sigma-2 and sigma-3 during exhumation of granitic body. The crack density of healed microcracks does not change with respect to the distance to the Nojima Fault, whereas that of sealed microcracks increases with approaching to the fault. Healed microcracks in quartz and feldspars are formed under relatively higher temperature condition (the homogenization temperatures range 160～280 ℃) probably before the rotation of SW Japan at 15 Ma, because sigma-1 direction (N-S) coincides with oceanic plate convergent direction (NW-WNW) before the 50° clockwise rotation of SW Japan. Sealed microcracks were probably formed under relatively lower temperature condition after the rotation of SW Japan at 15 Ma, because the sigma-1 direction (ENE-WSW) can move the Nojima fault dextrally as same as current fault movement.