No abstract

This study reports the results of paleostress analysis using the three-dimensional orientation and distribution of healed microcracks (HCs) in quartz within granitic bodies. Although the mechanical constraints of a universal stage together with the Terzaghi factor introduce sampling bias to the orientation and distribution of microstructures, few previous studies have taken this bias into consideration. We devised a series of corrections that allow the investigation of the orientation and distribution of HCs within the Ryoke granites, located along the Median Tectonic Line (MTL) in the Chubu region of Japan. Paleostress values were determined by fitting mixed Bingham distributions to the corrected distribution of each sample, resulting in the clustering of HC orientations in each data set into a few groups, which were then inverted to yield a paleostress state. The state corresponding to the group with the maximum number of HCs was considered to be the most significant stress for each individual sample, and the orientation of σ of this most significant stress in most samples is roughly perpendicular to the trend of the MTL. However, significant variations in the distribution of σ and σ were also identified, suggesting that the orientation of σ probably reflects the regional stress field, whereas the orientations of σ and σ are more dependent on the local stress state within the granitic body.

The Iragawa mylonite zone is ~ . km long (N-S) and ~ m wide, and occurs in the Cretaceous Shirakamidake granitic complex along the western coastline of southernmost Aomori Prefecture, Northeast Japan. As already reported and confirmed by magnetic susceptibility measurements, the Iragawa mylonite zone is not the northern extension of the Hatagawa fault zone, but exists within the Abukuma Belt. The center of the mylonite zone, which is ~ m wide, consists of ultramylonite locally overprinted by cataclasite. The mylonitic foliation strikes N-S and dips °-° to the east, while the mylonitic lineation plunges at °-° to the northeast. Asymmetric deformation microstructures indicate a sinistral normal shear. The lattice preferred orientation (LPO) and grain size of recrystallized quartz across the mylonite zone, as measured using SEM-EBSD, reveal that the most fine-grained ultramylonite displays a random LPO pattern and mean grain size of recrystallized quartz of . -. μm. The other mylonites mostly show LPO patterns indicating activity of the rhomb and/or prism systems, with a mean grain size of recrystallized quartz ofμm. The former suggests grain boundary sliding as the dominant deformation mechanism, whereas the latter suggests that dislocation creep took place at -.