Deformation microstructures developed in the Iragawa mylonite zone in the western part of the Shirakami Mountains, Northeast Japan

Abstract

The Iragawa mylonite zone is ~2.5 km long (N-S) and ~350 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 ~200 m wide, consists of ultramylonite locally overprinted by cataclasite. The mylonitic foliation strikes N-S and dips 40°-80° to the east, while the mylonitic lineation plunges at 30°-70° 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 7.8-9.2 μm. The other mylonites mostly show LPO patterns indicating activity of the rhomb <a> and/or prism <a> systems, with a mean grain size of recrystallized quartz of 13-250 μm. The former suggests grain boundary sliding as the dominant deformation mechanism, whereas the latter suggests that dislocation creep took place at 350-450℃.