Abstract
Dextral ductile shear zones are found within the NW-SE-striking granitic porphyry dikes (older dikes) that intruded the older Ryoke granitoids on Awaji Island. The occurrence of these small-scale shear zones in the dikes is significant when considering the mechanism of strain localization, the history of stress, and the tectonics of the Ryoke Belt. This paper aims to clarify the geotectonic history of mylonite zones in the older dikes on the basis of petrological and structural analyses together with geochronological data. The mylonitized granitic porphyry yields K-Ar hornblende ages of 79.8 and 81.0 Ma. The mylonitic foliation of granite porphyry dikes is subparallel to the intrusion surfaces (NW-SE) and not parallel to the foliation of weakly to moderately mylonitized older Ryoke granitic rocks, which strikes E-W to ENE-WSW. Quartz porphyroclasts in granitic porphyry are commonly highly elongate and recrystallized, and the axial ratio of the porphyroclasts ranges from 3:1 to 17:1. Most of the estimated strain ellipsoids lie in the uniaxial shortening field. In the northern Shio area, granite porphyry is strongly mylonitized and contains recrystallized quartz with a mean grain size of ~100 μm. Quartz c-axis fabrics of the granite porphyry mylonites are midtemperature type (400-550°C), whereas those of the older Ryoke granitoids are low-temperature to transition (300-450°C) types. On the basis of the meso- and microstructural evidence, together with K-Ar ages, the generation process of mylonite prior to the Miocene clockwise rotation of southwestern Japan is inferred to be as follows: (1) mylonitization in Granitic Rocks I occurred at 90-85 Ma, (2) EW-striking intrusion of granite porphyry dikes along extension cracks took place at 85-80 Ma, and (3) dextral mylonitization along the E-W-striking dikes occurred at 80-70 Ma in a NW-SE compressional regime.
