EVIDENCE OF STRAIN LOCALIZATION IN A LOWER-TO MIDCRUSTAL SHEAR ZONE (KOREA)

The Sunchang shaer zone (SSZ) is NE-striking subvertical shear zone exposed for over 200 km along strike and ranges 18 km thickness, where consists of Precambrian gneisses, metasedimentary rocks, and Jurassic deformed granites. Detailed microfabric analyses are summarized below: The mean recrystallized quartz grain size gradually decreases from 0.27 mm to 0.05 mm symmetrically in three zones within the SSZ. This indicates the existence of strain localization into three high-strain zones (HSZ) with the same thickness and interval about 3 km. In the HSZs, quartz c-axis fabrics from the deformed granitoid are characterized by a single girdle and a type I crossed-girdle patterns. In contrast, in low-strain zones (LSZ, ca. 3 km-width), it is characterized by a Y-point maximum and a type II crossed-girdle patterns. Typical microstructures of feldspar in the HSZs indicate lower deformation temperature (e.g., flame perthites). On the other hand, those in the LSZs indicate higher deformation temperature (e.g., myrmekites). The transition of deformation temperatures and microstructures/fabrics occurs at the boundaries between the LSZ and the HSZ. Evidence of chloritization and sericitic alteration throughout the HSZs suggests the importance of fluid circulation in the shear zone under lower-temperature condition. In addition, mylonites within the HSZs have a higher content (4-26 vol%) of muscovite (mica fish or flakes) in the matrix than in the LSZ (ca. 1 vol%). Based on the existence of the three HSZs and variation of microstructures and fabrics, deformation processes are considered to be temperature-and strain-rate-dependent with time and depth, and the original variation of the content of muscovite in protoliths and reaction-enhanced softening probably played important roles to produce initial stage of strain localization (a positive-feedback).