[S-MP28] Deformed rocks, Metamorphic rocks and Tectonics

[SMP28-P11] P-T condition of recrystallization during mylonitic shear zone using quartz microstructural analysis: a case study of Hoping mylonite, northeastern Taiwan

*Ya-Chu Tseng¹, En-Chao Yeh¹, Hiroaki Yokoyama², Sando Sawa², Jun Muto², Jian-Cheng Lee³ (1.Department of Earth Sciences, National Taiwan Normal University, 2.Department of Earth Science, Tohoku University, 3.Institute of Earth Sciences, Academia Sinica)

Keywords:Ductile Shear Zone, Dynamic Rcrystallization, Differential Stress, Deformation Temperature, Electron Backscattered Diffraction

Large earthquakes frequently occur at the base of the seismogenic zone, corresponding to the brittle-ductile transition zone in the continental crust. The generation of large earthquakes likely requires high differential stress. Assessments of the differential stress and deformation temperature conditions can provide valuable insights into understanding the occurrence of large earthquakes. Ductile shear zones containing mylonitic rocks record intense shearing resulting from ductile or mixed brittle-ductile deformation mechanisms, which are most likely activated at the base of the seismogenic zone. During ductile deformation, recrystallized grains form because of dynamic recrystallization process, with their grain sizes related to differential stress. Additionally, deformation temperature influences the types of dynamic recrystallization and correlates with recrystallized grain size and c-axis fabric patterns of quartz. This study aims to determine differential stress and deformation temperature using recrystallized grain size and c-axis fabric patterns of quartz in the Hoping ductile shear zone, northeastern Taiwan. Electron backscattered diffraction (EBSD) analysis provides an efficient technique for collecting crystallographic data, including grain size and crystallographic preferred orientation. The separation of recrystallized and relict grains is based on the Gaussian mixture model of Grain Orientation Spread values (Yeo et al., 2023).
Preliminary results indicate that recrystallized grain sizes in the Hoping ductile shear zone are smaller than in the wall rock, consistent with grain size reduction in mylonites. Grain sizes range from 13.1-18.0 μm in the shear zone and 14.9-22.0 μm in the wall rock. The estimated differential stress based on recrystallized grain size piezometer ranges from 88.0 MPa to 110.4 MPa in the shear zone and 76.5 MPa to 100.8 MPa in the wall rock (Cross et al., 2017). The opening angle of quartz c-axis fabric patterns can indicate deformation temperature (Law, 2014). In the Hoping ductile shear zone, most opening angles of the quartz c-axis fabric patterns range from 68°-77°, suggesting deformation temperatures of 520°C to 579°C.