[SSS17-P20] Determination of the deformation conditions of the Shajigami Shear Zone developed in Fukushima Prefecture, northeast Japan, based on deformation microstructures of mylonites
Keywords:Shajigami Shear Zone, Mylonite, Lattice preferred orientation
The granodiorite mylonite is distributed along the Shajigami Fault and mostly overprinted cataclasis. The mylonites show quartz LPO patterns suggesting activity of rhomb <a> and/or prism <a> system. The microstructures and LPO patterns suggest dislocation creep took place at about 400 °C (Takeshita, 1996; Passchier and Trouw, 2005). The mean grain size of recrystallized quartz ranges 13.8–21.1 μm.
The grain size (mean: 16.9–46.9 μm) of recrystallized calcite in the limestone mylonite is minimum along the fault. Asymmetric deformation microstructures indicate a dextral shear, but some calcite porphyroclast preserve a microstructure of former sinistral shear. The LPO is characterized by a maximum of the c–axes in the Z direction rotating clockwise (10–20°). The a–axes are distributed within a girdle in the XY plane. The twin geometry of calcite grains indicates the plastic deformation above 200 °C (Burkhard, 1993). Hisada and Takagi (1992) estimated the granodiorite cataclasites are formed up to 90 Ma.
In conclusion, the granodiorite mylonites are deformed at about 400 °C after 105 Ma (hornblende K–Ar ages; Agency for Natural Resources and Energy, 1990). After the strike–slip inversion, the limestone mylonites and granodiorite cataclasites were formed at 200–300 °C up to 90 Ma.
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