11:30 AM - 11:45 AM
[SCG62-04] The rheology of a ductile shear zone inferred from microstructures of granitic mylonites in the Hida Belt
Keywords:Hida belt, shear zone, granitic mylonite, microstructure, rheology
In the Hida Belt, mylonite zones occur in granitic and metamorphic rocks, which appear to have developed between the Triassic and Jurassic ages [1]. Few studies have been conducted on the microstructures of mylonites to reveal the deformation conditions and flow states within the shear zone in the Hida Belt. This study focuses on the microstructures of granitic mylonites exposed in the southwestern part of the Hida Belt, and discusses the structural development of the shear zone.
Field surveys and rock sampling of the granitic mylonites were conducted along the Miyagawa River in Furukawa Town, Hida City, Gifu Prefecture, Japan. Thin sections were prepared in a plane perpendicular to the foliations and parallel to the lineations (XZ plane), and microstructural observations were carried out using a polarized microscope. EBSD data were obtained using the SEM-EBSD method to analyze the crystallographic preferred orientations (CPO) and grain sizes of the quartz. To extract the dynamically recrystallized grain size of quartz from the EBSD data, two types of filtering were conducted using the MATLAB MTEX Toolbox: GBF filtering, which removes grains sharing a small proportion of boundaries with the same mineral phase, and GOS & Area filtering, which separates recrystallized grains from relict grains based on the intracrystalline strain and grain area.
Mylonitic granites consist mainly of quartz, plagioclase, and K-feldspar, with secondary minerals such as chlorite and white mica. Quartz formed polycrystalline aggregates with undulatory extinction within the grains. Feldspar was found in both porphyroclasts and polycrystalline aggregates. The asymmetric textures indicate dextral shear sense. The c-axis CPOs of quartz exhibited Y-maximum patterns. The grain size distributions of quartz were close to log-normal, and the average grain size showed no significant differences among the samples.
The c-axis CPOs patterns and evidence of plastic deformation in feldspar suggest that the deformation temperature can be approximately 500 °C [2][3]. By applying the average grain size to two paleopiezometers [4][5], a flow stress of approximately 35 MPa was obtained. Accordingly, the strain rate was estimated to be roughly 10-12 s-1 using the quartz flow law [6]. Based on these results, we discuss the rheology of the shear zone in the Hida Belt.
Reference
[1] Takehara & Horie 2019 Island Arc, [2] Takeshita 1996 J Geol Soc Japan, [3] Passchier & Trouw 2005 Springer, [4] Twiss 1977 Pure Appl Geophys, [5] Cross et al. 2017 Geophys Res Lett, [6] Tokle et al. 2019 Earth Planet Sci Lett.
Field surveys and rock sampling of the granitic mylonites were conducted along the Miyagawa River in Furukawa Town, Hida City, Gifu Prefecture, Japan. Thin sections were prepared in a plane perpendicular to the foliations and parallel to the lineations (XZ plane), and microstructural observations were carried out using a polarized microscope. EBSD data were obtained using the SEM-EBSD method to analyze the crystallographic preferred orientations (CPO) and grain sizes of the quartz. To extract the dynamically recrystallized grain size of quartz from the EBSD data, two types of filtering were conducted using the MATLAB MTEX Toolbox: GBF filtering, which removes grains sharing a small proportion of boundaries with the same mineral phase, and GOS & Area filtering, which separates recrystallized grains from relict grains based on the intracrystalline strain and grain area.
Mylonitic granites consist mainly of quartz, plagioclase, and K-feldspar, with secondary minerals such as chlorite and white mica. Quartz formed polycrystalline aggregates with undulatory extinction within the grains. Feldspar was found in both porphyroclasts and polycrystalline aggregates. The asymmetric textures indicate dextral shear sense. The c-axis CPOs of quartz exhibited Y-maximum patterns. The grain size distributions of quartz were close to log-normal, and the average grain size showed no significant differences among the samples.
The c-axis CPOs patterns and evidence of plastic deformation in feldspar suggest that the deformation temperature can be approximately 500 °C [2][3]. By applying the average grain size to two paleopiezometers [4][5], a flow stress of approximately 35 MPa was obtained. Accordingly, the strain rate was estimated to be roughly 10-12 s-1 using the quartz flow law [6]. Based on these results, we discuss the rheology of the shear zone in the Hida Belt.
Reference
[1] Takehara & Horie 2019 Island Arc, [2] Takeshita 1996 J Geol Soc Japan, [3] Passchier & Trouw 2005 Springer, [4] Twiss 1977 Pure Appl Geophys, [5] Cross et al. 2017 Geophys Res Lett, [6] Tokle et al. 2019 Earth Planet Sci Lett.