[S-CG50] Rheology, fracture and friction in Earth and planetary sciences

[SCG50-08] EBSD analyses on Kashio mylonite and rheology of mid-crustal shear zone

*Katsuyoshi Michibayashi¹, Takamoto Okudaira², Hiroto Endo³, David Mainprice⁴ (1.Department of Earth and Planetary Sciences, Nagoya University, 2.Osaka Metropolitan University, 3.Shizuoka University, 4.Université de Montpellier 2 / CNRS)

Keywords:mylonite, quartz, plagioclase, viscosity contrast

Ultramylonites are among the most extremely fault-related deformed rock types that occur commonly in the mid-crustal brittle–plastic transition and are mainly characterized by intensely sheared fine-grained microstructure and well-mixed mineral phases. Although the deformation mechanism of ultramylonites is a key to understanding the rheological behavior of a mid-crustal shear zone, its microstructural development is still quite controversial due to their intensely fine-grained microstructures. We investigated variously mixed quartz–plagioclase composites developed within a granitic mylonite in the Kashio shear zone along the Median Tectonic Line, which is the largest strike-slip fault in Japan. A high-quality phase-orientation map obtained by Electron backscattered diffraction showed not only a wide range of quartz–plagioclase mixing (10%–80% in quartz modal composition), but it also revealed a correlation between grain-size reduction and crystal-fabric weakening in quartz, indicating a change of deformation mechanism from dislocation creep to grain-size-sensitive creep in the mixed quartz-plagioclase composites. In contrast, plagioclase showed almost consistently fine grain size and weak to random crystal fabrics regardless of modal composition, indicating that grain-size-sensitive creep is dominant. Combined with laboratory-determined flow laws, our results show that the Kashio shear zone could have developed under deformation mechanisms, whereby the viscosity of quartz and plagioclase are nearly comparable, effectively within 10¹⁷–10¹⁹ Pa·s, thereby possibly enabling extensive shearing. We argue that such a shearing mechanism in ultramylonite could occur ubiquitously at plate or other tectonic boundaries or in large-scale fault zones in the middle crust.