[S-CG50] 地球惑星科学におけるレオロジーと破壊・摩擦の物理

[SCG50-08] 鹿塩マイロナイトのEBSD解析と大陸地殻延性剪断帯のレオロジー

*道林 克禎¹、奥平 敬元²、遠藤 弘人³、Mainprice David⁴ (1.名古屋大学 大学院環境学研究科 地球環境科学専攻 地球惑星科学系、2.大阪公立大学、3.静岡大学、4.モンペリエ大学・CNRS)

キーワード：マイロナイト、石英、斜長石、粘性比

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.