講演情報
[T4-P-6]沈み込みプレート境界における歪速度と剪断応力の推定：石英の変形機構とレオロジー特性からの知見
*駒木野 照太1、氏家 恒太郎1、イヨ トーマス1、重松 紀生2 (1. 筑波大学、2. 産業技術総合研究所)
キーワード：
石英、動的再結晶、レオロジー、深部スロー地震、EBSD
Quantitative estimation of strain rates and shear stresses along the subduction mélange is key to understanding the diverse slip behavior of megathrusts. We investigated the subduction mélange in western Kyusyu, Japan, which was deformed at ∼500 ℃ and ∼1.1 GPa under epidote-blueschist facies metamorphic conditions. Within the mélange, viscous shear is localized along multiple chlorite-actinolite schist (CAS) layers intercalated with metabasite and metasediments. The CAS layers exhibit composite planar fabrics and consistently show shear indicators aligned with megathrust movement. In the northern segment of the mélange, the contact between metapelite and CAS layers underwent albite metasomatism, supplying fluids to the CAS. Microscopic observations and electron backscatter diffraction analysis of quartz veins in both the CAS and surrounding rocks revealed spatial variations in quartz microstructures. In the northern segment, quartz in CAS layers show dynamic recrystallization by subgrain rotation, whereas in the southern segment, both subgrain rotation and grain boundary migration recrystallization are observed. Quartz in the surrounding rocks is mainly recrystallized via grain boundary migration, with occasional subgrain rotation. Rheological analysis using a quartz paleo-piezometer and flow law suggests that CAS in the northern segment experienced shear stresses of 41–52 MPa and strain rates of 2.5–6.8×10-10 s-1. In contrast, the southern CAS experienced lower shear stresses of 22–27 MPa and strain rates of 2.0–4.6×10-11 s-1, values that are comparable to those of surrounding rocks (22–25 MPa, 2.2–3.6×10-11 s-1, respectively). Notably, the higher strain rates and shear stresses estimated from the northern CAS coincide with the region where metasomatic dehydration is prominent. This suggests that stress amplification may result from elevated strain rates associated with metasomatic dehydration-enhanced viscous shear.