[S-SS10] Fault Rheology and Earthquake Physics

[SSS10-P21] Deformation mechanisms of quartz in plate boundary rocks deformed in a deep slow earthquake source region

*Shota Komagino¹, Kohtaro Ujiie¹, Thomas Yeo¹, Norio Shigematsu² (1.University of Tsukuba, 2.National Institute of Advanced Industrial Science and Technology)

Keywords:block-in-matrix fabric, quartz, dislocation creep, rheology, deep slow slip, EBSD

Block-in-matrix fabric, characterized by blocks embedded in a weaker matrix, is commonly observed in plate boundary rocks exhumed from deep slow earthquake source regions. Numerical models suggest that deep slow earthquakes arise from by heterogeneous deformation of mélange zones characterized by block-in-matrix fabric. However, the deformation mechanisms, shear strength, and strain rates during mélange formation remain unclear. Here, we investigate the Nishikashiyama mélange, which was exhumed from a deep slow earthquake source region, comparable to the Nankai subduction zone beneath Shikoku. Viscous shear in the mélange is localized along the chlorite-actinolite schist, with a shear direction consistent with north-northwest subduction along a NE-SW striking paleo-Pacific margin. Within the chlorite-actinolite schist, quartz veins lenses are embedded in the matrix. Microstructural observations and electron backscatter diffraction analysis of these quartz lenses reveal a strong crystallographic preferred orientation and high grain orientation spread values. Quartz exhibit microstructures indicative of recrystallization, primally through subgrain rotation, with lesser contributions from grain boundary migration. Dislocation creep and dislocation-accommodated grain boundary sliding are the primary deformation mechanisms, with basal-a slip and prism-c slip as the dominant active slip systems. The shear strength and strain rate of chlorite-actinolite schist, estimated from the quartz recrystallized grain size piezometer and quartz flow law are ~50 MPa and ~5.3×10^-10-7.6×10^-10 s^-1, respectively. In contrast, grain boundary migration mainly controlled the microstructure in quartz veins located outside the chlorite-actinolite schist. This contrast in microstructures between the inner and outer chlorite-actinolite schist suggests an increased strain rate along the chlorite-actinolite schist, which may correspond to the occurrence of slow slip events along the chlorite-actinolite schist.