[S-CG45] Science of slow-to-fast earthquakes

[SCG45-33] Stress amplification and increased strain rate along the chlorite-actinolite schist: Implications for deep slow slip event

*Yuto Yamasaki¹, Kohtaro Ujiie¹, Thomas Yeo¹, Shota Komagino¹, Norio Shigematsu² (1.University of Tsukuba, 2.National Institute of Advanced Industrial Science and Technology (AIST))

Keywords:chlorite-actinolite schist, microboudinage, deformation mechanisms, slow slip events

Chlorite-actinolite schist (CAS) is a key lithology accommodating viscous shear along megathrusts in subduction zones. Numerical models suggest that slow slip events (SSEs) may occur along shear zones composed of stronger lenses embedded in a weaker viscously deformed matrix, but remain poorly tested in nature. We examine the subduction mélange deformed at ~500 ℃ and ~1.1 GPa under epidote-blueschist facies metamorphic conditions, comparable to the source area of deep SSEs in the Nankai subduction zone. Viscous shear deformation in the mélange is concentrated along the multiple CAS layers, with shear directions consistent with north-northwest subduction along a NE-SW striking paleo-Pacific margin. A contrast in deformation mechanisms is observed between lenses and matrix within the CAS with the actinolite-dominated matrix deformed primarily by dissolution-precipitation creep, whereas quartz lenses deformed by dislocation creep ± grain boundary sliding. Rheological analyses of quartz and actinolite indicate that dissolution-precipitation creep of actinolite can coexist with quartz dislocation creep at shear stresses of ~50 MPa and strain rates of 5.3-7.6×10^-10 s^-1, under near lithostatic fluid pressure. These deformation conditions likely reflect enhanced viscous shear driven by dehydration associated with metasomatic reactions along the CAS. Adjacent to the CAS, glaucophane in the blueschist shows microboudinage, with sodic-calcic and/or calcic amphibole diffusing into the boudin necks, indicating deformation by diffusion creep at higher shear stress and lower strain rate than in the CAS. The dynamic recrystallization mechanism of quartz also varies spatially. For quartz not surrounded by CAS and those surrounded by CAS, recrystallization occurs via grain boundary migration and subgrain rotation respectively, consistent with increased strain rates. We suggest that the contrast in deformation mechanisms between matrix and lenses within the CAS is caused by stress amplification in the shear zones, and that increased strain rates along the CAS relative to the surrounding rocks represent the generation of SSEs.