[3poster01-18] T4. Deformation and reaction of rocks and minerals activities

entryECS

[T4-P-3] Deformation mechanisms and rheology of plate boundary rocks in subduction zones

*Yuto Yamasaki¹, Kohtaro Ujiie¹, Thomas Yeo¹ (1. University of Tsukuba)

Keywords:

deformation mechanisms,rheology,slow slip,chlorite-actinolite schist,blueschist

　Chlorite-actinolite schist (CAS) and blueschist are among the primary lithologies constituting the subduction plate boundary from the downdip end of the seismogenic zone to the vicinity of the mantle wedge. However, their deformation mechanisms and rheological properties remain controversial. We investigated a subduction mélange in western Kyushu, Japan, deformed at ~500 ℃ and ~1.1 GPa under epidote-blueschist facies metamorphic conditions, comparable to those in the source region of deep slow slip events (SSEs) in the Nankai subduction zone beneath Shikoku. The mélange exhibits localized viscous shear along multiple 2–60 cm-thick CAS layers intercalated with metabasite (or blueschist) and metasediments, with a shear direction consistent with that of megathrust shear. Microstructural and electron backscattered diffraction analyses of CAS reveal that actinolite exhibits aluminum zoning along its long axis, weak crystallographic preferred orientation (CPO), and low grain orientation spread (GOS) values. These features suggest deformation was primarily accommodated by dissolution-precipitation creep. In contrast, glaucophane in blueschist occurs as microboudins, with sodic-calcic to calcic amphiboles diffusing into boudin necks. Glaucophane displays weak CPO, low GOS values, and c-axis maxima aligned parallel to the shear direction. These observations indicate that blueschist deformed mainly via diffusion creep, limited by microboudinage. Rheological analysis using flow laws for dissolution-precipitation creep and diffusion creep suggests that blueschist is mechanically weaker than CAS, which is consistent with the observed localization of viscous shear along CAS layers. At estimated shear stresses along modern megathrust interfaces in the source region of deep SSEs (~10–30 MPa), viscous shear in CAS occurred at strain rates one to two orders of magnitude higher than those of blueschist, which range from 2.8×10^-13 s^-1 to 2.6×10^-12 s^-1. This suggests that, while blueschist accommodates aseismic creep, the relatively higher strain rates localized in multiple CAS layers may play a critical role in the generation of deep SSEs.