Role of Metasomatic Reactions of Metabasite on Plate-boundary Shear Localization at Slab-mantle Interface

Geophysical observations have shown that fluids play an important role on the generation of slow earthquakes along the subduction plate boundary near the mantle wedge corner. However, the plate boundary processes remain poorly understood. We examined the subduction mélanges in Kyushu Island, Japan, which were deformed at depths of ~ 25- 30 km and temperatures of ~450-500 ° C, comparable to environments of deep slow earthquakes. The subduction mélanges are composed of metabasite, pelitic schist, serpentinite, and chlorite-actinolite schist. The shear deformation is localized along the 0.02 - 5 m-thick chlorite-actinolite schist, resulting in strong shape and crystallographic preferred orientation of actinolite and chlorite. The sheared zone accompanied with ~0.1 -2 m-thick metasomatized metabasite. The metasomatic reactions in metabasite are characterized by decreased calcium (Ca) associated with consumption of actinolite and epidote and water release. The coexistence of metasomatized metabasite and localized shear zone suggest that the Ca-metasomatism and associated dehydration may lubricate the plate boundary fault by reaction-enhanced shear, resulting in the shear localization along chlorite-actinolite schist.