[S-CG44] Science of slow-to-fast earthquakes

[SCG44-P08] Change in slip mode along subduction megathrust controlled by metasomatic reactions near mantle wedge corner

*Naoki Nishiyama¹, Kohtaro Ujiie¹, Kazuya Noro¹, Yasushi Mori², Haruna MASUYAMA¹ (1.University of Tsukuba, 2.Kitakyushu Museum of Natural History and Human History)

Keywords:Metasomatic reactions, Dissolution-precipitation creep, Slow slip event

Subduction megathrust below the land Moho exhibits a change in slip mode with depth, from episodic slow slip events (SSEs) to stable sliding. However, the factors controlling the downward change in the slip mode remain unknown. The Mie mélange in Kyushu, Japan, was deformed along the slab-mantle interface near the mantle wedge corner in warm-slab environments, resembling the inferred conditions of SSEs in the Nankai subduction zone beneath Shikoku. The mélange recorded distributed shear along anastomosing foliations in antigorite serpentinite and subsequent mixing of mafic, ultramafic, and metasedimentary rocks. The mixing of rocks induced metasomatic reactions between ultramafic and mafic rocks, resulting in the release of water from metasomatized mafic rock and the production of fine-grained actinolite in ultramafic schist. The fine-grained metasomatic actinolite exhibits chemical zoning of aluminum via dissolution-precipitation creep, leading to viscous shear localization at shear strengths lower than that of the surrounding rocks. The absence of tensile hydrofracture in highly foliated ultramafic shear zones may indicate that water released by the metasomatic reactions flowed along foliations in permeable ultramafic shear zones, which could contribute to an increase in the strain rate by accelerating the dissolution-precipitation creep. The down-dip limit of the metasomatic reactions, determined from the stable condition of the assemblage of actinolite and chlorite in the ultramafic shear zone, is ~40–50 km depth, comparable to the lower limit of the SSEs region in the Nankai subduction zone. We suggest that the metasomatism-related localized viscous shear along the weak ultramafic schist at increased strain rates represents SSEs along the slab-mantle interface near the mantle wedge corner, whereas distributed shear in antigorite serpentinite reflects the stable creep in the deeper stable sliding zone. The metasomatic reactions in the subduction mélange may control the depth-related change in the megathrust slip mode from SSEs to stable sliding below the land Moho.