Megathrust Shear Modulated by Albite Metasomatism in Subduction Mélanges

Abstract Aseismic megathrust slip downdip of the seismogenic zone is accommodated by either steady creep or episodic slow events (SSEs). However, geological conditions defining rheology remain elusive. We examined exhumed subduction mélanges on Kyushu, Japan, which deformed at ∼370–500°C under greenschist to epidote‐amphibolite facies conditions, comparable warm‐slab environments. The recorded fluid release and viscous shear localization associated with metasomatic reactions between juxtaposed metapelitic metabasaltic rocks. Metasomatic caused albitization metapelite, resulting in depth‐dependent changes rheology. In a mélange ∼370°C, very fine grained reaction products (metasomatic albite) facilitated grain boundary diffusion stresses ∼45 MPa, less than those surrounding metabasalt. Mineralogical chemical during reactions, their field content, imply an onset albite metasomatism ∼350°C. Albite therefore potentially contributed decreased strength around inferred thermally controlled base zone. near mantle wedge corner ∼500°C, promoted local quartz vein formation localized strain rates, coarse‐grained behaved as relatively rigid blocks matrix. suggest that can facilitate mode depth may explain why from SSEs tremor increasing depth.