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Subduction megathrusts below the land Moho slip at either steady creep or episodic slow slip events (SSEs). However, deformation styles and mechanisms responsible for aseismic megathrust slip remain unknown. We examined the subduction mélange in Kyushu, Japan, which consists of ultramafic, mafic, and sedimentary rocks. The mélange deformed at ∼470 °C under epidote-amphibolite facies condition, comparable to the inferred conditions of SSEs near the mantle wedge corner in the Nankai subduction zone. Subduction-related viscous shear in the mélange was concentrated into antigorite serpentinite and ultramafic schist mainly composed of chlorite and fine-grained actinolite, which is characterized by anastomosing scaly foliations and S C fabric. The mixing of mafic and ultramafic rocks in the mélange induced metasomatic reactions, 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 and truncation of the zoning parallel to S surface through dissolution-precipitation creep. Water released by metasomatic reactions may assist the dissolution-precipitation creep. Rheological analysis indicates that the dissolution-precipitation creep of fine-grained actinolite in ultramafic schist accommodated plate convergence and SSEs at shear stresses of 0.3–5 MPa and 10–40 MPa, respectively, whereas antigorite serpentinite can accommodate slow slip rates at shear stresses of ≤43–94 MPa, much higher than inferred shear stresses during SSEs in active subduction zones. The down-dip limit of the metasomatic reactions, determined from the stable condition of metasomatic actinolite in the ultramafic schist, was ∼40–50 km depth, comparable to the lower limit of the SSEs region in the Nankai subduction zone. We suggest that while antigorite serpentinite only accommodated aseismic creep, dissolution-precipitation creep of metasomatic actinolite in weaker ultramafic schist can host more diverse slip behavior including aseismic creep and SSEs. The metasomatic reaction between mafic and ultramafic rocks at the slab-mantle interface is potentially one of the factors controlling the downdip limit of SSEs below the land Moho. • Mélange deformation was concentrated into serpentinite and ultramafic schist. • Metasomatic reactions produce fine-grained actinolite in ultramafic schist. • Metasomatic reactions weaken ultramafic schist by promoting pressure solution creep. • Pressure solution creep of metasomatic actinolite can facilitate megathrust slip. • Metasomatic reaction may control downdip limit of slow slip event. [ABSTRACT FROM AUTHOR]