Slow slip along subduction megathrust controlled by metasomatism-enhanced viscous shear

Slow slip events (SSEs) downdip of the locked seismogenic zone accommodate relative plate motions and may trigger megathrust earthquakes in subduction zones. However, the geological conditions and deformation mechanisms responsible for SSEs remain elusive. Here, we show that subduction mélanges deformed at ~370-500 °C in warm-slab environments record locally elevated viscous shear strains associated with products of metasomatic reactions between pelitic and basaltic rocks. The metasomatism is characterized by albitization of pelite associated with migration of sodium-bearing fluids. We constrain elevated strain rates using quartz paleopiezometry and existing quartz flow laws. In a mélange deformed at ~370 °C near the downdip limit of the seismogenic zone, very fine-grained metasomatic albite facilitates grain boundary diffusion creep in addition to quartz dislocation creep at strain rate of ~10^-13s^-1, one order of magnitude faster than surroundings. In a mélange deformed at ~500 °C near the mantle edge corner, the fluids released from metasomatic reactions led to hydrofracturing and viscous shear at increased strain rates of ~10^-10s^-1. Albitization is not recognized in shallower subduction mélanges deformed at temperatures lower than ~330 °C. Our findings suggest that metasomatism-enhanced viscous shear is a fundamental process that may generate deep SSEs in warm-slab environments.