Metasomatism and Mélange Development at the Conditions of Modern Deep Slow Slip: P-T-t Evolution of Metasomatic Rocks (Pimu'nga/Santa Catalina Island, CA)

The subduction interface hosts megathrust earthquakes and ductile creep, is fluid-rich and chemically dynamic, and produces metasomatic rocks that may host episodic tremor and slow slip (ETS). However, determining the depths at which these metasomatic rocks form and deform remains challenging. We reconstruct the pressure-temperature-time (P-T-t) evolution of epidote amphibolite-facies subduction interface metasomatic rocks suggested to host slow slip (Pimu'nga/Santa Catalina Island, California) using accessory phase petrochronology, thermometry, and thermodynamic modeling. Talc-, actinolite-, and chlorite-rich metasomatic rocks were produced from ultramafic, metasedimentary and metamafic protoliths by a combination of local chemical exchange, fluid infiltration and mechanical mixing. Rutile thermometry constrains the prograde initiation of local chemical exchange to near the mantle wedge corner (450–550°C) where the slab top and mantle were first juxtaposed. Metasomatism continued through peak metamorphic conditions at the depths of modern ETS (∼1 GPa, 550°C), constrained by carbonaceous material thermometry and the stability of albite and titanite in actinolite-rich rocks. Periodic influx of Ca-rich fluid released by dehydration of downgoing oceanic crust occurred near peak metamorphism and is recorded by the growth of titanite and development of actinolite-rich layers within talc-rich rocks. These results suggest that chemical exchange throughout the depths of modern ETS produced weak talc-rich rocks that may have hosted slow slip events under high fluid pressures produced by infiltrating Ca-rich fluids. Such complex chemo-mechanical interactions profoundly influence deformation and seismicity in subduction zones.