THE ROLE OF METASOMATISM IN EPISODIC TREMOR AND SLOW SLIP: STRESS AND STRAIN RATE VARIATIONS IN A CHEMICALLY REACTING SHEAR ZONE

Episodic tremor and slow slip (ETS) is imaged downdip of the subduction seismogenic zone and represents a key unconstrained process in the seismic cycle. ETS occurs at the fluid-rich and chemically reactive subduction interface, yet metasomatic processes have not been fully incorporated into conceptual models of ETS. Recent rheological modeling suggests that talc-rich metasomatic rocks, common at the subduction interface, may host slow slip under high pore fluid pressures [1]. We test this model by investigating metasomatic rocks from a subduction interface mélange on Pimu’nga/Santa Catalina Island (California). Talc-bearing rocks in this mélange evolved from an ultramafic protolith through concurrent deformation and chemical reaction with adjacent metasedimentary rocks. Field mapping, thermometry, and U-Pb geochronology of titanite and rutile constrain these processes to the depths and temperatures of ETS in modern subduction zones. Integrated electron back-scatter diffraction (mineral orientation) and energy dispersive spectroscopy (chemical mapping) of actinolite reveals microstructural differences between talc-free and talc-bearing samples. Actinolite is deformed by dissolution-reprecipitation creep and rigid body rotation in talc-free rocks and dislocation creep/cataclasis in the talc-bearing rock. This contrast is best explained by stress amplification in actinolite from the talc-bearing rock produced by high strain rates in surrounding weak talc. Higher strain rates in the talc-bearing sample record episodic slow slip, while lower strain rates in the talc-free samples occurred during intervening aseismic creep. Nearby quartz veins record evidence of high pore fluid pressures and multiple deformation fabrics record episodic deformation, which together support this interpretation. The prevalence of talc in subduction interface mélanges and the agreement among this geologic study, and experimentally derived constitutive relations suggests talc could host all deep episodic slow slip. This work demonstrates the importance of considering metasomatism in studying subduction zone slip behaviors and its likely role in episodic tremor and slow slip.

[1] French, M.E., Condit, C.B., 2019. Earth Planet. Sci. Lett. 528, 115828.