FORMING A SLOW SLIPPING SUBDUCTION INTERFACE: P-T-T-X HISTORY OF METASOMATIC ROCKS

The interface between subducting and overriding plates is fluid-rich, chemically dynamic, and seismogenic. Constitutive relationships and microstructural evidence indicate that talc-rich rocks produced by metasomatism at this interface host episodic tremor and slow slip (ETS), but the chemical pathways producing talc and other metasomatic minerals remain unclear. Constraining the pressure-temperature-time-chemical evolution of such metasomatic rocks can reveal the full extent of their rheologic impact. We combine field mapping, geochemistry, geochronology, and thermometry to determine the P-T-t-X history for a suite of slow slip-hosting talc-, chlorite-, and amphibole-rich metasomatic rocks from the Catalina Schist (California). These rocks have both metasedimentary and ultramafic protoliths and compose the matrix of an exhumed subduction interface shear zone stratigraphically overlying coherent oceanic crust. Local exchange of Na, LILE and REE from metasedimentary to ultramafic rocks, and Mg, Fe, Cr, and Ni from ultramafic to metasedimentary rocks was complemented by addition of Ca from infiltrating fluid and loss of K from the system. Epidote veins in underlying metabasalts suggest dehydration of these rocks released the infiltrating Ca-bearing fluid. This metasomatism converted slab metasedimentary rocks to chlorite and actinolite schists and mantle wedge ultramafic rocks to talc and actinolite schists. Zr-in-rutile thermometry and RSCM of the metasomatic rocks indicate syn-kinematic metasomatism began during prograde subduction and continued through peak conditions comparable to those of modern episodic tremor and slow slip. Petrochronology of rutile and titanite overgrowths suggests the rutile-to-titanite reaction was catalyzed by metasomatism and constrains the timing of metasomatism to ~100 Ma consistent with subduction of this portion of the Catalina Schist. Both local exchange between juxtaposed lithologies and external fluid infiltration were key in driving metasomatism of the subduction interface and this work suggests Mg loss as a new pathway for talc formation in ultramafic rocks. Metasomatism spanned prograde through peak subduction and the source region of ETS representing a dynamic and evolving system impacting interface rheology across a broad range of depths.