K‐Metasomatic Weakening of Oceanic Crust at Shallow Subduction Depths: Evidence From the Rodeo Cove Thrust Zone, Marin Headlands, California

Abstract Studies of exhumed subduction shear zones indicate that metamorphism and metasomatism of the oceanic lithosphere influence the composition, structure, and rheology of megathrust faults, particularly deep along the plate boundary (>30 km). However, less is known about the effects that fluid‐mediated chemical reactions have on the rheological evolution of oceanic crust at shallower depths, which may control diverse modes of fault slip and down‐stepping of the plate boundary into oceanic crust. Here, we present a structural and geochemical study of fault rocks from the Rodeo Cove thrust zone (RCT) in California to examine feedbacks between deformation and metasomatism of oceanic crust in a shallow subduction thrust environment (<15 km). At the RCT, deformation is accommodated by a dense network of reddish and greenish cataclasites, which surround altered basalt blocks containing abundant calcite veins and cement. Electron microprobe analyses show that the altered basalt is primarily composed of clinopyroxene, albite, chlorite, and pumpellyite, whereas the cataclasite is dominated by ferroaluminoceladonite (K‐ and Fe‐rich mica) and iron‐oxyhydroxides interlayered with well‐crystallized sheets of aluminoceladonite. Our findings suggest that subduction‐related faulting and cataclasis increased permeability within the basalt‐hosted shear zone, promoting extensive K‐metasomatism, first by oxidizing seawater and later by hydrothermal fluids sourced from subducted sediment and/or altered oceanic crust at greater depths. Moreover, contrasting deformation mechanisms between the less altered basalt and strongly K‐metasomatized cataclasite, combined with their constitutive properties quantified from deformation experiments, indicate that K‐metasomatism significantly decreased the frictional strength of oceanic crust causing strain to localize in the RCT.