The role of metasomatic alteration on frictional properties of subduction thrusts: An example from a serpentinite body in the Franciscan Complex, California

Fluid–rock interaction within accretionary prisms drastically changes the frictional strength and slip stability of the fault zone. In order to understand the effect of ultramafic components on the degree of interplate coupling, we present data on frictional properties of a reaction zone between serpentinite and argillite in a tectonic mélange of the Franciscan Complex, central California. Field and petrographic observations indicate that the argillite and serpentinite along the lithological boundary are metasomatized to tremolite and talc schists, respectively, forming a ductile shear zone. Simulated gouges made from fault and wall rock samples collected are sheared at effective normal stresses (σneff) of 60–180 MPa and temperatures (T) of 20–400C∘, using a hydrothermal ring shear machine. The serpentinite is frictionally strong (steady-state friction coefficient μ_ss=0.5–0.6), and exhibits both velocity-weakening and velocity-strengthening behavior. The μ_ss of the argillite and tremolite schist increases up to 0.7 and 0.6, respectively, with increasing σneff and T, while (a−b) transitions from positive to negative with increasing T. At a given experimental condition, μ and (a−b) values are lower and higher for the tremolite schist than for the argillite. In contrast, the talc schist exhibits very low friction (μ_ss=0.1–0.2) and velocity-strengthening behavior at all experimental conditions tested. Our results demonstrate that Si-metasomatism along the argillite–serpentinite contact results in reduced friction and stabilized slip, forming frictionally weak, velocity-strengthening regions in the megathrust zone.