Effect of Metasomatic Alteration on Frictional Behavior of Subduction Megathrusts

Abstract:

Along-strike variations in seismicity of subduction megathrusts can be attributed to the frictional properties of the fault-zone material, which is affected by the distribution of weak clays (smectite and illite) within sediments on the incoming plate. In addition, metasomatic alteration of the subducting sediments may result in significant changes in fault strength and slip stability of the megathrust.

We examined an exhumed subduction thrust that separates serpentinite from tectonic mélange (argillite) of the Franciscan Complex, central California. The serpentinite represents a cataclastic shear zone, consisting of angular fragments in a fine-grained talc matrix. Talc schist also developed near the fault in beds up to 2 m thick. The argillite away from the fault displays a scaly fabric, composed of illite/muscovite and chlorite, while it is altered near the fault, characterized by the overgrowth of tremolite and minor chlorite along the previous foliation.

We determined the frictional characteristics of these samples by performing rotary shear experiments at pore fluid pressures of 40–120 MPa, effective normal stresses (σ_n^eff) of 60–180 MPa, temperatures (T) of 20–400°C, and sliding velocities of 0.3–100 μm/s. The serpentinite was frictionally strong (friction coefficient, μ, 0.6) and exhibited velocity strengthening only at 150°C. The talc schist showed a low μ of 0.1–0.2, characterized by velocity-strengthening behavior at all experimental conditions tested. Argillite showed μ ranging from 0.4 to 0.6 with increasing T and σ_n^eff and a transition from velocity strengthening to velocity weakening behavior as T increased above 300°C. The tremolite schist had a weaker normal-stress dependence of μ than argillite, with μ of 0.4–0.5 and a velocity strengthening to velocity weakening transition occurring at 400°C.

We propose that intense fluid–rock interactions took place during movement of the investigated fault. The serpentinite–argillite contact is frictionally unstable, capable of seismic slip, while metasomatic alteration will result in a temporal evolution of slip stability from unstable to stabile sliding behavior. Thus, fluid-assisted metasomatic changes are key in lowering fault strength and forming aseismically-slipping patches in the megathrust.