Grain size and chemical controls on the ductile properties of mostly frictional faults at low-temperature hydrothermal conditions

A conceptually simple process which establishes a steady grain size distribution is envisioned to control the ductile creep properties of fault zones that mainly slip by frictional processes. Fracture during earthquakes and aseismic frictional creep tend to reduce grain size. However, sufficiently small grains tend to dissolve so that larger grains grow at their expense, a process called Ostwald ripening. A dynamic stedy state is reached where grain size reduction by fracture is balanced by grain growth from Ostwald ripening. The ductile creep mechanism within fault zones in hard rock is probably pressure solution where the rate is limited by diffusion along load-bearing grain-grain contacts. The diffusion paths that limit Ostwald repening are to a considerable extent the same as those for pressure solution. Active Ostwald ripening thus implies conditions suitable for ductile creep. An analytic theory allows estimation of the steady-state mean grain size and the viscosity for creep implied by this dynamic steady state from material properties and from the width, shear traction, and long-term slip velocity of the fault zone. Numerical models were formulated to compute the steady state grain size distribution. The results indicate that ductile creep, as suggested in the companion paper, is a plausible mechanism for transiently increasing fluid pressure within mostly sealed fault zones so that frictional failure occurs at relatively low shear tractions, 10 MPa. The relevant material properties are too poorly known, however, for the steady state theory (or its extension to a fault that slips in infrequent large earthquakes) to have much predictive value without additional laboratory experiments and studies of exhumed faults.