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 di stribution is envisioned to control the ductile creep properties of fa ult zones that mainly slip by frictional processes. Fracture during ea rthquakes and aseismic frictional creep tend to reduce grain size. How ever, sufficiently small grains tend to dissolve so that larger grains grow at their expense, a process called Ostwald ripening. A dynamic s tedy state is reached where grain size reduction by fracture is balanc ed by grain growth from Ostwald ripening. The ductile creep mechanism within fault zones in hard rock is probably pressure solution where th e rate is limited by diffusion along. load-bearing grain-grain contact s. The diffusion paths that limit Ostwald ripening are to a considerab le extent the same as those for pressure solution. Active Ostwald ripe ning thus implies conditions suitable for ductile creep. An analytic t heory allows estimation of the steady-state mean grain size and the vi scosity for creep implied by this dynamic steady state from material p roperties and from the width, shear traction, and long-term slip veloc ity of the fault zone. Numerical models were formulated to compute the steady state grain size distribution. The results indicate that ducti le creep, as suggested in the companion paper, is a plausible mechanis m for transiently increasing fluid pressure within mostly sealed fault zones so that frictional failure occurs at relatively low shear tract ions, similar to 10 MPa. The relevant material properties are too poor ly known, however, for the steady state theory (or its extension to a fault that slips in infrequent large earthquakes) to have much predict ive value without additional laboratory experiments and studies of exh umed faults.