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Occurrence of instability in crustal faults depends in part on the small-magnitude dependence of frictional strength on slip rate and slip history. Rate dependence of friction reflects the operation of thermally activated mechanisms at points of contact along fault surfaces and is expected to change in space and time owing to variations in environmental conditions and slip rams during the seismic cycle. Several lines of evidence suggest solution-precipitation processes in fault zones may be activated during interseismic periods when slip rates are small and may contribute to fault healing. We develop a constitutive model for faulting at hypocentral conditions that is capable of describing the variation in frictional properties as different slip mechanisms are activated in response to changes in temperature or slip rate. This model is based on the assumption that slip mechanisms are thermally activated and follow an Arrhenius relationship between temperature and slip rate, which allows the addition of temperature dependence to existing rate-and state-dependent friction constitutive laws. Multiple slip mechanisms are treated as operating independently and concurrently, where each mechanism is described by the rate-, state-, and temperature-dependent friction constitutive relation. The constitutive model is used to analyze tdaxial friction experiments on ultrafine-grained quartz gouge at temperatures to 600øC, effective confining pressure of 150 MPa, and water-saturated or room-dry conditions. These experiments investigated the stress relaxation response and slip history effects during slide-hold-slide tests with hold times up to 105 s. The microstructure of the deformed quartz gouge and the transient friction behavior define at least two distinct frictional slip regimes: a low-temperature regime characterized by cataclastic mechanisms with significant slip history effects, and a high-temperature regime characterized by solution-precipitation-aided cataclastic flow with large-magnitude rate dependence and insignificant slip history effects. In the model the parameters of the friction constitutive relation (e.g., a, b, and L) are treated as constants for each slip mechanism but are different for the different mechanisms. This model accurately describes the frictional behavior within each regime and across the transition between regimes. The analysis suggests that the greatest-magnitude rate weakening behavior occurs at 100 ø to 300øC under wet conditions at laboratory slip rates. Significant solution-precipitation is activated at temperatures above 300øC at laboratory slip rates or at lower slip-rates and lower temperatures. The high-temperature solution-precipitation regime is described by a large-magnitude rate strengthening (a -b = 0.03) and an apparent activation energy of ap-kJ moF . The constitutive analysis suggests that the solution-precipitation-aided flow mecha-proximately 44 nism could be important during interseismic periods at hypocentral conditions and low shear stress but apparently is not characterize...

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An elastic-plastic material model, with strain-hardening or -softening, and volumetric strains, implemented within a general-purpose finite-element system (SAVFEM TM ), is shown to reproduce the stress -strain relationships and localized to de-localized (brittle to ductile) changes in strain response that have long been observed in typical laboratory experiments on common porous rocks. Based on that validation of the implementation, SAVFEM TM is then used to create numerical simulations that reproduce the patterns of localized shear zones, and their growth history, that occur in experimental (physical) models of fold-fault systems in layered rocks. These simulations involve a progressive evolution of the mechanical state, illustrating a geometrically dominated type of localization behaviour. Part of the deformation simulated here represents a crestal graben system. Analysis of the evolving mechanical state in the system of simulated faults poses challenges to some longstanding ideas concerning the way that faults operate, suggesting the need for a new fault-process paradigm.

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