Evidence is presented to support the conclusion that pore fluid pressures tresses PF during regional metamorphism are generally greater than or equal to the minimum principal compressive stress S3. The resultant very low effective stresses σ lead to significantly increased porosity and permeability, even at moderate to high metamorphic temperatures. Permeabilities between 10−18 and 10−15 m2 and are considered to be common, resulting in rapid fluid migration and the dominance of advective (infiltrative) over diffusive mass transport, even over relatively small distances. In view of the importance of intergranular mass transport to rock deformation during metamorphism, a mobile, high‐pressure fluid will have substantial rheological effects, especially in polyphase rocks. The fluid is capable of influencing the rate of dislocation creep in a number of ways. More importantly, advective mass transport along fluid pressure gradients can give rise to a solution transfer deformation mechanism that competes with conventional pressure solution. The rate of deformation by advective mass transport could be controlled by a number of processes, including dissolution kinetics, advective transport rates, and the rate of crack growth. A specific deformation model, based on advective transport rate control,is developed, which can produce strain at competitive rates but with stress and temperature dependences of unusual form.

The internal structures of the Wattle Gully Fault provide insights about the mechanics and dynamics of fault systems exhibiting fault valve behavior in high fluid pressure regimes. This small, high‐angle reverse fault zone developed at temperatures near 300°C in the upper crust, late during mid‐Devonian regional crustal shortening in central Victoria, Australia. The Wattle Gully Fault forms part of a network of faults that focused upward migration of fluids generated by metamorphism and devolatilisation at deeper crustal levels. The fault has a length of around 800 m and a maximum displacement of 50 m and was oriented at 60° to 80° to the maximum principal stress during faulting. The structure was therefore severely misoriented for frictional reactivation. This factor, together with the widespread development of steeply dipping fault fill quartz veins and associated subhorizontal extension veins within the fault zone, indicates that faulting occurred at low shear stresses and in a near‐lithostatic fluid pressure regime. The internal structures of these veins, and overprinting relationships between veins and faults, indicate that vein development was intimately associated with faulting and involved numerous episodes of fault dilatation and hydrothermal sealing and slip, together with repeated hydraulic extension fracturing adjacent to slip surfaces. The geometries, distribution and internal structures of veins in the Wattle Gully Fault Zone are related to variations in shear stress, fluid pressure, and near‐field principal stress orientations during faulting. Vein opening is interpreted to have been controlled by repeated fluid pressure fluctuations associated with cyclic, deformation‐induced changes in fault permeability during fault valve behavior. Rates of recovery of shear stress and fluid pressure after rupture events are interpreted to be important factors controlling time dependence of fault shear strength and slip recurrence. Fluctuations in shear stress and transient rotations of near‐field principal Stresses, indicated by vein geometries, are interpreted to indicate at least local near‐total relief of shear stress during some rupture events. Fault valve behavior has important effects on the dynamics of fluid migration around active faults that are sites of focused fluid migration. In particular, fault valve action is expected to lead to distinctly different fluid migration patterns adjacent to faults before, and immediately after, rupture. These fluid migration patterns have important differences with those predicted by models for dilatancy‐diffusion effects and for poroelastic responses around reverse faults.

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[1] There is widespread evidence indicating that faults regain a portion of their strength during the interseismic period. Here, we present experiments designed to understand and quantify the interseismic cohesive strengthening resulting from fluid-rock reactions in fault zones. The triaxial experiments consisted of fracturing cores of Fontainebleau sandstone under dry conditions, forming a localized shear failure zone (stage 1). The specimens were then reacted hydrothermally under isostatic conditions, allowing the fault damage zone to compact, consolidate and strengthen (stage 2). Following reaction, the specimens were then reloaded to failure under nominally dry conditions, so that the increase in cohesive strength of the fault could be measured (stage 3). Experiments show that cohesion increase is positively correlated to temperature and pore pressure during reaction. After 6 hours of reaction at the highest temperatures (927°C) and pore pressures (200 MPa), cohesion increases by as much as 35 MPa. Microstructural examination of the specimens showed that the gouge particles within the fault compacted and cemented together, exhibiting textures typical of pressure solution and that fractures in the surrounding damage zone had healed. A theoretical treatment of the data was conducted using these experiments in combination with results on time-dependent changes in fault cohesion presented by Tenthorey et al. (2003). We find that the rate-controlling process in our experiments has an activation energy (Q) of approximately 70 kJ mol À1. We use this information to develop a model for time-dependent cohesive strengthening in fault zones within the continental seismogenic regime. We conclude that significant cohesive strengthening of fault zones can occur during the interseismic period of medium to large earthquakes given the presence of reactive pore fluids.

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