FAULTING PROCESSES AT HIGH FLUID PRESSURES - AN EXAMPLE OF FAULT VALVE BEHAVIOR FROM THE WATTLE GULLY FAULT, VICTORIA, AUSTRALIA

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 degrees 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 degrees to 80 degrees 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.