Evolution of Deformation Processes, Stress States and Pore Fluid Factors During High Fluid Flux Fault-Valve behavior Near the Base of the Seismogenic Regime - an Example From the Archaean Argo Fault System, Kambalda, Western Australia

The development of low displacement, moderate to high-angle reverse faults during the formation of the Argo fault system involved a four stage evolution of deformation processes and associated hydrothermal alteration within a tholeiitic gabbro host-rock. Fault zone evolution was associated with high fluid flux, fault- valve behavior in a stress regime in which the maximum principal stress was approximately east-west and horizontal, and the minimum principal stress was sub-vertical. The fault system developed at approximately 400°C in a transitional brittle-ductile regime. Initial (Stage 1) deformation involved predominantly viscous shear and the development of potassic (biotite-rich) alteration assemblages and associated reaction- weakening; minor quartz extension veins were formed. Stage 2 is marked by onset of predominantly brittle shear failure, the widespread development of vein matrix-supported breccias in fault zones, and sodic (albite- carbonate-quartz) alteration styles. Extension veins have limited development. Stage 3 is characterized by a change to quartz-carbonate assemblages in fault-fill veins and breccias, and the widespread development of large extension vein arrays. In Stage 4, widespread sub-horizontal quartz-carbonate-biotite extension veins were developed, but shear failure was limited. Failure mode diagrams in pore fluid factor and differential stress space are used to illustrate how the structural evolution of the Argo fault system was a response to progressive changes in relative rates of change of pore fluid factor and differential stress during individual fault-valve cycles. High fluid fluxes and rapid rates of recovery of fluid pressures, relative to rates of recovery of shear stress after slip events, maintained the system at near-lithostatic fluid pressures and very low differential stresses during fault activity.