Fluid Flow Controlled By Progressive Fault Triggering And Aftershock Distributions In Archean Fault Systems

Permeability enhancement associated with aftershock sequences potentially plays an important role in controlling localization of fluid flow and fluid redistribution between crustal fluid reservoirs. Co-seismic positive static stress changes trigger clusters of aftershocks on fault networks around a mainshock. Likewise, gold mineralization associated with two Archean fault systems in the Kalgoorlie greenstone terrane, Western Australia, is hosted in domains of small-displacement fault networks adjacent to unmineralized high- displacement faults. Field evidence demonstrates that high pore fluid factors were repeatedly attained in the mineralized small-displacement faults and that the fluids were out of chemical equilibrium with the host rock. It has been shown that the distribution of gold deposits broadly correlates with domains of positive static stress change generated by mainshocks expected on the high-displacement faults. Here, we take the analysis further by using fault geometries and slip directions to calculate static stress changes from both the arrest of mainshock ruptures at fault step-overs, plus the triggering of subsequent ruptures. On this basis, it is shown that large aftershocks significantly modify the initial distribution of positive static stress change and that this modified distribution of stress change has a strong correlation with the distribution of mineralization. Thus large aftershocks exerted a modifying control on the final distribution of small-magnitude aftershocks, which in turn modified the fluid flow pathways through an aftershock zone and controlled where the highest fluid fluxes developed. The permeability enhancement from repeated triggering of aftershock faulting occurred up to 15 km away from the fault step-overs. The field evidence implies that overpressured fluids contribute to the rupture of small-displacement faults already promoted towards failure by static stress changes. These fluids were derived from breached, deep-level, overpressured reservoirs rather than local wall-rock porosity.