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Abstract

In strike-slip fault systems, Coulomb failure stress changes due to mainshocks can trigger large aftershocks or further earthquakes.
The combination of static stress changes from mainshocks and large aftershocks potentially has a profound influence on the final
distribution of aftershocks and crustal-scale fluid redistribution. Because mineralization acts as a high fluid flux indicator the
interaction of static stress changes, fault triggering and fluid flow can be studied from mineralized fossil fault systems. Two examples
are presented from separate fault systems in the Kalgoorlie greenstone terrane,Western Australia (the Black Flag and Boulder-Lefroy
Fault systems). Using mapped fault geometries, slip directions and the known distribution of fault-hosted gold mineralization we
show that the repeated arrest of mainshock ruptures, at both dilational and contractional fault step-overs, controlled aftershock-related
fluid flow. Importantly, the largest aftershocks or subsequent triggered earthquakes exerted a very strong control on where the highest
fluid fluxes occurred through small-event aftershock fault networks (at distances up to ∼15 km away from the step-overs). Fluid flow
through mid-crustal fault systems in crystalline rock is spatially localised in regions where repeated clusters of aftershocks cause
permeability enhancement. It is dependent on the seismogenic behaviour of the system, rather than a passive exploitation of the
internal structure and fabrics developed by faults or damage zones. Field evidence implies that high pore fluid factors were repeatedly
attained in the aftershock-related mineralized faults and that the fluids were derived from deep-level, overpressured reservoirs, rather
than local wall rock porosity. It is apparent that high-pressure fluids, possibly released in a pulse after a mainshock, contribute to the
rupture of structures already promoted towards failure from static stress changes.

Item Type:	Article
Authors/Creators:	Micklethwaite, S and Cox, SF
Keywords:	Faults and fluids; Mineralization; Coulomb failure stress change; Aftershocks; Triggering
Journal or Publication Title:	Earth and Planetary Science Letters (EPSL)
DOI / ID Number:	10.1016/j.epsl.2006.07.050
Additional Information:	The definitive version is available at http://www.sciencedirect.com
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