Porosity reduction in post-rupture ultramylonitic fault rocks – implications for fluid transport in Archean granitic shear zones

Abstract:

Dynamic permeability can focus fluids into ductile shear zones and brittle faults and lead to chemical modification and mechanical weakening of dry igneous and metamorphic rock. The availability of fluids in granitic crust and the capability of ductile shear zones to act as large-scale conduits is not well constrained. We use microscale X-ray computed tomography, electron microscopy and high-definition X-ray fluorescence imaging to characterize granitic fault rocks from the Neoarchean Moyagee Fault in the Yilgarn Craton of Western Australia. The Moyagee Fault formed when a crustal scale transpressional ductile shear zone in granitic host rocks experienced dynamic rupturing under amphibolite facies metamorphic conditions during the late stages of the c. 2730-2660 Yilgarn Orogeny. Rupturing was followed by ultramylonite formation before movement in the shear zone ceased overall. Uncharacteristically for ductile shear zones, ultramylonite formation reduced rather than increased micro-scale porosity in the fault rocks – we measured a reduction by more than two orders of magnitude. Sequence and timing of deformation processes suggest that a limited availability of hydrous fluid in the late-orogenic environment restricted dynamic weakening, resulting in the fault zone becoming a fluid sink and ultimately stop deforming. Our results demonstrate that while actively deforming ductile shear zones in mid-crustal environments are likely to focus fluids, they are not necessarily capable of fluid transport. Ductile shear zones that were active in the granitic middle crust during the late stages of crustal evolution in the Yilgarn Craton are therefore unlikely to have provided sufficient hydraulic conductivity to feed hydrothermal systems related to orogenic gold deposits.