Palaeopermeability structure within fault-damage zones: A snap-shot from microfracture analyses in a strike-slip system

Abstract

Understanding fault zone permeability and its spatial distribution allows the assessment of fluidmigrationleading to precipitation of hydrothermal minerals. This work is aimed at unraveling theconditions and distribution of fluid transport properties in fault zones based on hydrothermally filledmicrofractures, which reflect the ‘‘frozen-in’’ instantaneous advective hydrothermal activity and recordpalaeopermeability conditions of the fault-fracture system. We studied the Jorgillo Fault, an exposed20 km long, left-lateral strike-slip fault, which juxtaposes Jurassic gabbro against metadiorite belongingto the Atacama Fault System in northern Chile. Tracings of microfracture networks of 19 oriented thinsections from a 400 m long transect across the main fault trace was carried out to estimate the hydraulicproperties of the low-strain fault damagezone, adjacent to the high-strain fault core, by assuming pennyshapedmicrofractures of constant radius and aperture within an anisotropic fracture system. Palaeopermeabilityvalues of 9.1*1011 to 3.2*1013 m2 in the gabbro and of 5.0*1010 to 1.2*1013 m2 in themetadiorite were determined, both decreasing perpendicularly away from the fault core. Fractureporosity values range from 40.00% to 0.28%. The Jorgillo Fault has acted as a left-lateral dilational faultbend,generating large-scale dilation sites north of the JF during co-seismic activity.