Fault zones permeability structure within the brittle crust: Microstructural estimations at the time of fracture sealing

Abstract

Understanding fault zone structure allows the assessment of fluid-flow leading to the precipitation of hydrothermal minerals at depth. This work is focused on unravelling palaeo fluid-flow conditions and distribution in an exhumed fault zone with clear evidence of crustal fracturing — fluid interaction — sealing. These estimations are based in hydrothermally filled microfractures, which reflect the instantaneous flux of fluids, recording palaeopermeability condition of the fault-fracture system. We have chosen the Jorgillo Fault (JF), a 20 km long, left-lateral strike-slip fault, which juxtaposes Jurassic intrusive rocks, belonging to the Atacama Fault System in northern Chile. A detailed microfracture tracing on 19 oriented thin sections from 400 m long transect across the main fault trace was carried out in order to estimate intrinsic paleopermeability and fracture porosity of the fault damage zone by assuming disk-shaped microfractures of constant radius and aperture within an anisotropic fracture system. Paleopermeability and fracture porosity values decrease perpendicularly away from the fault core, both constructing an anisotropic permeability structure of the JF zone, as a respond to higher flux of fluids coming from the east in a co-seismic fluid-flow of high fluid pressure with the consequent sealing of the microfracture network and strengthening of the fault zone.