Paleofluid Flow in the Upper Crust: A Study Case from the Atacama Fault System

Abstract:

Fault zones and fault-related permeability structures have long been recognized to play a first-order role on fluid flow migration through the crust. However, the nature of the spatial relationship between fault zones and fluid-flow and the evolution of such permeability structures in terms of paleofluid flow quantification is less well understood. This work presents a microfracture analysis aimed to unravel the nature of the fault plumbing system and estimate paleo permeability parameters within the Jorgillo Fault (JF), a subvertical strike-slip fault exposed in the Atacama Fault System in northern Chile. The JF is a ca. 18 km long, NNW striking strike-slip fault with sinistral horizontal separation of ca. 4 km. The JF juxtaposes orthogranulites to the west (JFW) with gabbros to the east side (JFE) wherea 200 m fault-perpendicular transect was mapped and sampled. The fault core consists of ca. 1 m cataclasite bounded by fault gouge zones of ca. 80 cm to the west and 30 cm to the east. A ca. 50 m wide symmetrical damage zone can be observed at field scale. The damage zone in the JFW is characterized by shear fractures and cataclasites whereas the JFE consists of open fractures and veins. The JFW microfracture analysis reveal mm-wide cataclasitic/ultracataclasitic bands orientated subparallel to the JF, calcite veins in a T-fracture orientation, and minor polydirectional chlorite veins. In the JFE, 1-20 mm wide chlorite, quartz-epidote and quartz-calcite veins, can be observed. Chlorite conjugate veins show syntaxial growth textures suggesting dilatational fracturing. Microfractures distribution show shear fractures distributed only in JFW while open fractures are mainly in the JFE. Using models off paleofluid flow, cumulative permeability (i.e. permeability due to all types of microfractures present in the samples) was estimated to range from 2.761x10^-11 to 4.89x10^-9 m² on the JFE, and 8.04x10^-12 to 4.39x10^-8 m² on the JFW.