Correlation of electrical conductivity and structural damage at a major strike‐slip fault in northern Chile

Abstract

Large‐scale strike‐slip fault zones are often imaged as electrically conductive structures in the brittle crust. However, the relationship of conductivity and internal architecture of the fault zone remains largely unclear. This paper presents results of a study designed to compare the record of structural deformation across a fault zone with its electrical conductivity image. Two high‐resolution magnetotelluric profiles trend perpendicularly across the West fault, a branch of the Precordilleran fault system in northern Chile. The magnetotelluric and geomagnetic response functions in the frequency range from 1000 Hz to 0.1 Hz clearly image a fault zone conductor (FZC) about 350 m wide and 1500 m deep, trending along the surface trace of the fault. The position of the FZC and its geometric properties (width, dip) correlate with a region of intense fluid alteration and the orientation of fault‐related damage elements (minor faults, fractures). According to estimates of fluid salinity and rock porosity, the conductivity anomaly results from meteoric water penetrating into a permeable zone. This suggests that the increase in electrical conductivity is causally related to a mesh of minor faults and fractures, acting as a pathway for fluids. The FZC reflects the central and most fractured part of the damage zone. In view of similar published magnetotelluric studies, we infer a dependency of a fault's state of activity and the characteristics of the FZC such as its conductance, width, and depth extent. Ongoing deformation is the controlling factor for maintaining a permeable and electrically conductive fracture network.