Internal structure of the Precordilleran fault system (Chile) — insights from structural and geophysical observations

Geological field work and audiomagnetotelluric data from two profiles crossing the West Fissure Zone in northern Chile were used to describe the geometry and structure of the fracture damaged zone surrounding this strike-slip fault system. The W–E profiles show that the width of the West Fissure Zone based on the fracture density distribution derived from aerial photographs is 4000 m on profile A and 7000 m on profile B. The estimated widths correspond roughly to the region where the fault is kinematically uniform. The ratio of the fault width to the fault length ranges from 0.024 to 0.041. This ratio compares favourably with the measured ratio of small (metre-scale) natural faults. frequency magnetotelluric imaging shows low electrical resistivity zones (resistivity of ∼5–30 Ωm) coincident with the mapped surface traces of the fault. However, these zones are very narrow (width about 100 m) and only 50 m (profile A) and 200 m (profile B) deep, respectively. On profile B the shallow high conductive zone is underlain by a resistive zone (∼1000 Ωm). The conductivity enhancement in these shallow and narrow zones contrasts with the broad process zone revealed by structural analysis. We assume that the conductivity enhancement is due to meteoric water entering a zone of ruptured rocks along the fault trace (fault core). At present, we have no indication for seismic slip along the investigated segments of the West Fissure Zone. Fault models have shown that during aseismic periods of fault evolution fault healing (i.e. strength recovery) due to compaction and cementation is active. Fluids and fluid transport, which are probably responsible for enhanced conductivity, are confined to the remaining fractures of the fault core at shallow depth.