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

Geological field work and audiomagnetotelluric data from two profiles cross ing the West Fissure Zone in northern Chile were used to describe the geome try 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 Zon e based on the fracture density distribution derived from aerial photograph s is 4000 m on profile A and 7000 m on profile B. The estimated widths corr espond 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. Th is ratio compares favourably with the measured ratio of small (metre-scale) natural faults. Audio-frequency magnetotelluric imaging shows low electrical resistivity zo nes (resistivity of similar to5-30 Omegam) coincident with the mapped surfa ce traces of the fault. However, these zones are very narrow (width about 1 00 m) and only 50 m (profile A) and 200 m (profile B) deep, respectively. O n profile B the shallow high conductive zone is underlain by a resistive zo ne (similar to 1000 Omegam). The conductivity enhancement in these shallow and narrow zones contrasts with the broad process zone revealed by structur al 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 ase ismic 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 re maining fractures of the fault core at shallow depth. (C) 2001 Elsevier Sci ence Ltd. All rights reserved.