THE ROLE OF BASEMENT FAULTS IN CONTROLLING ANDEAN GEOTHERMAL SYSTEMS: INSIGHTS FROM THE TOCOMAR GEOTHERMAL SYSTEM, CENTRAL PUNA, NW ARGENTINA

A distinctive feature of the Southern Central Andes (15°-33°S) is the widespread volcanic activity and the development of crustal-scale fault systems oriented parallel and oblique to the trench. The Calama-Olacapato-El Toro (COT) is a clearly identifiable transverse structure consisting of several NW- to WNW-trending fault strands, such as the left-lateral transpressive Chorrillos fault (CF), associated with late Miocene-Recent magmatism and hydrothermal activity. Notably, the CF has been suggested to exert a first-order control on the development of the high-temperature Tocomar geothermal field.

This study presents the first characterization of the CF zone based on detailed geological mapping, fracture transects at different distance from the fault, and SEM/CL microstructural analysis. We have studied the fault zone where it cuts through basement Ordovician granitoids, the last fault activity was Quaternary reworking in some cases Paleozoic fabrics. Our data suggest that the internal architecture of the CF consists of multiple clay- to silty-rich core zones made of foliated cataclasites, ultracataclasites with elongated lithons and intensely fractured rocks. Given the emergence of cold springs on the opposite side of the fault to the Tocomar geothermal field, we infer the fault core must act a barrier to fluid flow. The damage zone is characterized by intensely to moderately fractured domains with fault breccia and minor faults. The fracture patterns can be grouped into 4 sets: i) E-W, ii) NW-SE, iii) N-S and, to a lesser extent, iv) NE-SW. The fracture intensity decreases non-linearly with distance from the fault, as far as 2200 m, and is consistent with a power-law distribution. The microstructural analysis shows barren, sealed and unfilled fractures along with microbreccia veins. Considering the host rock are granitoids we infer that both the permeability structure and hence the fluid flow parallel to the fault is controlled predominantly by the macrofractures. The fault-related fracture mesh enhances the permeability, allowing not only the infiltration, circulation and accumulation of hydrothermal fluids but also, given the low-permeability rock matrix, favours water-rock interaction processes. This process, coupled with long-term circulation, is interesting not only for geothermal exploration but also for lithium and/or precious metal enrichment.