Fault-controlled development of shallow hydrothermal systems: Structural and mineralogical insights from the Southern Andes

Abstract

Paleofluid-transporting systems can be recognized as meshes of fracture-filled veins in eroded zones ofextinct hydrothermal systems. Here we combined meso-microstructural analysis of 107 fractures andmechanical modeling from two exhumed exposures of the faults governing regional tectonics of theSouthern Andes: the Liqui˜ne-Ofqui Fault System (LOFS) and the Andean Transverse Faults (ATF). The ATFspecific segment shows two tectonic solutions that can be modeled as Andersonian and non-Andersoniantectonic regimes: (1) shear (mode II/III) failure occurs at differential stresses > 28 MPa and fluid pres-sures < 40–80% lithostatic in the Andersonian regime; and (2) sporadic hybrid extensional + shear (modesI + II/III) failure occurs at differential stresses < 20 MPa and anomalously high fluid pressures > 85–98%lithostatic in the non-Andersonian regime. Additionally, the LOFS exposure cyclically fails in extension(mode I) or extension + shear (modes I + II/III) in the Andersonian regime, at differential stresses < 28 MPaand fluid pressures > 40–80% lithostatic. In areas of spatial interaction between ATF and LOFS, these con-ditions might favor: (1) the storage of overpressured fluids in hydrothermal systems associated with theATF faults, and (2) continuous fluid flow through vertical conduits in the LOFS faults. These observationssuggest that such intersections are highly probable places for concentrated hydrothermal activity, whichmust be taken into consideration for further geothermal exploration.