Tectonic constraints leading to hydrothermal fluid flow: insights from the Liquiñe-Ofqui Fault System and the Andean Transverse Faults, Southern Volcanic Zone of the Andes (39ºS)

Abstract

Fault and fracture meshes exert a first-order control on the nature and development of hydrothermal circulation in the upper crust. The hydrostructural behavior of a fault zone critically depends on geodynamic constraints, which are fundamental to assess favorable conditions for fluid flow. Tectonics of the Southern Volcanic Zone are controlled by the right-lateral Liquiñe-Ofqui Fault System (LOFS) and the left-lateral Andean Transverse Faults (ATF). The LOFS is a NS-striking long-lived trench parallel, right-lateral group of faults built as a response to the subduction of the Cenozoic Nazca plate beneath the South American plate; whereas the ATF are a set of subparallel NW-striking faults and morphological lineaments, that are believed to be pre-Cretaceous in age as a result of a global plate reorganization during late Triassic. With the aim of determining strain patterns (P and T axes) that might favor fluid flow, we collected brittle fault-slip and vein orientation data in the Liquiñe area (39ºS) from both the LOFS and ATF. Fault-slip data were inverted using the software Faultkin 8.0.7, and calculated tectonic solutions where then compared to orientation patterns drawn from vein data. Preliminary results reveal two independent solutions of the PT axes at both sites: one has a subhorizontal NE-trending P axis and a related subhorizontal NW-trending T axis, whereas the other has a subhorizontal WNW-trending P axis and a related subhorizontal NNE-trending T axis. The poles to epidote-filled veins cluster about a NW- and SE-trending ca. 20-40º plunge at both sites, whereas the poles to zeolite-filled veins cluster about a NW-trending ca. 30º plunge in the LOFS site, and about NNE-striking ca. 60º dip plane in the ATF site. Vein orientations and strain field solutions argue that faults from the LOFS and ATF record the long-term shortening imposed by the subducting plate and favored emplacement of epidote-bearing fluids. Zeolite-filled veins were favorable emplaced under a similar regime at both sites, but likely with significantly different fluid pressures.