EXTRACTING THE CONTRIBUTIONS OF MAGMATIC HEATING, FAULT BLOCK EXHUMATION, AND GEOTHERMAL FLUID FLOW FROM MODELING OF ZIRCON AND APATITE (U-TH)/HE DATA

Magmatism, fault-controlled exhumation, and geothermal fluid circulation are key processes in arc crust. Resolving the signature of each process can yield insights into the dominant mechanism(s) operating in a particular tectonic setting. Thermochronology is a valuable tool to accomplish this task. Here we apply forward and inverse models to help interpret recently acquired zircon (ZHe) and apatite (AHe) (U-Th)/He dataset gathered along the Liquiñe-Ofqui Fault System (LOFS) of the Southern Andes. The LOFS is an active intra-arc strike-slip fault associated with a geothermal system. Plutonic rocks astride the LOFS were emplaced at 160 Ma, 135 Ma, 115 Ma, and 7 Ma. Single-grain ZHe and AHe dates range from 6.9–2.1 Ma and 3.2–0.3 Ma respectively regardless of U-Pb zircon crystallization age. We forward modeled a wide range of thermal histories of multi-stage pluton intrusion, denudation, and fluid circulation using Heat3D to calculate the extent of He loss. The depth and temperature conditions of each magmatic pulse are taken from thermobarometric data. Background geothermal gradient ranged from 20ºC/km to 35ºC/km. In all cases, post-intrusive temperatures rapidly decreased to <450ºC. However, thermal relaxation to temperatures low enough to permit He retention in zircon and apatite was more protracted (>3-5 Myr). Comparison of forward-modelled and measured data reveals that ZHe dates astride the LOFS are best matched by 8 km depths (20ºC/km). The measured 3.0 to 1.8 Ma AHe dates cannot be explained by pluton intrusion. Accordingly, we evaluated post-magmatic cooling through inverse HeFTy models. All models require cooling through the apatite partial retention zone (APRZ) between 5-1.5 Ma. Spatial patterns in AHe are best explained by differing levels of exhumation along steeply dipping LOFS faults. AHe dates of <1.5 Ma appear to record reheating within the last 1 Myr. We attribute this to circulation of late geothermal fluids along the LOFS following a shift in σ₁ from NE-SW to E-W at 3.6-1.6 Ma. We show that ZHe and AHe together record the combined influence of shallow Miocene plutonism, initial exhumation of tectonic blocks along the LOFS, and younger geothermal fluid circulation.