THERMAL EFFECTS OF CORDILLERAN PROCESSES ON THE MAGALLANES-AUSTRAL RETROARC FORELAND BASIN: A CASE STUDY USING THERMOCHRONOMETRY, VITRINITE REFLECTANCE, AND CARBONATE CLUMPED ISOTOPE ANALYSIS

We present new detrital thermochronometry, vitrinite reflectance, and carbonate clumped isotope data to investigate the post-depositional thermal history of the Paleogene Magallanes-Austral Basin in response to sedimentary burial, orogenic growth, and fluid flow during Patagonian orogenesis. The retroarc foreland basin is unusually hot due to formation on attenuated lithosphere and a plate margin history involving two asthenospheric slab windows. We focus on the Paleogene erosional unconformity at Sierra Baguales and Cerro Castillo (51-50 °S) across which previous work has documented a significant thermal contrast. Danian strata below the unconformity yield zircon (U-Th)/He (ZHe) dates 1- 200 Ma that suggest post-depositional heating and partial He loss. In contrast, Eocene strata above the unconformity yield ZHe dates 43-107 Ma suggesting minimal He loss. Thus, Danian strata preserve sediment source and post-depositional burial histories while Eocene strata preserve sediment source histories. Apatite (U-Th-Sm)/He (AHe) dates from both strata range from 6-22 Ma suggesting AHe dates are fully reset, indicating an Oligo-Miocene event. Vitrinite reflectance data range from 0.39-1.17 %Ro and reveal an increase in heating along the boundary of the Paleogene unconformity. This incursion indicates hydrothermal fluid flow. Inverse thermal history modeling of Jurassic zircons in conjunction with ZHe, AHe, and %Ro above the unconformity reveal sediment-source cooling 100-60 Ma associated with Late Cretaceous orogenic unroofing. Modeled post-depositional heating of Danian strata below the unconformity indicate maximum burial temperatures of 125–155 °C (48-59 Ma), which suggests 3-4.5 km of missing section and 55-98 °C (22-12 Ma). For Eocene strata, modeled burial temperatures are consistent with calcite cement formation temperatures (~62-92 °C) from carbonate clumped isotopes. These data, modeling results, and cement textures refine the magnitude and timing of thermal evolution across the unconformity. Findings highlight the role of Paleocene basin heating related to thermal fluids during fold and thrust belt activity instead of heating related to an excess of missing section. These findings show how important multiple indices are for unraveling thermal events during unconformity development.