EOCENE EXHUMATION OF THE HIGH ANDES DIFFERENTIATED BY DETRITAL MULTIMETHOD U-Pb-He THERMOCHRONOLOGY AND THERMAL HISTORY MODELING

The southern Central Andes exhibit a complex tectonic history crucial for understanding orogenic processes in subduction-related orogens yet debate on the timing and mechanisms of early Cenozoic topographic growth persists. We present double-dated detrital zircon U-Pb and (U-Th)/He thermochronology data from the early Oligocene–Miocene Bermejo Basin at ~30°S to investigate source unroofing during development of the High Andes. (U-Th)/He results yield dates of ca. 565–16 Ma (n=73), with distinct detrital modes that indicate a mixing of sediment sources characterized by variable cooling and exhumation histories. We employ a novel approach for modeling detrital thermochronology data that leverages the shared basin subsidence history of multiple detrital modes to resolve provenance and source unroofing histories. Results from the lower Oligocene Vallecito Formation (northwestern Argentina) reveal that detritus was sourced from Permian–Triassic Choiyoi Group rocks that underwent rapid late Eocene cooling, indicated by short lag time (2–5 m.y.) between source cooling and deposition. Our findings are consistent with bedrock studies of Eocene exhumation in the High Andes and establish source-to-basin connectivity during this time. Other detrital modes with pre-Cenozoic cooling histories were derived from Carboniferous Elqui-Colangüil and Choiyoi Group rocks or recycled from Paleozoic basins. We propose that an early Oligocene drainage divide in the High Andes was located west of the Punilla-La Plata fault, an active thrust front at ~30°S. These findings challenge Paleogene neutral stress-state models for the Andes and underscore the importance of improved knowledge of erosion and deformation histories for refining models of Andean orogenesis.