DETRITAL THERMOCHRONOLOGIC SIGNAL OF SYNDEPOSITIONAL PALEOGENE EXHUMATION IN THE HIGH ANDES, SOUTHERN CENTRAL ANDES

Detrital geochronology provides a diverse toolset to quantify crustal deformation, reheating, and unroofing during orogenesis. However, a persisting challenge in provenance analysis is to distinguish between sediment derived directly from crystalline versus recycled sources. We present new detrital thermochronology data from the Central Andes, a long-lived Cordilleran margin, that captures two modes of cooling dates not resolved in the published geochronology record. Here, the paucity of Paleogene sedimentary records may reflect Andean stasis and diminished plate coupling during subduction. New (U-Th)/He thermochronology (ZHe) of detrital zircons previously dated using U-Pb geochronology from the Oligocene-Miocene Bermejo foreland basin yield ZHe dates ~16-565 Ma (n=80). Predominance of ~340-220 Ma ZHe dates from the Paleozoic and Proterozoic U-Pb age modes preserve signals of Late Paleozoic – Triassic thermal events in the Central Andes and Sierras Pampeanas. Zircon U-Pb age spectra and eastward paleoflow point to the High Andes as sediment sources to the retroarc depocenter. Potential sources include the Late Cretaceous and Cenozoic Andean Arc, Cretaceous sedimentary rocks, Permo-Triassic Choiyoi Group rhyolitic volcanics and shallow intrusions, and the Carboniferous-Permian Elquí-Colangüil granitic batholiths. Notably, however, ZHe data from the Choiyoi Group grains show two distinct modes: (1) grains with rapid post-emplacement Permo – Triassic dates, and (2) grains with reset and latest Eocene – Oligocene dates.

We suggest that these modes represent source areas within two different tectonomorphic units of the Choiyoi Group, or younger recycled sedimentary units, characterized by different thermal histories. Latest Eocene – Oligocene detrital ZHe dates are synchronous with deposition, indicating short lag time between cooling through the ZHe closure window and deposition. These results are consistent with published structural and thermochronological evidence from the Frontal Cordillera that suggests rapid cooling of the Choiyoi Group during a late Eocene constructional phase in the High Andes. Taken together, our findings motivate the need to reconcile hinterland unroofing and contractional deformation with low foreland subsidence rates within a neutral stress-state model.