MID-PLEISTOCENE CHANGES IN GLACIAL EROSION RATES IN THE MID-LATITUDE PATAGONIAN ANDES REVEALED BY DETRITAL THERMOCHRONOLOGY OF OCEAN SEDIMENTS

Erosion of mountain belts reflects combined effects from climate and tectonics. Changes in erosion rates through time allow us to investigate the contribution and dynamics of these external forcings. In the mid-latitude Patagonian Andes, various hypotheses about the relative importance of climate and tectonics have been proposed to explain young (i.e., Miocene and younger) bedrock thermochronological ages. These hypotheses make different predictions about cooling ages of detrital minerals in foreland sediments, which integrate erosion rates at the landscape scale. We measured new detrital apatite (U-Th)/He cooling ages from core sediments collected during Ocean Drilling Program leg 141, drilled adjacent to the Chile Triple Junction (CTJ). We also re-calculated apatite fission-track cooling ages from these cores using published fission-track data and modern fission-track annealing kinetics. We estimated the depositional ages of the core sediments, which are of Pliocene to Pleistocene age, using a suite of available age-depth models from magneto- and bio-stratigraphy and a new model based on modern cyclostratigraphy. From the thermochronometric ages and depositional age models we calculated lag times, or the difference between the cooling and depositional ages. We find that the lag times are longer for the most recently deposited sample we studied, deposited at ~0.20 Ma, compared to the samples deposited at ~1 Ma and earlier. The increasing lag times towards shallow stratigraphic levels seem to rule out a younging prediction of the detrital cooling ages made by CTJ-related crustal deformation hypotheses. Instead, the change of lag times indicates a slow-down of erosion rates on the windward side of the Patagonian Andes that temporally coincides with the widely-reported Mid-Pleistocene Transition of glacial cycles. We therefore tentatively interpret the observed detrital thermochronological data as reflecting a climate-modulated erosion of the mid-latitude Patagonian Andes.