PALEOTOPOGRAPHIC EVOLUTION OF THE SOUTHERN CENTRAL ANDEAN FORELAND: A VIEW TO THE EAST

It is commonly viewed that the Central Andes rose during the Miocene and the large block uplifts of the foreland were uplifted in the last 10 million years. Here we present a >50 million year record of volcanic glass (Fennell et al. 2023 EPSL) and organic biomarker δ²H data from the Malargüe basin, Argentina, to provide new constraints on the topographic evolution of the Central Andean foreland. We use modern water isotopes and the Orographic Precipitation Isotope model (OPI) to quantify the relationship between water isotopes and orographic lifting. Our analysis yields three key results: 1) moisture reaching the Malargüe basin is solely derived from the northeast and is fractioned by orographic lifting over large foreland basement highs and along the east side of the Cordillera. Moist airmasses from the west cross the Andes at this latitude, but do not contribute appreciable moisture to the foreland due to downslope flow on the eastern side, which is associated with warming, undersaturation and inhibition of precipitation. 2) Glass and biomarker δ²H values are strongly depleted (-50 to -90‰) through the entire Cenozoic. This depletion can only be generated by progressive rainout from an airmass during upslope flow of moist winds on the eastern side of the Andes. 3) From 55-15 Ma, the magnitude of isotopic fractionation is similar to modern, indicating stable topography in the Andean foreland. Glass and biomarker data record a 50% decrease in isotopic fractionation from 15 to 10, consistent with a 50% reduction in the upwind foreland topography, and an equivalent increase from 10 to 0 Ma. In total, these data record stable topography from 55-15 Ma, subsidence of the foreland and associated block uplifts from 15-10 Ma, and a rebound to modern orographic lifting between 10 and 0 Ma. Miocene subsidence of the Andean foreland recorded here is evidenced by the Miocene Paranaense transgression and numerical modeling of dynamic topography associated with subduction of the Nazca plate.