NEW EVIDENCE FOR LONG-LIVED CENOZOIC TOPOGRAPHY IN THE PATAGONIAN ANDES

The long-held paradigm for the evolution of the Patagonian Andes is that the region was low-lying until the middle Miocene, when it underwent substantial surface uplift. Here we present new paleoclimate data that suggest the Andes in this region have been a substantial topographic feature since at least the Paleocene. Precipitation δD was reconstructed from volcanic glasses from three well-dated sedimentary sections on the eastern side of the Andes between 45°S and 48°S. Using the steady, westerly wind-dominated climatology of Patagonia and the benthic foraminiferal calcite δ¹⁸O compilation, we estimate the effect of climate on precipitation δD and subtract it from the reconstructed δD signal. The resulting climate-adjusted record shows that the strength of the rain shadow in eastern Patagonia has remained roughly constant since the Paleocene, suggesting the Andes are a long-lived topographic feature. This is in agreement with the presence of Paleogene synorogenic sediments east of the Andes (Suárez and De la Cruz, 2000) and the observation that the majority of the shortening occurred in late Cretaceous time (Fosdick et al., 2011). It also fits with genomic evidence for the presence of cold-adapted flora in the Patagonian Andes during the Paleogene. A review of geological data indicates the presence of a rain shadow prior to Miocene tectonism, which was relatively minor. The age of marine sediments within the Andes, timing of magmatism forming the Patagonian Batholith, and our new data constrain the timing of uplift to the late Cretaceous, and suggest that it followed a period of batholithic magmatism and crustal shortening.