UNRAVELING STABLE ISOTOPE SIGNALS OF CENOZOIC CLIMATE, TECTONICS, AND TOPOGRAPHY IN A HIGH-ELEVATION HINTERLAND BASIN, SOUTHERN CENTRAL ANDES (~31–33ºS)

The fractionation of stable isotopes in response to temperature and phase changes has been used to reconstruct What about “... past topographic, latitudinal, and climatic histories in ancient mountain ranges. We use δD measurements from hydrated volcanic glass to examine the relative effects of climate and tectonics on isotopic signatures preserved in the long-lived, high-elevation Villa Nueva basin (southern central Andes, ~31ºS). We analyzed 12 volcaniclastic samples from a ~45 to 7 Ma stratigraphic section that is currently located in a hinterland (wedge-top) position at 2030 m elevation. Our samples span 45–7.8 Ma in age, and δD measurements corrected for precipitation (δDprecip) range from -81.7 to -47.7‰ with a pronounced trend toward heavier compositions during the Miocene (from -80‰ at 20 Ma to around -45‰ at 5 Ma). Our δDprecip values are markedly heavier than modern isotopic signals derived from westerly moisture sources and influenced by fractionation over the Andes, and thus point to contribution from eastern air masses derived from the Atlantic. The Miocene trend toward heavier δDprecip signals is opposite of the response predicted during Cenozoic global cooling and Andean shortening/topographic relief development documented during the Miocene (which should result in lighter precipitation values), but aligns with trends toward heavier isotopes predicted in response to (1) the 3º northward movement of South America over the same timeframe and (2) possible dynamic subsidence and marine ingression during development of the middle to late Miocene Paranaense Seaway. Our research improves understanding of the complex factors influencing stable isotope fractionation and suggests that regional tectonic processes (changes in latitude; dynamic subsidence), rather than local topographic relief or climate, had greater influence on the signals observed in the Villa Nueva hinterland basin.