COMPARING STABLE ISOTOPE SIGNALS OF CENOZOIC CLIMATE, TECTONICS, AND TOPOGRAPHY IN HIGH VS. LOW ELEVATION RETROARC BASINS, SOUTHERN CENTRAL ANDES (~31°S)

The stable isotopic composition of ancient precipitation is recorded by volcanic glass and provides critical informaiton to reconstruct past topographic, tectonic, and climatic histories in ancient mountain ranges and sedimentary basins. In this study, we use stable isotope geochemistry to examine the effects of climatic, topographic, and/or tectonically driven changes on the isotopic records preserved within high (~2030 m) vs. low (~750 m) elevation basins spanning the Argentine Precordillera fold-thrust belt (Southern Central Andes, ~31°S). We measured the δD of hydrated volcanic glass collected from (1) 14 samples from the low-elevation Mogna basin, located at the toe of the orogenic wedge and filled by predominantly fluvial mudstones and sandstones, and (2) 21 samples from the high-elevation Villa Nueva basin, which is comprised of mostly volcaniclastic sedimentary rocks and occupies a modern-day hinterland (wedge-top) position. The intervening Precordillera fold-thrust belt accommodated ~65–100 km of shortening from ~16 to 2 Ma and reaches present-day elevations >3000 m. Mogna samples encompass 10–1.4 Ma and provide a low-elevation baseline, yielding consistent δD glass values of -80‰ to -60‰ over the sampled time interval. Data from the higher-elevation Villa Nueva basin span >45 to 7.5 Ma and demonstrate a pronounced mid-Miocene trend toward lighter compositions characterized by a decrease in δD glass values from -80‰ to -60‰ at 20 Ma, to -112‰ at 18 Ma, followed by a return to -80‰ to -60‰ at 15 Ma. The timing of this isotopic excursion aligns with the onset of major shortening and topographic relief development in the Precordillera, possible dynamic subsidence and marine ingression during development of the middle to late Miocene Paranese Seaway, and with the Middle Miocene Climatic Optimum (MMCO); we note that lighter δD precipition values are predicted in response to topographic uplift, while heavier isotopic values are predicted in response to both seaway ingression and global warming. Additional volcanic glass δD measurements of older (~18–17 Ma) deposits preserved in the low-elevation Mogna basin may help clarify the role of global climate vs. local tectonic/topographic factors on the observed signals.