Orographic barrier development along the eastern flanks of the southern central Andes, Argentina: new insights from stable hydrogen isotopes in hydrated volcanic glass

Abstract:

The Andes constitute one of the most important orographic barriers in the S hemisphere, which impacts atmospheric circulation, the amount and distribution of rainfall, and resulting surface processes. In the central Andes of Argentina the orogenic Puna Plateau and the intermontane basins within the adjacent Eastern Cordillera (EC) constitute archives that furnish spatiotemporal information on the uplift of the orogen and ensuing paleoenvironmental changes. Presently, rainfall in NW Argentina is focused along the windward flanks of the EC, while its intermontane basins and the Puna constitute high-elevation regions with decreasing rainfall and steep topographic gradients. The influence of topography on hydrologic conditions is reflected in the stable isotopic composition of meteoric water. Importantly, in light of these systematic present-day isotope characteristics, proxy materials retrieved from basin strata may record analogous conditions in the past and provide insight concerning topographic growth through time. Here, we present isotopic data of volcanic glass (δD_g), extracted from volcanic ash deposits interbedded with strata in different sedimentary basins across the E flank of the plateau. Combined with geochronology, our data show clear spatiotemporal variations in δD_g within the different basins that are correlated with regional topographic growth and associated orographic effects during the Mio-Pliocene. For example, the Humahuaca basin (23.5°S) shows a gradual δD_g-decrease that documents uplift of the basin catchment and possibly enhanced rainout along a paleo-Andean front, followed by a strong shift towards modern δD_g values (–100‰) after 3 Ma, associated with aridification due to range uplift and orographic shielding to the E. In the Angastaco basin (25.5°S), δD_g is more variable prior to 3.5 Ma (–70 to –95‰). Conditions similar to today (–65‰) were established by at least 1 Ma. Isotopic and sedimentologic data from the Toro basin (24.5°S) suggest a similar pattern, indicating topographic growth along the E Andean front. Isotope data from the broken foreland to the east seem unaffected by large isotopic shifts, which may ultimately help to reconstruct relative elevation changes across the Andes to better understand the topographic evolution of the orogen interior.