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

Tuesday, 16 December 2014
Heiko Pingel1, Andreas Mulch2, Manfred R Strecker1, John M Cottle3, Alexander Rohrmann1 and Ricardo N Alonso4, (1)University of Potsdam, Potsdam, Germany, (2)Biodiversity and Climate Research Centre, Frankfurt, Germany, (3)University of California Santa Barbara, Santa Barbara, CA, United States, (4)Universidad Nacional de Salta, Departmento de Geología, Salta, Argentina
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 (δDg), 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 δDg 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 δDg-decrease that documents uplift of the basin catchment and possibly enhanced rainout along a paleo-Andean front, followed by a strong shift towards modern δDg 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), δDg 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.