Hydrogen stable isotopes from hydrated volcanic glass record orogenic growth and climate change at the eastern Puna Plateau margin, NW Argentina

Abstract:

Intermontane basin strata along the E flanks of the Puna Plateau in NW Argentina are ideal archives to investigate the interaction between tectonics, topography, and changes in climate. In particular, these strata record the fragmentation of a formerly contiguous foreland by range uplifts, ensuing intra-basin deformation, and surface uplift. These changes were often accompanied by a transition from humid to semiarid conditions as windward range uplift exceeded orographic threshold elevations. The E Andean flanks comprise steep gradients in topography, rainfall, and surface-process rates. Rainfall is focused along the E flanks of the plateau, while the orogen interior is arid. These gradients are mirrored by the stable isotope ratios of modern rainfall, and therefore, in the stable isotope composition of proxy materials that incorporate this water. We present D/H ratios of volcanic glass (δDg) from dated tuffs in Mio-Pleistocene sediments of intermontane basins in the Eastern Cordillera between ~23 and 26°S (Humahuaca, Toro, and Angastaco basins). We document a strong co-varying relationship between tectono-sedimentary events in the basins and corresponding δDg values. Initial D-depletion trends in the Toro and Angastaco basins constrains the onset of surface uplift to 6.5 and 7 Ma, respectively. Strong positive δDg shifts of >15‰ in Humahuaca at ~3 Ma and <2 Ma in the Toro basin are apparently caused by enhanced evaporation. In this tectonic setting the observed relationships may be related to the attainment of orographic threshold conditions and ensuing hinterland aridification. δDg values in Angastaco, additionally, appear to be episodically influenced by enhanced convective rainfall during the Plio-Pleistocene, similar to modern conditions.