224 Paleoclimatology/Paleoceanography

Wednesday, 12 October 2011
8:00 AM - 12:00 PM
Room 200FG

PALEOELEVATION OF THE PUNA PLATEAU (NORTHWESTERN ARGENTINA) INFERRED FROM GEOCHEMICAL ANALYSES OF VOLCANIC GLASS

Paper number: 224-9
Program: Technical Sessions
Time: 10:00 AM – 10:15 AM

Authors

Abstract

Evidence for recent uplift (< 10 Ma) of the Andes has largely come from studies of the Altiplano Plateau in Bolivia. However, this region represents a relatively small portion of the Andes and is geographically distinct from the Puna Plateau to the south, which is significantly higher (average elevation = 4.4 km) and drier (precipitation <0.5 m) than the Altiplano (average elevation = 3.4 km; precipitation = 0.6 to 1.2 m). The Altiplano today is also very smooth topographically whereas the Puna Plateau is more ‘basin and range’ style with numerous intermontane basins, resulting in more regional climate and environmental variability. In addition, the two regions exhibit unique structural deformation and volcanic geochemistry, suggesting a more complex uplift history for the Puna Plateau. Previous geochemical studies suggest that the Altiplano in Bolivia underwent rapid uplift from 10 - 6 Ma. (Garzione, 2008) Alternatively, recent climate modeling has suggested a more gradual uplift history of the Andes beginning around 25 Ma. (Ehlers and Poulsen, 2009) No geochemical proxies for paleoaltimetry exist further south in the Puna Plateau of NW Argentina.

In order to resolve possible differences in uplift history between the Altiplano and the Puna Plateau we collected volcanic tuff and ignimbrite samples spanning the late Eocene to Recent from across the Puna Plateau (between 24° and 26° south latitude) of NW Argentina. Hydrated volcanic glass has been shown to retain the isotopic signature of meteoric waters once fully hydrated (thought to occur <103 yrs). The isotopic values of these meteoric waters are determined by precipitation, which decreases with gain in elevation. Age control for our deuterium values is based on zircon U-Pb dating of the same volcanic samples. Deuterium values of modern precipitation across NW Argentina decrease by about 1.8‰/100m. Applying this same lapse rate to our data and assuming that water within the glass was not reset diagenetically, our results suggest that the Puna Plateau had reached its modern elevation prior to the uplift previously proposed for the Altiplano.