A PALEOELEVATION HISTORY OF THE PATAGONIAN ANDES FROM HYDRATED VOLCANIC GLASS

In this study, we utilize the evolution of topography over geologic time scales to determine tectonic processes and paleoclimate. Provided that the source of precipitation is steady, the orographic effect on stable water isotopes (δD and δ¹⁸O) has been used as a proxy for the strength of a rain shadow and thus the height of a mountain range. In Patagonia, where the landscape is bathed in a constant stream of westerly winds (Garreaud et al., 2013), there is a steady precipitation source of Pacific Ocean water. Patagonia has also not changed latitude significantly during the Cenozoic (Pardo-Casas & Molnar, 1987). While geochemical (Blisniuk et al., 2005) and structural (Lagabrielle et al., 2004) data suggest a period of significant mountain building in the Miocene, little is known about the pre-Miocene history in this region.

To assess the topographic history of the Patagonian Andes, we constructed a water isotope record from hydrated volcanic glasses contained in Cenozoic sediments from sections at Lago Jeinimeni (Paleocene-early Eocene), Gran Barranca (Eocene-Miocene), and Lago Posadas (early Miocene), all located on the eastern side of the Patagonian Andes. We extracted glasses from these sediments and measured their H isotopic composition. The precipitation δD values reconstructed from those glasses for the Paleocene-Eocene samples were concentrated at values of -85‰, Eocene-Oligocene samples average -100‰, and early Miocene samples cluster around -110‰. This steady progression towards more negative water isotope values from the Paleocene to the Miocene implies that (1) the Patagonia Andes had a protracted history of mountain building that began well before the Miocene and (2) that history appears likely to have been one of gradual mountain building during that period.