TOPOGRAPHIC EVOLUTION OF THE SOUTHERN CENTRAL ANDES FROM STABLE ISOTOPE PALEOALTIMETRY OF HYDRATED VOLCANIC GLASS

The Southern Central Andes (27º-46º30’S) are part of one of the largest orographic barriers to moisture transport on Earth, exerting a far-reaching influence on atmospheric and precipitation patterns along South America. While there are numerous models for the mechanisms and timing of orogenesis, there is considerable debate over when the Andes grew to their current size and how the orogen has changed through time. Some authors argue that they were constructed entirely from the Miocene onwards, although recent research has highlighted the importance of a Late Cretaceous shortening phase. However, the contribution of both contractional stages to present topography and the transition between them remain unresolved.

The stable isotopic composition of precipitation reflects the degree of isotopic distillation during rainout as an airmass moves across a landscape. Thus, materials that record this geochemical variable can provide an indication of the present or past elevations along a moisture transport pathway. Cenozoic volcanism along the length of the Andes has resulted in near-continuous production of felsic ashes for more than 65 million years. As ashes are deposited on a landscape, they readily hydrate, providing a record of the isotopic composition of ambient water over a timescale of several thousand years. Following hydration, water uptake ceases and this signature is “locked in”, providing a long-term record of stable hydrogen isotopes (δD) of paleo-precipitation.

We analyzed the δD of volcanic glasses preserved within strata in the Malargüe basin (35ºS) in Southern Mendoza Province, Argentina, to constrain changes in precipitation over the past ~55 Ma.

δD values suggest high-standing topography since at least the beginning of the Cenozoic, along with an uplift phase during middle Eocene to Oligocene times and a decrease in topography in the middle Miocene. While the first event coincides with low subsidence rates during lacustrine and distal fluvial deposition in the foreland, the second episode overlaps with high subsidence rates and coarse-grained sedimentation. These results support the hypothesis of a pre-Neogene orographic barrier and could be interpreted to reflect topographic change associated with long-timescale growth and removal of a dense lithospheric root underneath the Andean orogen.