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Geological Sciences Seminar: Mark Brandon, Yale University - "Cenozoic evolution of climate, topography, and tectonics in the Patagonian Andes"
I will report on work at Yale using precipitation isotopes to study the evolution of topography in the Patagonian Andes. This 1500 km long mountainous range is commonly thought to have formed over the last 15 Ma, due to back-arc thrusting and “collision” of the Chile triple junction and associated Nazca-Pacific spreading center. In contrast, stratigraphic studies indicate that the range may have been established by during the Late Cretaceous.
There are two aspects of our study: 1) Dynamic modeling of orographic precipitation and isotopic fractionation of modern precipitation, which provides a direct estimate of the sensitivity of water isotopes to changes in topography and climate. 2) Measure of deuterium isotopes in hydrated volcanic glass collected from several Cenozoic sections in the lee of the Patagonia Andes.
- Our modeling study demonstrates that the fractionation observed in precipitation isotopes vary linearly with changing topography, but is also significantly influenced by changing surface temperature. For example, a 5°C increase in surface temperature and a 20 percent decrease in topography would produce equivalent shifts in isotopic fractionation associated with orographic precipitation.
- Our dynamic modeling provides a specific representation of average atmospheric flow during orographic precipitation in the Patagonian Andes. We determine that the relationship of isotopic fractionation to orographic lifting is ~5 ‰/km for 18O and ~38.5 ‰/km for 2H, which is about twice the gradient estimated in empirical studies of global precipitation.
- Our volcanic ash study indicates that the Patagonian Andes have maintained a steady height for the last 60 Ma. We infer that the topography was formed at ~100 Ma in association with emplacement of the Patagonian batholith, which underlies the core of the range.