How long have California's Sierra Nevada Mountains been high? This question is one of the most persistent challenges in unraveling the geological history of North America. Many studies argue that the Sierra Nevada reached high elevation by the Late Cretaceous (about 65 to 100 million years ago) and experienced subsequent limited surface elevation increases. An opposing array of studies argue that the Eocene (about 35 to 55 million years ago) Sierra Nevada had low elevation and weak relief and that a post-Eocene pulse of surface uplift linked to the Pliocene delamination of a crustal root led to the modern elevation of the range. These divergent views embody fundamentally different theories of North American tectonic evolution. The primary sources of evidence for the interpretation that the Sierra Nevada have been a region of persistent high topography since the Eocene are stable isotope and leaf-shape paleoaltimetry techniques. However, a series of recent atmospheric dynamics studies challenge these interpretations. These studies show that current frameworks for interpreting proxy records of precipitation isotopic ratios are based on assumptions about the atmosphere that are not generally valid for the Sierra Nevada or that do not take proper account of paleoclimate variation. The goal of this project is to merge the latest methods from geodynamical studies, atmospheric dynamics, and paleoclimate modeling, to substantially improve interpretations of paleoaltimetric data from the Sierra Nevada and thereby advance understanding of the surface elevation history of the range. The project will rigorously evaluate air parcel trajectories around the Sierra Nevada and associated precipitation isotopic and enthalpy distributions for a range of atmospheric conditions. These will be derived from suites of water-isotope enabled atmospheric General Circulation Model simulations of paleoclimate scenarios from the Eocene through the present, and for a range of proposed topographic settings for western North America over that interval. The result will be improved quantitative constraints on the topography of the Sierra Nevada throughout the Cenozoic and in particular on the potential for Late Cenozoic surface uplift.
The proposed study is a new, interdisciplinary approach to tectonics that leverages modern atmospheric modeling techniques and applies them to one of the most persistent problems in geology, the surface uplift history of the Sierra Nevada. While the approach draws on dynamical meteorology and isotope geochemistry techniques, the results will directly impact our understanding of the geological evolution of North America. A suite of new techniques for interpreting paleoaltimetry proxy records that will be of use in a wide range of settings around the world will be developed. Once proven in the limited context of the Sierra Nevada, these techniques can be applied to other regions and time intervals to better interpret uplift histories and disentangle the mixed elevation/climate signals in paleoclimate records. The results of this study will be of interest to the atmospheric sciences community because it will advance our knowledge of static stability in several paleoclimate scenarios and will elucidate the nonlinear interactions between topography and climate. This study will thus have important implications for both the solid earth and climate change communities.
