THERMOCHRONOLOGY AND SURFACE UPLIFT: USING LARGE DATASETS TO MEASURE PALEOTOPOGRAPHY (AN EXAMPLE FROM THE SIERRA NEVADA)

Like many high mountain ranges, the Sierra Nevada is both the subject of a spirited paleotopography debate and the site of increasingly dense thermochronologic sampling. The paleotopography debate is focused on how much of the present topography was uplifted since the Neogene, when there were major changes in plate convergence and the structure of the lithosphere, including delamination. Much of the thermochronology was performed with this question in mind, but many samples were collected for other reasons as well. Here, we use the example of the Sierra Nevada to illustrate how a comprehensive, structural analysis of thermochronologic data can resolve paleotopography. This result is contrary to the usual argument that thermochronology can only resolve exhumation and not surface uplift. In fact, uplift may be resolved whenever there is an adequate constraint on vertical rock velocity. The tilting of isochrones—defined as surfaces of equal cooling age—provide this constraint in the Sierra Nevada. Our analysis allows long-wavelength (i.e., mountain-scale) topography to evolve independently from local relief. In this way, the tilting trend observed in the full dataset constrains the rock velocities responsible for long-wavelength topography, while individual ages constrain exhumation and the formation of local relief. Inversion of the 3-D thermal-kinematic model Pecube provides estimates of both parameters in the past, as well as estimates of their uncertainties. Results indicate that the westward tilting of the Sierra Nevada accounts for 2 km of uplift since 20 Ma. This finding suggests that the Sierra Nevada lost elevation during the Late Cretaceous and Paleogene, following the extinction its volcanic arc, but regained much of its initial elevation beginning in the Neogene. The approach presented here should be applicable to other mountain ranges where large thermochronologic datasets are available.