TOPOGRAPHIC EVOLUTION OF THE SIERRA NEVADA MEASURED DIRECTLY BY INVERSION OF LOW-TEMPERATURE THERMOCHRONOLOGY

摘要

At present, there are at least two competing ideas for the topographic evolution of the Sierra Nevada. One idea is that the Sierra Nevada was formed as a monocline in the Cretaceous, marking the transition from the Great Valley forearc basin to the west, and a high Nevadaplano plateau to the east, similar to the west flank of the modern Altiplano of the Andes. Both the thermochronologic imprint of local relief and the stable isotopic evidence of a topographic rain shadow support this hypothesis. A second idea, supported by geomorphic observations, suggests that the Sierra gained a large fraction of its present elevation as recently as the Pliocene. This recent surface uplift could have been driven by convective removal of the lower part of the lithosphere, the isostatic response to Basin and Range faulting, and/or by changes in dynamic topography associated with deep subduction of the Farallon plate. Here we present the first comprehensive analysis of low-temperature thermochronology in the Sierra Nevada, which indicates that both ideas are likely correct. We show that thermochronology can resolve surface uplift as well as exhumation by measuring long-wavelength topographic evolution relative to sea level. Uplift measurements are possible whenever there is sufficient constraint on vertical rock velocity. The tilting of isochrones, defined as surfaces of equal cooling age, provide this constraint in the Sierra Nevada. Three new factors further distinguish our analysis. The first is that we allow for the local relief and the long-wavelength topography to evolve independently. Second, we use Al-in-Hb barometry to constrain the initial depth of emplacement for the Sierra Nevada plutons. Third, our analysis is tied to a sea-level datum by using the paleo-bathymetric record of the Great Valley basin, where it transitions to the Sierra Nevada batholith. Results indicate that westward tilting of the Sierra Nevada accounts for 2 km of uplift since 20 Ma. Topographic relief increased by a factor of 2. These findings suggest that the Sierra Nevada lost elevation through most of the Tertiary but regained much of its initial elevation following the onset of surface uplift in the Miocene.