TOPOGRAPHIC EVOLUTION OF THE SOUTHEASTERN SIERRA NEVADA, CA: PROSPECTS FOR INSIGHTS FROM MULTICHRONOMETER DETRITAL GEOCHRONOLOGY AND THERMOCHRONOLOGY

The antiquity of topographic relief in the Sierra Nevada mountain range has been the focus of research for over a century, yet debate remains as to how much of it was established in the Cretaceous and how much is of Cenozoic age. Thermochronologic studies aimed at constraining uplift traditionally rely on the interpretation of bedrock apparent age-elevation transect datasets, with assumptions made to convert cooling histories to uplift histories. Some limitations to this approach include limited access to appropriately steep traverses, locally low relief, or locally rapid cooling rates which may complicate the interpretation of bedrock age-elevation relationships in terms of uplift. Recognizing that active streams draining an orogen carry sediments that, collectively, preserve a rich record of the bedrock cooling history in a catchment, we have developed an alternative approach to studying catchment-by-catchment bedrock cooling histories for large segments of mountain ranges by combining detrital geochronology and thermochronology techniques. While many bedrock age-elevation studies focus on variations by elevation of cooling dates using one or two thermochronometers, our approach is to use multichronometer datasets to attain a richer sense of cooling history over longer timeframes. We demonstrate this approach using hornblende ⁴⁰Ar/³⁹Ar and paired apatite and zircon U/Pb and (U-Th)/He data for four catchments along a ~100 km segment of the eastern flank of the southern Sierra Nevada. Using a version of the method described by Gallagher and Parra (2020) modified for multi-chronometer datasets, we modeled cooling histories for the catchments that suggest rapid bedrock cooling in the catchments at ca. 80-75 Ma, followed by much slower cooling. This signal persists regardless of crystallization ages of Sierran plutons and is thus likely to be due to rapid bedrock uplift in the Upper Cretaceous. For example, in one catchment, thermal models suggest that catchment bedrock – with an apparent ²⁰⁶Pb/²³⁸U crystallization age of 164.45 ± 0.32 Ma based on detrital zircons – cooled slowly (at ca. 2 °C Myr-1) before cooling rapidly beginning at ca. 80 Ma; this period of accelerated cooling persisted for ca. 10 Myr. Subsequent slow cooling until at least 18 Ma ± 26 Ma implies limited uplift-related cooling over that interval.