Collaborative Research: Development of topography in the northern Sierra Nevada: Reconciling differences in uplift history and causes through multi-source thermochronologic data

Regents Of The University Of Idaho, Moscow ID

Investigators

Abstract

The Sierra Nevada of eastern California is a rugged mountain range home to deeply incised canyons and the highest peak in the conterminous United States. These features and a variety of studies have led many researchers to conclude that the high elevations are relatively recent (young tectonic uplift model). Other datasets, however, indicate a long-standing Sierra Nevada that has retained high elevations for the past 70 million years or more (old tectonic uplift model). Most studies favoring the young uplift model are from the southern part of the Sierra, whereas those favoring the old uplift model stem from studies of ancient gold-bearing river gravels and volcanic rocks only present in the north. This project will use low-temperature thermochronology (which can reveal the timing of exhumation), together with study of these ancient river deposits preserved on the western range flank, to address discrepancies between uplift models. Importantly, this work will create a range-wide thermochronologic dataset that will test whether conflicting interpretations are due to fundamental north to south changes in the geology of the mountain range or if the range shares a unified uplift history. Undergraduate and graduate students from three universities will be supported by this project and will receive mentoring from both their peers and principal investigators from all involved institutions. Three cohorts of high-school students will also be engaged in this research through a TRIO-INSPIRE STEM-Access summer internship program. This project aims to constrain the exhumation history of the northern Sierra Nevada and its Cenozoic sediment sources to test hypotheses for possible along-strike variability in the history and causes of topographic uplift. Most thermochronologic and tectonic geomorphology studies are focused in the southern Sierra and support a model of recent (post-Miocene) tectonic uplift. In contrast, paleoaltimetric and detrital zircon (DZ) studies of Cenozoic strata preserved in the northern Sierra Nevada suggest development and maintenance of high topography since the Late Cretaceous. These spatially separated and often contrasting data have hindered agreement on an uplift theory for either part of the range. This study will use 1) basement (U-Th)/He data along two range-perpendicular transects in the northern Sierra, with a focus on sampling both modern valley and paleovalley bottoms, the latter immediately below the Eocene fluvial gravels, and 2) laser-ablation (U-Th)/(He-Pb) double dates coupled with Hf isotope data on targeted DZ sub-populations in the basal Eocene gravels to constrain incision timing and discriminate local vs. extra-regional sediment sources, which is not possible with the DZ U-Pb data alone. The integrated basement-detrital datasets will determine whether (U-Th)/He patterns are similar or different across the range, implying shared or separate uplift histories, and the Eocene position of the northern drainage divide, which informs the large-scale geometry of topography and fluvial drainages from the Sierra eastward to an elevated plateau. The researchers will test competing hypotheses for the uniformity, timing, and causes of Sierra Nevada uplift. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.