EXHUMATION HISTORY OF THE SIERRA NEVADA RANGE FROM LOW-TEMPERATURE THERMOCHRONOLOGY

This study explores the exhumation history of the Sierra Nevada Range (SNR) using new and published low-temperature thermochronologic data. Although the range is well studied, competing models for its Late Cretaceous to Cenozoic uplift and erosion history suggest there is little consensus on the timing, rates, and drivers of exhumation and cooling.

Here we present new (U-Th)/He data derived from 46 samples collected from Mesozoic granitoids covering a broad area of the central SNR, between the American and Merced rivers. Samples were collected in west-to-east transects through major river drainages in this corridor and cover the full range of elevations, from the western foothills to near the crest of the range. An additional set of samples was collected from interfluves between these drainages. We obtained 106 zircon (U-Th)/He dates, with mean ages for samples ranging 61.0±4.9 to 95.9±7.7 Ma. Apatite He dates (n=176) were also acquired and, excluding six samples thermally reset by Miocene volcanics, mean ages range from 31.0±0.6 to 103±6.2 Ma. Age-elevation relationships using the new ages and published data from northern and southern SNR indicate the youngest ages are generally along the range crest; however, age differences between drainages and interfluves are negligible. Age distributions suggest that exhumation of the range mainly occurred from 70-60 Ma, although Eocene exhumation is documented for the southern SNR. Calculated cooling rates also systematically increase from ~3°C/My to >20 °C/My between the early and latest Cretaceous.

The thermochronology data from the SNR appear to be broadly compatible with published cooling ages from the western Great Basin, where two distinct periods of cooling are documented: one at ~65 Ma and another at ~55-50 Ma. Late Cretaceous cooling has been linked to a several possible causes (e.g. refrigeration, magmatic arc cooling, contractional deformation, and extensional deformation); however, the high rates of cooling documented by this study and across the region are inconsistent with models that invoke conductive cooling of the crust as a primary cause. Instead, the cooling at ~65 Ma most likely relates to tectonic exhumation associated with known structures in the region. The Eocene cooling, however, remains enigmatic and potential causes require further exploration.