CENOZOIC DENUDATION AND EROSION OF THE NORTHERN SIERRA NEVADA, CALIFORNIA: INSIGHTS FROM (U-TH)/HE THERMOCHRONOLOGY

We present new (U-Th)/He cooling ages in both apatite and zircon from samples collected along an east-west transect across the northern Sierra Nevada, California. These ages are used to constrain the history of Sierran exhumation and erosion since ca. 90 Ma and are compared with previously reported apatite He ages from the southern part of the range. (U-Th)/He ages in apatite decrease from 80 Ma along the low western range flanks to 46 Ma in the higher elevations to the east. (U-Th)/He ages in zircon also show a weak inverse correlation with elevation, decreasing from 91 Ma in the west to 66 Ma in the east. Rocks near the range crest, sampled at elevations of 2200-2500 m, yield the youngest apatite helium ages (46 – 55 Ma), whereas zircon helium ages are more uniform across the divide. These data suggest a two-stage model of Sierran evolution which includes an early (~ 90 – 60 Ma) pulse of relatively rapid exhumation (0.2 – 0.8 km/My), followed by a long period of slower exhumation (0.02 – 0.04 km/My) from the early Paleogene to today. This second stage is reflected in the low-relief paleomorphology of the northern Sierra Nevada, where an Eocene erosional surface has long been identified. A long period of slow exhumation is also consistent with the occurrence of widespread lateritic paleosols found at the base of Eocene depositional units. We interpret this exhumation history as recording a late Cretaceous to early Cenozoic period of relatively rapid uplift and unroofing followed by tectonic quiescence and erosional smoothing of Sierran topography through the Neogene. Well-documented recent incision appears to have little effect on (U-Th)/He ages, suggesting that less than ~ 3 km has been eroded from the Sierra Nevada since the early Pliocene.