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

New (U-Th)/He ages from granodioritic rocks of the northern Sierra Nevada are used to constrain the history of Sierran exhumation and erosion since ca. 90 Ma. These ages are compared with previously published low-temperature thermochronologic data from the southern Sierra in order to examine the post-orogenic topographic evolution of the range. (U-Th)/He ages in apatite reported here decrease from 80 Ma along the low western range flank to 46 Ma in the higher elevations to the east. (U-Th)/He ages in zircon also show 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, the first stage of which is marked by an early (~90 – 60 Ma) pulse of relatively rapid exhumation (0.2 – 0.8 km/My). This stage immediately follows implacement of the youngest plutons in the northern Sierra and indicates a period of intense erosional denudation associated with crustal thickening and orogenic shortening of the ancestral range. We describe the second stage as a long period of slower exhumation (0.02 – 0.04 km/My) from the Paleogene to today. This second stage is reflected in the low-relief paleotopography of the northern Sierra, where an Eocene erosional surface has long been identified. A long period of slow erosion is also consistent with the occurrence of widespread lateritic paleosols found at the base of Eocene depositional units. The range-wide dominance of Paleogene apatite helium ages suggests that less than ~ 3 km has been eroded from the Sierra Nevada since that time. Climatic and tectonic models, including that of the Sierra as an orogenic plateau, are evaluated in order to account for such limited erosion from what was presumably an orogenic belt in the earliest Cenozoic.