PALEOGEOGRAPHY AND PALEOTOPOGRAPHY OF THE SIERRA NEVADA FROM DETRITAL GEO- AND THERMOCHRONOLOGY OF EOCENE FLUVIAL DEPOSITS

Eocene and Oligocene fluvial deposits in the ancestral Yuba river on the west flank of the Sierra Nevada provide an opportunity to constrain paleotopographic and paleogeographic aspects of the range. These coarse-grained auriferous sediments were deposited by west-flowing rivers prior to ~40 Ma (from overlying Oligocene volcanics), and their stable isotope compositions have been used to suggest high (>2.2 km) topography in the Eocene Sierra. We measured U/Pb and (U-Th)/He ages of zircons and apatites in clasts and sand-sized fractions from two locations: Eocene deposits at Orleans Flat (1.2 km a.s.l.) and Oligocene deposits downstream at the Blue Lead Mine near Smartville (~0.2 km a.s.l.)

U/Pb ages of zircons from clasts at Orleans Flat and Blue Lead mine are all 340-380 Ma and 160-165 Ma, respectively, suggesting local derivation. Zircons from sands at Blue Lead mine show two major Sierran magmatic peaks at 90-120 Ma and 140-160 Ma, and <5% Phanerozoic and Precambrian (1.8 Ga and 2.3-2.7 Ga) grains characteristic of Sierran accreted terranes and roof pendants.

The Eo-Oligocene gravels show a broad distribution of detrital AHe ages, from 44-104 Ma (80% from 50-85 Ma), and are older (65-101 Ma) in the higher elevation Eocene sample. Surprisingly, this age distribution is similar (but slightly skewed towards older ages) to that of modern AHe ages from the central and northern Sierra (~40-80 Ma). This requires both relatively rapid Eo-Oligocene exhumation (~0.1-0.3 km/Myr), but also some regions with much slower time-averaged erosion rates, similar to today's (~0.04 km/Myr). This could be explained by large erosion rate variations in either space or time (or both) in the ancestral Sierra Nevada. A widespread temporal change in erosion rates would require a pulse of rapid erosion in the early Paleogene, which is not supported by thermochronology of bedrock samples. A large (~10x) spatial variation in erosion rates, on the other hand, could explain these data. The prevalence of younger detrital AHe ages (and shorter lag times) in the western sample implies faster erosion towards the west, despite lower inferred paleoelevations there, possibly indicating a slope and/or climatically controlled cross-range erosion rate gradient. This is consistent with high paleotopographic relief in the Eocene-Oligocene Sierra Nevada.