CONSTRAINING LANDSCAPE EVOLUTION IN THE SIERRA NEVADA THROUGH DETRITAL ZIRCON PROVENANCE ANALYSIS OF CENOZOIC FLUVIAL SEDIMENTS

U-Pb analyses of single zircon grains from Cenozoic fluvial deposits found on the western flank of the Sierra Nevada provide temporal and spatial constraints on the development of a Sierran range front. There has been considerable recent debate surrounding the timing and magnitude of bedrock and surface uplift of the Sierra Nevada. Traditional models have the bulk of surface uplift occurring in the last 3 – 5 Myr, while a collection of recent studies support a model of the Sierra Nevada existing as a topographic range throughout the Cenozoic. We use the provenance of detrital zircon grains deposited in fluvial sediments in the central and northern parts of the range to test these two end-member models of Sierran landscape development. Zircon grains with age signatures from basement terranes east of the modern Sierra support the traditional model of a low-elevation, low-relief landscape pre-Neogene uplift. Zircon grain populations with a dominant Sierra-age signature support a model of sustained high Cenozoic topography and an existing range front near to or the same as the modern drainage divide.

Preliminary results from sands within the Eocene gold-bearing gravel deposits indicate that the majority of grains (~94%) are of Sierran-arc affinity, with ages ranging between 91 and 166 Ma. Relatively few grains (~5%) are Precambrian in age. The youngest zircon(s) in a sample are Eocene (40Ma) and are used to constrain the maximum age of deposition. Because of the paucity of Precambrian zircons, there are no statistically significant old age peaks, which is problematic as those old ages may make it possible to distinguish between sources in the Sierra Nevada western foothills metamorphic belt and Nevada sources.