PALEOTOPOGRAPHY OF THE CENTRAL - NORTHERN SIERRA NEVADA AND IMPLICATIONS FOR UPLIFT AND TILTING OF THE SIERRAN BLOCK

Because of its high elevations and rugged relief, the Sierra Nevada has commonly been considered an example of youthful topography, yet available paleoaltimetric and thermochronologic data suggest topographic antiquity of the range. We address this issue by using the geo- and thermo-chronology of Eocene fluvial deposits, preserved along the western flank of the central and northern Sierra Nevada, to trace the sourcing of detritus in paleoriver systems and to constrain the development of the Sierra Nevada range front. U-Pb ages of detrital zircons from the ancestral Yuba, American, Mokelumne, and Stanislaus Rivers, are measured with the aim of constraining the paleotopography of source regions, and the development of regional drainage systems. U-Pb ages are bimodal, with a dominant age peak between ~110 – 90 Ma, and lesser, but significant, peaks in the Jurassic (between ~ 170 and 150 Ma), a distribution that matches that of pluton ages in the central and northern Sierra. Pre-Mesozoic aged grains represent a small fraction (up to 6%) of the total population from a given sample locality, and are a good match to local metasedimentary belts. A subset of zircons (n = 11) from one locality were double-dated using U-Pb and (U-Th)/He methods. Zircon crystallization ages range from ~ 2500 Ma to 90 Ma and have corresponding (U-Th)/He ages which do not correlate with U-Pb age, and which range from 114 Ma to 74 Ma, consistent with cooling ages reported for the northern Sierra Nevada. Our results suggest that Eocene rivers were sourced locally within the Sierra and were likely shorter, steeper, and draining smaller catchments than was commonly thought. Furthermore, our data suggest that the Sierra Nevada was a major topographic barrier that had an established drainage divide through the early – mid Cenozoic. Given geologic and paleoelevation data, which indicate moderate to high mid Cenozoic elevations for the adjacent Great Basin, the Sierran crest was likely high (>2 km) in the Eocene, with a western flank characterized by a gradient similar to the modern one. Our results, therefore, are consistent with a model of Sierran topographic antiquity, and preclude major post-Eocene crestal uplift and /or westward block tilting.