­DISEQUILIBRIUM RIVER NETWORKS DISSECTING THE WESTERN SLOPE OF THE SIERRA NEVADA, CALIFORNIA RECORD SIGNIFICANT LATE CENOZOIC TILTING AND ASSOCIATED SURFACE UPLIFT

The timing, rates, and spatial patterns of elevation change in the Sierra Nevada, California, USA, have long been the subject of vigorous debate owing to their importance in constraining the tectonic history of western North America and models of orogenesis. Here we present a systematic analysis of river profiles from the western slope of the range from the Feather River south through the Tehachapi River and interpret these observations using 1-D landscape evolution modeling based on the stream power fluvial erosion rule with a rate parameter calibrated from a large dataset of millennial-scale erosion rates. We demonstrate that rivers from the Yuba south through the Kings River are in a disequilibrium state that is consistent with the transient fluvial response expected from a short-lived (<5 Myr) and rapid pulse of down-to-the-west tilting that began <11 Ma and slowed between 2 and 6 Ma. Assuming rigid-block tilting, we show that late Cenozoic tilting likely resulted in ~500-1,300 m of surface uplift at the crest from the Yuba through the Stanislaus Rivers (0.3-0.8° tilt) and 1,000-1,400 m of surface uplift at the crest (0.6-0.9° tilt) from the Tuolumne south through the Kings River. We show that the transient fluvial response to tilting in the Yuba and American Rivers is heavily modulated by heterogeneous lithology in a manner that reconciles the high spatial variability in basement incision observed in numerous prior studies. However, we find that heterogeneous lithology alone cannot explain both the transient state and observed patterns in channel steepness, which seem to require late Cenozoic changes in tectonic forcing. River profiles of basins at the southern end of the Sierra Range, including the Kaweah, Kern, Tule, and Tehachapi Rivers, also have disequilibrium forms that are consistent with late Cenozoic tectonic forcing but not consistent with a simple rapid rigid-block tilting event.