LATE CENOZOIC UPLIFT AND ASSOCIATED LANDSCAPE EVOLUTION OF THE SIERRA NEVADA, CALIFORNIA

We present new data from combined field and computational methods that place tight spatial and temporal constraints on Late Cenozoic Sierra Nevada uplift and landscape evolution. Additionally, our data constrains temporal and spatial development of the northern Sierra Nevada Frontal fault system (FFS). Field relations of six Late Cenozoic volcanic units ranging in age from ~5.0 Ma to 0.4 Ma, combined with exposures of Tertiary auriferous stream gravels, in the central Feather River watershed indicate uplift initiation of the northern Sierra Nevada beginning just before ~2.8 Ma. Mehrten (~5.0 Ma) and Yana (2.8 Ma) Fms that once occupied channels and broad valleys presently cap the highest peaks of the northern Sierra Nevada crest indicating significant uplift-related stream incision. A suite of younger terrace-like basalt remnants records temporal variation in the pace of uplift-related incision which peaks between 1.2 and 0.59 Ma (0.26 mm/yr), and continues into present at 0.23 mm/yr. While rates of incision in the northern Sierra are similar in magnitude to those calculated by previous workers in the central and southern parts of the range, peak uplift rates occur more than 1.5 Ma later than determined for areas father south. Landscape modeling based on knickpoint distribution in the central Feather River watershed indicates mean upland paleorelief of 164 m, consistent with field-based estimates from previous work in the northern Sierra, but significantly less than estimates for areas farther south. Additionally, hypsometric analyses indicate that the northern Sierra Nevada landscape (i = 0.53) is considerably more youthful than the range-wide average (i = 0.33). Further landscape analyses reveal that the Feather River watershed sits higher than the range-wide average, and has more relief than any area north of Hetch Hetchy Valley. These data suggest that timing of most recent Late Cenozoic uplift is much younger in the northern Sierra. Similarly, vertical separation north and south of the North Fork, 260 m and 801 m respectively, suggests that Late Cenozoic slip along the FFS has propagated northward along the rangefront, accommodating prior uplift. For these reasons, we argue that a uniform block-tilt model for the Sierra Nevada cannot account for variation in timing and magnitude of uplift throughout the range.