Paper No. 21-2
Presentation Time: 8:55 AM
LATE CENOZOIC UPLIFT OF THE NORTHERN SIERRA NEVADA IS NOT SUPPORTED BY THE GEOMORPHIC EVIDENCE
Substantial uplift ( > 500 m) of the Sierra Nevada in the late Cenozoic has been a dominant paradigm in California geology over the past 100 years. The ramp-like profile of the range’s western slope and the deep canyons cutting across its surface give the northern half of the Sierra the appearance of an old landscape rejuvenated by recent uplift. Attempts to estimate the timing and magnitude of this uplift in the northern half of the range have relied on geomorphic analyses which are found to be flawed. For example, one approach, first employed in 1911, is based on the reconstruction of Eocene-Oligocene rivers from channel remnants that are often separated by many kilometers. Several studies have compared the gradients of reconstructed reaches trending east-west (i.e., normal to the strike of the range) with those trending north-south (i.e., parallel to the range) and concluded that the range-normal reaches are steeper due to tectonic uplift and westward tilting. Among other problems, this approach does not account for the fact that channels adjust their gradients to the strength of the bedrock; indeed, the low-gradient range-parallel reaches apparently flowed through relatively erodible rock while the steep range-normal reaches were cut into resistant lithologies (e.g., granite). A second technique to estimate the timing and magnitude of Sierran uplift uses the dip of sedimentary beds. One study employing this approach determined that there had been 1.4° of westward tilt since ~5 Ma, a result implying that Eocene fluvial deposits near the Yuba River and presently at 700 masl were once 500 m below sea level. Because Cenozoic marine deposits have not been found in association with these river gravels, this technique is of dubious value. Finally, with the assumption that uplift leads to valley deepening, several studies have attempted to constrain the ages of channel incision by various means and have argued that the canyons of the Sierra Nevada were only cut in the past few million years. Miocene and Pliocene-age volcanic rocks deep within some valleys challenge this interpretation. We conclude that the geomorphic evidence is too flawed to support the claim of significant late-Cenozoic uplift in the northern Sierra and, thus, tectonic quiescence over this time period should be considered the default hypothesis.