Using tracer thermochronology to measure modern relief change in the Sierra Nevada, California

Abstract

We examine relief change in two major Sierra Nevada drainages, the Kings and the San Joaquin. The modern distribution of bedrock cooling ages is derived by fitting isochrones to a dense distribution of existing cooling ages. This analysis indicates southwest tilting of the range by 3.4° ± 0.8°, which is consistent with other geologic evidence for tilting after ∼ 5 Ma. The detrital cooling ages from the Kings show an excess of young ages relative to what would be expected from uniform erosion, whereas the San Joaquin drainage shows excess old ages. These results indicate increasing relief in the Kings. The results in the San Joaquin are more difficult to interpret due to the dense network of dams, which trap sediment. However, we analyze the effect of storage behind the dams and suggest steady or declining relief in the San Joaquin. The residence time of sediment in these drainages indicates that these relief estimates are averaged over the last 100 to 10 ka. We suggest that relief change is related to localized surface uplift in the southern Sierra Nevada, including the Kings drainage, caused by ongoing lower lithosphere removal. The San Joaquin drainage may be north of the lithospheric anomaly, which is consistent with stagnating or decaying relief there.