DETRITAL THERMOCHRONOLOGIC STUDIES OF STEEP FLUVIAL CATCHMENTS: EXAMPLES FROM THE SOUTHEASTERN SIERRA NEVADA, CALIFORNIA

In their simplest application, detrital thermochronologic techniques rely on the assumption that detrital data sets faithfully represent bedrock ages from within a given catchment area. Given this framework, both bedrock and detrital data sets from regions with constant uplift and uniform erosion should yield similar estimated exhumation rates. Deviations from this expectation likely imply non-uniform uplift or non-uniform erosion in a catchment. We report here an attempt to explore the relationships between bedrock- and detritus-derived exhumation rates for two similar, adjacent catchments in the southeastern Sierra Nevada of California using both conventional (single crystal) and laser ablation (U-Th)/He thermochronometry of zircon. Thus far, samples analyzed from steep bedrock age-elevation transects in the two transects yielded statistically indistinguishable conventional and laser ablation (U-Th)/He results: a range of from 58.1 ± 7.0 Ma to 73.7 ± 1.8 Ma, implying relatively rapid Late Cretaceous to Paleocene exhumation. Somewhat surprisingly, laser ablation data from near the mouth of the catchment yielded substantially broader apparent age ranges – 20.1 ± 0.6 Ma to 109.2 ± 3.9 Ma, and indicate much lower exhumation rates than those suggested by the bedrock cooling ages. The difference in apparent age ranges implies that either: 1) the sediment from the mouth of the catchment may have been contaminated by zircons with both older and younger (U-Th)/He zircon dates, perhaps due to sediment transport from farther north along the Owens Valley; or 2) similar non-uniform uplift or non-uniform erosion in both catchments. While we are not yet sure which of these alternatives may be correct, we suspect that valuable clues will come from comparisons of the U/Pb ages of the detrital and bedrock zircons (Horne et al., Geochim. Cosmochim. Acta, 2016).