TESTING SPATIALLY UNIFORM EROSION OF STEEP MOUNTAIN CATCHMENTS USING DETRITAL APATITE (U-TH)/HE COOLING AGES

The way in which steep catchments erode, i.e., predominately by uniform grain-by-grain attrition or by point source mass wasting, is fundamentally important to understanding how mountain landscapes evolve. We investigate the spatial variability of erosion in steep catchments draining the eastern Sierra Nevada, California, using detrital apatite (U-Th)/He thermochronometry in modern river sediments. Previous work shows that bedrock-derived apatite (U-Th)/He ages predate development of the eastern escarpment and suggests slow cooling through nearly horizontal isotherms. Thus, mineral cooling ages act as particle tracers, identifying the elevation where sediment grains were shed from bedrock and entered the fluvial system.

We dated detrital sediment from unglaciated Inyo Creek (3.1 km²) and adjacent glaciated Lone Pine Creek (30.7 km²). Contrasting glacial histories result in distinctly different hypsometries; the hypsometry of Lone Pine Creek is shifted toward higher elevations, reflecting glacial sculpting of wide, flat basins and cirques. We compare measured cooling age probability density functions (PDFs) with model PDFs constructed from catchment hypsometries and a local bedrock-derived age-elevation transect. Apatite (U-Th)/He ages from Inyo Creek sediment (n=52) range from 23.6 to 77.4 Ma, with a peak at ~33 Ma. Close correlation between the measured and model PDFs suggests that the remarkably steep Inyo Creek catchment is eroding nearly uniformly, a finding consistent with our field observations of grussified hillslopes. In contrast, ages from Lone Pine Creek sediment (n=15) range from 33.3 to 56.0 Ma, with a peak centered at ~44 Ma. The span of measured ages does not capture the full range predicted by the model PDF, and the measured peak is shifted toward younger ages. Although the mismatch may simply result from an insufficient number of analyses, it more likely relates to storage of older grains in glacial moraines and lakes, destruction of older grains by abrasion, and/or focused erosion of glacially oversteepened hillslopes at intermediate elevations in the catchment. These results highlight the potential sensitivity of detrital apatite (U-Th)/He dating to quantifying the distribution of erosion in both fluvially and glacially sculpted catchments.