OLD FANS, SMOOTH SURFACES: THE AGING OF ALLUVIAL SURFACES IN THE MOJAVE DESERT

Cosmogenic nuclide concentrations measured on abandoned alluvial fan surfaces along the Mojave section of the San Andreas Fault provide insight into the surface evolution of these fans and suggest that sediment is generated, transported, and removed from the fans on the order of 30–40 kyr.  We sampled five progressively older abandoned fan surfaces, substituting space for time, and measured in situ produced cosmogenic ¹⁰Be, and in some cases ²⁶Al, in boulders (n=15), surface sediment (n=15), and in one depth profile (n=9). Our data reveal that nuclide concentrations in surface sediments and boulders underestimate fan ages, suggesting that ¹⁰Be accumulation is largely controlled by the geomorphic processes that operate on the surfaces of the fans and not by their ages.

Field observations, grain-size distribution, and cosmogenic nuclide data suggest that over time, boulders weather into grus and the bar sediments diffuse into the adjacent swales. As fans age the relief between bars and swales decreases, the sediment transport rate from bars to swales decreases, and the surface processes that erode the fan become uniform over the entire fan surface. The nuclide data therefore show that, over time, the difference in ¹⁰Be concentration between bars and swales increases to a maximum (180 to 220 ka) until the topographic relief between bars and swales is minimized, resulting in a common surface lowering rate (10 to 15 mm ky^-1) and common ¹⁰Be concentrations across the fan.