Isotopic insights into smoothening of abandoned fan surfaces, Southern California

Cosmogenic nuclide concentrations measured on abandoned fan surfaces along the Mojave section of the San Andreas Fault suggest that sediment is generated, transported, and removed from the fans on the order of 30-40 kyr. We measured in situ produced cosmogenic ¹⁰Be, and in some cases ²⁶Al, in boulders (n = 15), surface sediment (n = 15), and one depth profile (n = 9). 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 grow older 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 suggest that, over time, the difference in ¹⁰Be concentration between bars and swales increases to a maximum until the topographic relief between bars and swales is minimized, resulting in a common surface lowering rate and common ¹⁰Be concentrations across the fan. During this phase, the entire fan is lowered homogeneously at a rate of 10-15 mm kyr^-1.