Isotopic insights into smoothening of abandoned fan surfaces, Southern California

Ari Matmon*, Kyle Nichols, Robert Finkel

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

50 Scopus citations


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.

Original languageAmerican English
Pages (from-to)109-118
Number of pages10
JournalQuaternary Research
Issue number1
StatePublished - Jul 2006

Bibliographical note

Funding Information:
We thank Marith Rehies and Bruce Harrison for very constructive comments. We thank Y. Kolodny for great advice. We thank Ori Dor and Rachel Menashe for field assistance. This study was funded by the USGS-EHZ. This work was performed under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48.


  • Al
  • Be
  • Nuclide concentration


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