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Isotopic insights into smoothening of abandoned fan surfaces, Southern California

Published online by Cambridge University Press:  20 January 2017

Ari Matmon ,
Kyle Nichols  and
Robert Finkel
Ari Matmon*
Affiliation:
U.S. Geological Survey, 345 Middlefield Rd., MS #977, Menlo Park, CA 94025, USA The Institute of Earth Sciences, Hebrew University, Givat Ram, Jerusalem, Israel 91904
Kyle Nichols
Affiliation:
Department of Geosciences, Skidmore College, 815 North Broadway, Saratoga Springs, NY 12866, USA
Robert Finkel
Affiliation:
Lawrence Livermore National Laboratory, Livermore, CA 94550, USA
*
⁎Corresponding author. The Institute of Earth Sciences, Hebrew University, Givat Ram, Jerusalem, Israel 91904. E-mail address: arimatmon@cc.huji.ac.il (A. Matmon).
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Abstract

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.

Keywords

10Be26AlNuclide concentration
Type
Research Article
Information
Quaternary Research , Volume 66 , Issue 1 , July 2006 , pp. 109 - 118
Copyright
University of Washington

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