EP031-0015
The Influence of Topographic Disequilibrium on Erosion Rates from the San Bernardino Mountains, California
Thursday, 10 December 2020
Poster
Marina Olivia Argueta1, Seulgi Moon1, Kimberly Blisniuk2, Lee B Corbett3, Paul R Bierman3, Nathan Brown4 and Susan R H Zimmerman5, (1)University of California Los Angeles, Department of Earth, Planetary, and Space Sciences, Los Angeles, CA, United States, (2)University of California Berkeley, Berkeley, CA, United States, (3)University of Vermont, Burlington, VT, United States, (4)University of California, Los Angeles, Department of Earth, Planetary, and Space Sciences, Los Angeles, CA, United States, (5)Lawrence Livermore Nat''l Lab, Livermore, CA, United States
Abstract:
10Be concentrations in river sand have been used to determine the basin-averaged erosion rates over millennial scales (hereafter
10Be-derived erosion rates) in order to infer the controls of tectonic, climate, and topography on erosion rates from hillslope and river processes. However, landscapes in topographic disequilibrium can respond to surface processes in complicated ways, making
10Be-derived erosion rates difficult to interpret. Located along the restraining bend of the San Andreas fault, the San Bernardino Mountains (SBM) have been subject to varying stages of block uplift and are an ideal site for assessing
10Be-derived erosion rates in a disequilibrium landscape. Previous studies showed a general increase in erosion and uplift rates from north to south within the SBM based on cosmogenic radionuclides and apatite-helium thermochronometry, respectively, but show appreciable scatter in
10Be-derived erosion rates.
In this study, we determined 10Be-derived erosion rates from 43 basins including 12 new basins from this study and 31 basins compiled and recalculated from previous studies. These 10Be-derived erosion rates are then compared with basin-averaged topographic metrics of slope, normalized channel steepness (ksn), chi metric, and local relief with radii 1500 m and 500 m. A comparison of 10Be-derived erosion rates to topographic metrics reveals that there are two distinct trends in the relationships between 10Be-derived erosion rates and ksn. In addition to an overall nonlinear trend between 10Be-derived erosion rates with ksn, some samples have low 10Be-derived erosion rates despite the high ksn. This deviation is mostly observed from basins in the San Gorgonio block of the SBM, which has relict surfaces. We show that the different trend in 10Be-derived erosion rates and ksn is likely due to the topographic disequilibrium between main channel and upland areas, which is from lagged response of hillslope relative to main channel. Our work implies that we should carefully assess the equilibrium state of landscapes to better interpret 10Be-derived erosion rates and how they reflect topographic and erosional response to tectonics.
