Deriving basin-wide denudation rates from cosmogenic radionuclides, San Bernardino Mountains, California

Abstract

As increasing emphasis is placed upon the role surface processes play in regulating tectonic behaviour, the need for accurate measurements of denudation rate has become paramount. The quantity and quality of denudation rate studies has grown with the advent of cosmogenic radionuclide techniques, capable of recording denudation rates over timescales of 100 to 1000000 years. This study seeks to utilise cosmogenic 10Be concentrations measured in alluvial sediments in order to further develop this method and to investigate rates of basin-wide denudation in the San Bernardino Mountains, an active orogen associated with the San Andreas Fault system. The theory which underpins measurements of basin-wide denudation rates with cosmogenic radionuclide analysis is evaluated in light of recent understanding of production mechanisms. Field testing of the assumptions required by the basinwide denudation rate model highlights the importance of sampling thoroughly mixed sediments. Denudation rates ranging over three orders of magnitude are measured by applying the cosmogenic radionuclide technique in thirty-seven basins throughout the San Bernardino Mountains. Results show a relationship between denudation rate and slope which provides quantification of the threshold slope angle in high relief granitic environments and suggests tectonic activity is the first order control of denudation rates in these mountains. Mean annual precipitation is shown to exert no significant influence over the rates measured in the San Bernardino Mountains. Questions concerning denudation rates recorded over differing timespans are addressed using the cosmogenic technique, (U-Th)/He thermochronometry, incision into dated horizons and modern day sediment flux data. This comparison reveals that a decrease in rates with distance from the San Andreas Fault has been consistent throughout the lifespan of the San Bernardino Mountains and provides further evidence that a tectonic mechanism is driving denudation in this region. The relevance of both spatial and temporal scale in geomorphic studies is considered in light of these results, highlighting the need for a greater appreciation of their role in the interpretation of basin-wide denudation rates.