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.