『Abstract
Basin-averaged cosmogenic 10Be concentrations, apatite
(U-Th)/He thermochronometry and incision into a dated palaeosurface
constrain spatial and temporal variations in the rates of denudation
experienced during the early-stages of orogenesis in the San Bernardino
Mountains, California. Cosmogenic 10Be analysis measures
denudation over intermediate (〜102-104 years)
time scales and records rates which decrease from a maximum of
2700±500 mm ka-1 in the south to a minimum of 52±5
mm ka-1 in northern catchments. Corresponding rates
from (U-Th)/He and incision into a dated palaeosurface measure
long-term (〜106 years) denudation rates which decrease
from between 1200±400 mm ka-1 in the south to a minimum
of 30±20 mm ka-1 in the north. The temporal consistency
observed in the broad-scale patterns of denudation rates probably
results from the persistent imprint of the initial crustal architecture
and drainage network. These have maintained an influence on slope
distributions, and are thus fundamental factors controlling the
gross patterns of denudation throughout the early stages of orogenesis.
Where variations between the denudation rates measured over different
time scales are apparent the intermediate-term rates are found
to be consistently greater than the long-term, with the increase
being more pronounced around the fault bounded peripheries of
crustal blocks relative to quiescent block interiors. This provides
empirical support for a model of mountain building whereby topographic
development is dictated by the headward retreat of drainage systems
that propagate away from zones of displacement. Our findings indicate
that recent localised increases in denudation rates in young fault
block orogens may be explained by a progressive denudational response
to prior tectonic uplift, rather than a consequence of climatic
change.
Keywords: denudation rates; topographic evolution; cosmogenic
nuclides; orogenesis; San Bernardino Mountains』
1. Introduction
2. Field area
3. Methods
3.1. Cosmogenic 10Be derived denudation rates
3.1.1. Sample collection and measurement
3.1.2. Denudation rate calculations
3.1.3. Cosmogenic measurement averaging period
3.2. Interpretation of long-term denudation rates
4. Results
4.1. Temporal and spatial patterns of denudation
5. Discussion
5.1. Denudation rate controls
5.2. Denudation rates in threshold topography
5.3. Variations of denudation rates with time
5.4. Implications for orogenesis
6. Conclusions
Acknowledgements
Appendix A
References