Patterns of denudation through time in the San Bernardino Mountains, California: Implications for early-stage orogenesis

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.