Tectonic and climatic controls of denudation rates in active orogens: The San Bernardino Mountains, California

The relative importance of climatic and tectonic factors in driving rates of denudation in mountain ranges has long been debated, with both precipitation and rates of crustal uplift cited as first order controls in a variety of different mountainous settings. Few studies, however, have explicitly considered the influence of climatic and tectonic processes on denudation rates during the early stages of orogenesis. Using basin-wide denudation rates derived from in-situ cosmogenic ¹⁰Be, and published modern precipitation rate data, the significance of rainfall and snowfall on rates of denudation is evaluated for the San Bernardino Mountains, California. Denudation rates vary between 52mmka^-1 in the arid northern regions of the mountains and 2700mmka^-1 in the more humid southern sector. We select three basins where the influence of precipitation on denudation rates can be isolated from the effects of crustal uplift and find that there is no apparent relationship between denudation rates and precipitation. Denudation rates differ more than five-fold on the opposing slopes of Mill Creek, a valley bisected by a major splay of the San Andreas Fault, and we propose that this denudation rate disparity can be best explained by variations in uplift rates across the fault. The results suggest that crustal uplift is the mechanism that underpins the ~50-fold variability in denudation rates we have measured in the San Bernardino Mountains, but that this must be facilitated by sufficient precipitation to remove valley-fill deposits.