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The relationship between tectonic uplift and chemical weathering rates in the Washington Cascades: Field measurements and model predictions

Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305

hren{at}stanford.edu

Tectonic uplift is one of the key factors controlling the supply of material to weathering environments. At present however, there is debate about whether tectonics or climate plays a larger role in controlling chemical weathering rates on a global scale over geologic time. We measured riverine weathering fluxes from twelve catchments along the Skykomish River in the Washington Cascades, where long-term exhumation rates have been constrained by (U-Th)/He thermochronometry, to examine the effect of tectonic uplift on chemical weathering rates. We show that in the western Washington Cascades, dissolved Si fluxes increase from approximately 1900 to 3000 mol ha^–1 yr^–1 from west to east across the range and are correlated with rock exhumation rates. We use dissolved Si flux data from these catchments, depth to bedrock measurements, and long-term exhumation rates to test a steady-state weathering model that describes the relative importance of reaction kinetics and rock supply on weathering rates. Characterization of study basins with respect to two key variables in this model, weathering zone depth and exhumation rate, allows us to examine the relative effects of climate and tectonics on chemical weathering by analyzing the ratio of the timescale of reaction kinetics and the residence time in the weathering zone. Evaluation of this model shows that chemical weathering rates in the Cascades are controlled by the rate of rock erosion and factors that influence the depth of the active weathering zone. However, the model predicts that in tectonically inactive areas, the supply of material limits the overall rate of chemical weathering while in active tectonic or erosional environments, the rate of supply exceeds the ability of a system to weather that material resulting in a landscape in which weathering rates are expected to respond strongly to changes in climatic conditions.