Landscape response to tectonic forcing: Digital elevation model analysis of stream profiles in the Mendocino triple junction region, northern California

The topographic evolution of orogens is fundamentally dictated by rates and patterns of bedrock-channel incision. Quantitative held assessments of pro cess-based laws are needed to accurately describe landscape uplift and denu dation in response to tectonics and climate. We evaluate and calibrate the shear stress (or similar unit stream-power) bedrock-incision model by study ing stream profiles in a tectonically active mountain range. Previous work on emergent marine terraces in the Mendocino triple junction region of nort hern California provides spatial and temporal control on rock-uplift rates. Digital elevation models and field data are used to quantify differences i n landscape morphology associated with along-strike northwest to southeast changes in tectonic and climatic conditions. Analysis of longitudinal profi les supports the hypothesis that the study-area channels are in equilibrium with current uplift and climatic conditions, consistent with theoretical c alculations of system response time based on the sheer-stress model. Within uncertainty, the profile concavity (theta) of the trunk streams is constan t throughout the study area (theta approximate to 0.43), as predicted by th e model. Channel steepness correlates with uplift rate. These data help con strain the two key unknown model parameters, the coefficient of erosion (K) and the exponent associated with channel gradient (n). This analysis shows that K cannot be treated as a constant throughout the study area, despite generally homogeneous substrate properties. For a reasonable range of slope -exponent values (n), best-fit values of K are positively correlated with u plift rate. This correlation has important implications for landscape-evolu tion models and likely reflects dynamic adjustment of K to tectonic changes , due to variations in orographic precipitation, and perhaps channel width, sediment load, and frequency of debris flows. The apparent variation in K makes a unique value of n impossible to constrain with present data.