Quantifying differential rock-uplift rates via stream profile analysis

Despite intensive research into the coupling between tectonics and surface processes, our ability to obtain quantitative information on the rates of t ectonic processes from topography remains limited due primarily to a dearth of data with which to test and calibrate process rate laws. Here we develo p a simple theory for the impact of spatially variable rock-uplift rate on the concavity of bedrock river profiles. Application of the analysis to the Siwalik Hills of central Nepal demonstrates that systematic differences in the concavity of channels in this region match the predictions of a stream power incision model and depend on the position and direction of the chann el relative to gradients in the vertical component of deformation rate acro ss an active fault-bend fold, Furthermore, calibration of model parameters from channel profiles argued to be in steady state with the current climati c and tectonic regime indicates that (1) the ratio of exponents on channel drainage area and slope (m/n) is similar to0.46, consistent with theoretica l predictions; (2) the slope exponent is consistent with incision either li nearly proportional to shear stress or unit stream power (n = 0.66 or n = 1 , respectively); and (3) the coefficient of erosion is within the range of previously published estimates (mean K = 4.3 x 10(-4) m(0.2)/yr). Applicati on of these model parameters to other channels in the Siwalik Hills yields estimates of spatially variable erosion rates that mimic expected variation s in rock-uplift rate across a fault-bend fold. Thus, the sensitivity of ch annel gradient to rock-uplift rate in this landscape allows us to derive qu antitative estimates of spatial variations in erosion rate directly from to pographic data.