Characteristics of steady state fluvial topography above fault‐bend folds

In steady state convergent orogens, erosion balances lateral as well as vertical bedrock motions. For simple geometrical reasons, the difference between the total steady state erosion flux and its vertical component is up to 30% for typical fluvial slopes and bedrock streamline inclinations, suggesting that lateral advection is also likely to be expressed topographically. In order to understand these geomorphologic consequences, we focus on steady state topography developed on active fault‐bend folds. First, we derive an analytical solution for the slopes of detachment‐limited streams that incorporates lateral advection. Next, we conduct experiments using a numerical two‐dimensional landscape evolution model (Channel‐Hillslope Integrated Landscape Development model (CHILD)) incorporating linear diffusion on hillslopes and detachment‐limited stream channel incision above a fault‐bend fold. The concavity and steepness indices of steady state long profiles are functions of bedrock velocity magnitude and direction, streamflow direction, and fluvial erosivity. Asymmetry of mountain range profiles varies as a function of fluvial erosivity or bedrock velocity only if we account for the lateral velocity component. This asymmetry is equally sensitive to this lateral component, fluvial incision, and hillslope diffusion. However, the effect of diffusion on drainage divide position is significant only at high diffusivities, short length scales, low bedrock advection rates, or relatively low fluvial erosivity. Thus in most mountain ranges and fault blocks, drainage divide migration is expected to be dictated by stream channel erosion. Model results are shown to be consistent with topography in the Siwalik Hills, Nepal, which overlie fault‐bend folds produced above the frontal fault systems in the Himalayan foreland.