Growth and lateral propagation of fault‐related folds in the Siwaliks of western Nepal: Rates, mechanisms, and geomorphic signature

Abstract

We study the controls on drainage development in tectonically active regions using a numerical tectonic surface processes model combining tectonic uplift caused by fault‐related folding with erosion by fluvial incision, hillslope diffusion, and landsliding. Our model shows the fundamental control exerted by the dip of the detachment underlying the folds on drainage evolution. When the detachment is horizontal, the relative rates of tectonic uplift and fluvial incision control the evolution. For a nonzero dip, in contrast, the lateral displacement gradient associated with fold propagation sets up a lateral slope behind the active structure, which deflects the stream network. We also demonstrate the importance of landsliding for the attainment of realistic and steady state topography. In contrast, discontinuous tectonic movements do not appear to influence the system. We apply our model to the Dundwa fault‐related fold ridge (Siwalik, Nepal). We estimate a remarkably low mean propagation rate for different segments of the structure. This finding, together with the structure and morphology of the ridge, leads us to propose that the ridge developed by linkage of several component segments. The drainage evolution predicted when modeling this scenario compares favorably with field observations. Our models provide insights into the dynamics of fault‐related fold propagation: In particular, as a result of fault segment linkage, estimates of propagation rate of these structures may be strongly scale‐dependent, and the observed morphology and drainage patterns on fault‐related fold ridges may be controlled by fault geometry rather than by the relative rates of tectonics and surface processes.