We have studied drainage development on active folds by using a numerical model that couples the kinematics of fault-propagation folding to a physical description of surface processes. Modeling results suggest that the dip of the underlying detachment exerts a major control on drainage development. For a nonzero detachment dip, the lateral displacement gradient sets up an axial slope behind the growing fold that diverts drainage so that the fold is shielded from rivers with large incision capacity. In this case, the characteristic fault-segment length, rather than the relative rates of fold uplift and incision, may control the spacing of transverse streams. We apply our model to the foothills of the Nepal Himalayas and suggest that striking differences in drainage patterns between central and western Nepal are controlled to a first order by variations in the dip of the underthrusting Indian plate.

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