Topographic advection on fault-bend folds: Inheritance of valley positions and the formation of wind gaps

Drainage basins formed along the flanks of any individual linear mountain range or fault block are commonly observed to be self-similar in planform, uniformly spaced, and in some cases aligned with drainage basins on the opposing flank. Data from the Siwalik Hills, Nepal, illustrate this organization where drainage basins and valleys are aligned across the main divide. We suggest that valley alignment is a consequence of advection of topography across the divide. To explore this hypothesis, numerical experiments were conducted using a landscape evolution model of a fault-bend fold that simulates detachment-limited stream incision and linear hillslope diffusion. Results show that the presence of incised valleys and a lateral component of bedrock motion are necessary and sufficient conditions for advection of relief across the divide, a mechanism by which wind gaps form and valley spacing on the far side of a ridge is inherited from that on the near side. This topographic inheritance is promoted by low rock erodibility, low precipitation rates, fast bedrock velocities, and/or intermediate fault dips.