Evolution of a Canyon-Fan Complex: Interacting Bedrock-Alluvial Morphodynamics

Abstract:

Arid, tectonically active basins are riddled with canyon-fan complexes. A common case is the vicinity of a normal fault, where one side is uplifting and the other side subsiding (relative to each other). We consider the common case of a canyon incising into the uplifting zone, terminating in an upstream-migrating knickpoint, followed by a depositional basin (fan or bajada) in the subsiding zone. A case in point, and the motivation for our work, is Rainbow Canyon and Panamint Valley, USA, as seen in the Figure. We apply purely alluvial morphodynamics in the fan, linked to tool-driven bedrock-alluvial dynamics in the canyon. We capture the knickpoint using a moving boundary formulation. We show that the canyon-fan system can reach a quasi-steady state, where the knickpoint migrates upstream at a nearly constant rate, canyon long profile is linear, and fan long profile is upward concave. Canyon and fan interact with each other; this is controlled in part by the contribution of sediment from failing canyon walls and the width of the fan-bajada zone. Sustained purely alluvial dynamics cannot penetrate into the uplifting zone, because a complete alluvial cover cuts off incision in response to uplift. If, however, the relative uplift between the canyon and the fan is too large, there is not enough sediment to completely fill the basin as uplift proceeds, and a hanging valley forms.