Topographic Response to Horizontal Advection in Normal Fault-Bound Mountain Ranges

Abstract

Tectonic displacements consist of vertical uplift or subsidence, and horizontal advection. We consider the effects of tectonic advection on mountain range topography, surface drainage patterns and drainage divides. Through numerically modeling a normal fault and uplifted footwall, we find that advection promotes the elongation of catchments and reduction in outlet spacing at mountain fronts. We demonstrate an erosional disequilibrium associated with the advection-induced transfer of mass from the proximal mountain front toward the distal mountain front. Our modeling also demonstrates the development of an erosion rate disequilibrium around fixed geomorphic features such as drainage divides, that we deem characteristic of advection. In our model, steady-state is achieved when the divide migration occurs at the same rate as advection; advection moves topography away from the fault, meaning drainage migration becomes a mechanism for balancing advection. Topographic observations in a mountain range bound by a low angle normal fault, the Sierra la Laguna (Mexico), are consistent with our modeling results when advection is included. These observations reveal a strong influence of advection on the development of the Sierra la Laguna's topography and suggest the main drainage divide is migrating toward the fault to counterbalance advection. Tectonic advection exerts a significant control on erosion rate distribution, topographic and drainage patterns and the stability of drainage divides. This work also highlights that the Gilbert metrics and across-divide χ disequilibrium cannot always be interpreted in terms of growing or shrinking catchments in settings with a large advection component of tectonic displacement.