Footwall topographic development during continental extension.

Abstract

We examine the progressive development of footwall topography associated with a set of active normal faults in the northeastern Basin and Range Province of the western United States. Fault length and displacement increase monotonically from northeast to southwest in the study area, allowing us to track both variations in footwall morphology with increasing displacement and along-strike changes in morphology on a single fault. We show that patterns of catchment area, footwall relief, and catchment outlet spacing vary predictably and are related to the growth of the range-bounding normal fault array. In this semiarid region, full parsing of footwall drainage area and removal of antecedent topography do not occur until fault arrays grow beyond two crustal-scale segments. Multiple-segment faults with lengths of up to 150 km have footwall relief that is limited to ∼1000 m in the center of the footwall and that decays to zero at the fault tips over a length scale of ∼15 km. We hypothesize that this relatively uniform footwall relief is erosionally limited and reflects the efficacy of surface processes in removing footwall material in the center of the footwall. If the fault array grows by relatively steady propagation of the tips, we suggest that the 15 km length scale required to reach uniform relief is related to a timescale of relief generation by the fault tip propagation rate. While such propagation rates are poorly known, an average rate of 10 mm yr−1 would imply footwall relief generation over a timescale of ∼1 Myr.