EVOLUTION OF DRAINAGE AREA AND CHANNEL STEEPNESS IN AN EXTENSIONAL TECTONIC DOMAIN

It has been well established by previous research that tectonic signals and past climate remain imprinted in topography. Although topographic evolution has been heavily studied, determining landscape sensitivity to tectonics stands to be challenging. Specifically, a quantitative framework of the competing and complementary effects of uplift and drainage reorganization is needed to understand how landscapes evolve. Normal faults of southwestern Montana present an excellent natural experiment to explore river steepening due to the onset of active faulting and the evolving spatial footprint of the drainage basins. We utilize a series of faults, the Red Rock, Blacktail, Sweetwater, and a new unidentified active structure in the Ruby Mountains, that present a gradient in tectonics (age and slip rate) over an extensional domain. These faults offer the proper conditions and area to study the temporal and spatial changes of faults through catchment analysis. We present a preliminary analysis of (1) drainage basin area, (2) the quantity χ, and (3) mean channel steepness values of a collection of watersheds to determine their response to different slip rates and displacement from the sequence of normal faults. We explore steepness patterns and χ values along-strike of a single fault and regionally along faults. Considering that large-scale deformation remains engraved in watersheds, drainage basin morphometric analysis is essential in order to determine deformation style and rates along the fault system. The patterns reveal the dynamic nature of drainage area in the footwall of an active normal fault. We observe that early in the fault history, drainage basins flowing from the footwall to the down dropping headwall are quite small and grow over time. Using a normalized steepness map, we noticed that steepness values increase moving westward toward larger, older faults. A χ map, constructed to evaluate drainage area exchange, presents a behavior analogous to the steepness map. There is more evidence of drainage basin instability toward the western side representing a wavelength of transient topography. We then address these results to better comprehend landscape response to tectonic processes. An analysis of how steepness and drainage reorganization develop with the initiation and evolution of faults provides a way to assess the adjustments that lead a system into equilibrium and to improve the understanding of landscape evolution processes.