Evolution of Drainage Divide Networks in Normal Faulted Landscapes of Southwest Montana

Abstract:

River basins are a key component of most landscapes, and the dynamic elements governing drainage divide reorganization are a fundamental feature of landscape evolution. The quantitative analysis of river network geometry can help uncover the equilibrium state of river basins which is necessary for understanding the relationship between uplift, erosion processes, and topography. Although drainage divide mobility has been researched extensively, assessing how and why divide migrating occurs stands to be exceptionally challenging. A natural setting of normal faults in southwestern Montana offers an opportunity to examine the response of drainage reorganization to tectonic perturbations. We compare watersheds disturbed by spatiotemporal variations in faulting to evaluate which topographic metrics are indicative of drainage divide migration. We exploit a sequence of faults over an extensional domain, the Stone Creek, Sweetwater, and Blacktail in the Ruby Mountains, that depict a contrast in age and slip rate. We calculate the hypsometric integral, mean channel steepness, Gilbert metrics (e.g. mean upstream relief, mean upstream gradient, and channel head elevation), and χ over several catchments along-strike each of the three normal faults. The normalized steepness map and a χ map, show that both metrics’ values increase westward in the direction of older faults. Hypsometric integral values decrease moving westward, coinciding with the higher steepness values and more developed watersheds of the larger faults. Cross-divide contrast in Gilbert metrics and χ values reflect divide instability along-strike. Multiple metrics indicate that the main divide for the given faults is moving away from the faults; however, discrepancies among the different metrics do exist. We incorporate cosmogenic nuclide data from previous work to explore which metrics best predict contrasts in erosion rates across divides and thus divide migration. The results expose the dynamic nature of drainage networks along the footwall of normal faults. An analysis of how the motion of drainage divides develops with the onset and growth of faults presents an opportunity to gauge the changes that lead a system into equilibrium and the understanding of landscape interaction between tectonics and river networks evolution.