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  1. Valla, Pierre (Ed.)
    Abstract Over the past few decades, tectonic geomorphology has been widely implemented to constrain spatial and temporal patterns of fault slip, especially where existing geologic or geodetic data are poor. We apply this practice along the eastern margin of Bull Mountain, Southwest Montana, where 15 transient channels are eroding into the flat, upstream relict landscape in response to an ongoing period of increased base level fall along the Western North Boulder fault. We aim to improve constraints on the spatial and temporal slip rates across the Western North Boulder fault zone by applying channel morphometrics, cosmogenic erosion rates, bedrock characteristics, and calibrated reproductions of the modern river profiles using a 1-dimensional stream power incision model that undergoes a change in the rate of base level fall. We perform over 104 base level fall simulations to explore a wide range of fault slip dynamics and stream power parameters. Our best fit simulations suggest that the Western North Boulder fault started as individual fault segments along the middle to southern regions of Bull Mountain that nucleated around 6.2 to 2.5 Ma, respectively. This was followed by the nucleation of fault segments in the northern region around 1.5 to 0.4 Ma. We recreate themore »evolution of the Western North Boulder fault to show that through time, these individual segments propagate at the fault tips and link together to span over 40 km, with a maximum slip of 462 m in the central portion of the fault. Fault slip rates range from 0.02 to 0.45 mm/yr along strike and are consistent with estimates for other active faults in the region. We find that the timing of fault initiation coincides well with the migration of the Yellowstone hotspot across the nearby Idaho-Montana border and thus attribute the initiation of extension to the crustal bulge from the migrating hotspot. Overall, we provide the first quantitative constraints on fault initiation and evolution of the Western North Boulder fault, perhaps the farthest north basin in the Northern Basin and Range province that such constraints exist. We show that river profiles are powerful tools for documenting the spatial and temporal patterns of normal fault evolution, especially where other geologic/geodetic methods are limited, proving to be a vital tool for accurate tectonic hazard assessments.« less
  2. Abstract. Landscape morphology reflects drivers such as tectonicsand climate but is also modulated by underlying rock properties. Whilegeomorphologists may attempt to quantify the influence of rock strengththrough direct comparisons of landscape morphology and rock strengthmetrics, recent work has shown that the contact migration resulting from the presence of mixed lithologies may hinder such an approach. Indeed, this work counterintuitively suggests that channel slopes within weaker units can sometimes be higher than channel slopes within stronger units. Here, we expand upon previous work with 1-D stream power numerical models in which we have created a system for quantifying contact migration over time. Although previous studies have developed theories for bedrock rivers incising through layered stratigraphy, we can now scrutinize these theories with contact migration rates measured in our models. Our results show that previously developed theory is generally robust and that contact migration rates reflect the pattern of kinematic wave speed across the profile. Furthermore, we have developed and tested a new approach for estimating kinematic wave speeds. This approach utilizes channel steepness, a known base-level fall rate, and contact dips. Importantly, we demonstrate how this new approach can be combined with previous work to estimate erodibility values. We demonstrate thismore »approach by accurately estimating the erodibility values used in our numerical models. After this demonstration, we use our approach to estimate erodibility values for a stream near Hanksville, UT. Because we show in our numerical models that one can estimate the erodibility of the unit with lower steepness, the erodibilities we estimate for this stream in Utah are likely representative of mudstone and/or siltstone. The methods we have developed can be applied to streams with temporally constant base-level fall, opening new avenues of research within the field of geomorphology.« less