Abstract
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 the 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.
Highlights
The evolution of topography within mountainous landscapes is driven by the competition between two major processes: (1) the tectonic uplift of rock [1] and (2) climatically and lithologically controlled erosion [2,3,4]
Steepness increases by an average factor of 2.9 from relict (109 ± 44 m1.08 average) to transient (286 ± 96 m1.08 average) reaches (Table 3), which should roughly reflect the difference between the relict (Uinitial) and new (U) rates of base level fall depending on the slope exponent (n) value [15]
The eastern margin of Bull Mountain in the Northern Basin and Range province contains a steep, dissected transient landscape that is most likely attributed to the recent base level fall of the Western North Boulder fault
Summary
The evolution of topography within mountainous landscapes is driven by the competition between two major processes: (1) the tectonic uplift of rock [1] and (2) climatically and lithologically controlled erosion [2,3,4]. When a tectonic perturbation like normal faulting causes a relative drop in the base level, relief is generated, and bedrock rivers respond by incising into the landscape undergoing relative uplift. The flanking highlands of these intermontane basins, such as Bull Mountain, are remnants of the Cordilleran fold-and-thrust belt [38] which were primarily formed under three tectonomagmatic events that overlap in space, namely, (1) Sevier fold and thrust belt deformation, (2) calc-alkaline magmatism, and (3) Laramide basement uplifts [37,38,39]. The basement-cored Laramide uplifts and Sevier fold-and-thrust belt development are the two primary tectonic components responsible for the development of the Cordilleran thrust belt that began in the late Jurassic and continued its development into the late Paleocene to early Eocene [38]. Acting as the roof of the Boulder Batholith, the Elkhorn Mountain volcanics are primarily composed of ignimbrites, dacites, rhyolites, and volcaniclastic sedimentary rocks, while the Boulder Batholith is primarily composed of intrusive bodies of granodiorite and quartz monzonite [41]
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