Data for "Quantifying normal fault evolution from river profile analysis in the Northern Basin and Range province, Southwest Montana, USA"

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 preform 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 southern to middle region of Bull Mountain that nucleated around 3.5 Ma. This was followed by the nucleation of other fault segments in the northern region around 1.5 Ma. We re-create 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 36 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 with the timing of the northward propagation of Basin and Range extension and the migration of the Yellowstone Hotspot across the Idaho-Montana border. Furthermore, our range of calibrated bedrock erodibility values are comparable to those calculated from our cosmogenic erosion rates as well as values reported in other studies. Overall, we show that tectonic geomorphology can be used to improve spatial and temporal estimates of fault slip, especially where other geologic or geodetic constraints are poor, proving to be a vital tool for accurate tectonic hazard assessments. ​