Tectonic geomorphology of the Red Rock fault, insights into segmentation and landscape evolution of a developing range front normal fault

The Red Rock fault, an active normal fault in southwest Montana, is reevaluated for fault segmentation using a multiproxy approach. Field characterization of soils coupled with radiocarbon ages in offset hanging wall fans and terraces constrain the rupture history at a number of locations along strike. These data are coupled with morphometric analysis of dated alluvial fan deposits and adjacent 2nd and 3rd order footwall drainage basins to further constrain fault kinematics and segmentation. Morphometric analysis of the Big Sheep Creek catchment, which drains a large area of the footwall range, provides a control on the spatial extent of fault influence on footwall topography. Three fault segments are recognized by this study, compared with the two segments recognized previously. Data from offset alluvial surfaces were synthesized with hanging wall fan and footwall drainage basin data to demonstrate a southward increase in displacement along the Red Rock fault since at least the latest Pleistocene, at a maximum rate of 1.5 mm/yr. Red Rock ruptures were mainly confined to segments, suggesting a fault behaving as several discrete slip surfaces; however, simultaneous rupture of adjacent segments during one event indicates some fault zone coalescence. A youthful Red Rock fault is supported by the absence of tectonic influences on footwall topography and is consistent with an eastward progression of normal fault development along the northern arm of the Yellowstone hot spot.