The Case against a Gravity-Slide Origin for Low-Angle Faults in the Mormon – Beaver Dam Mountains (MBDM) Extensional Corridor, Southeastern Nevada and Southwestern Utah
Three well-exposed low-angle faults of the MBDM area have been explained as rooted normal faults that cross-cut or reactivate older thrusts of the Sevier orogen. These models are geologically consistent at both local and regional scales, and explain all existing data. They include balanced, step-wise retrodeformable cross sections that yield typical foreland thrust-belt geometries and several independent estimates of initial fault dips at paleodepths to ~7 km. In contrast, gravity-slide models hypothesize steep, basin-forming faults at depth, for which evidence is sparse. The required gravity-slide toes, where slides override basin deposits, are hypothesized to be buried in the basins. For the domed Mormon Peak detachment, radial sliding is inferred from a small number of fault striations (~7 of ~30 total) that are not near the average WSW extension direction; these are probably due to natural scatter, as is common in major, shallow fault zones. In fact, non-rooted sliding to the north and south is precluded by lateral bedrock continuity of the detachment upper plate with adjacent ranges in those directions. A thin, very locally developed conglomerate along the detachment is taken as evidence that the slide overrode the land surface. However, this conglomerate is not far from carbonate spar-filled vugs up to ~1 m high along the detachment; both were probably deposited in caves that formed in the brecciated Paleozoic carbonates along the detachment before erosion dissected the upper plate into klippen. The only reconstructed cross section advocating steep normal faulting fails to adequately restore thrust-belt structures and Paleozoic isopachs.