The Stillwater Scarp, Central Nevada, USA; Coseismic Gravitational Failure on a 1.200-M-High Range-Front Escarpment

The Stillwater scarp bounds one side of a Quaternary range-crest graben in the northern Stillwater Range, central Nevada. The scarp was reactivated in the 1915 M7.3 Pleasant Valley and 1954 M6.8 Dixie Valley earthquakes, the only such occurrence known in the Central Nevada Seismic Belt. The scarp is a partial reactivation of an older, north-striking, west-dipping Neogene normal fault that crops out at the range crest. This section of the east-facing range-front escarpment had previously failed in a 2.8 km(2) dipslope landslide. Our field mapping extended the known length of the Stillwater scarp from 1.5 to 3.8 km and identified additional landslide elements in the range-front escarpment below. We dug a 3.2-m-deep trench across the Stillwater scarp where it offsets and dams the graben drainage outlet. The trench exposed a series of six stacked sag pond deposits, each overlain by a coarsening-upward package of alluvium, all in normal fault contact with Triassic limestone of the range crest. Based on hanging-wall sedimentology and a series of four nested fissure fills along the fault zone, we infer four prehistoric displacement events between 1954 CE and 35-45 ka, with vertical displacements ranging from 40 to 120 cm (mean 71 cm). We conclude that periodic late Quaternary earthquakes on nearby active faults triggered additional ridge-crest spreading and incipient slope failures on the escarpment, and future occurrences should be expected. Conversely, the rest of the range does not have the same lithology, structural attitudes, and topography favorable for ridge crest spreading, so we do not anticipate spreading hazards there.