OBSERVATIONS OF TECTONIC DEFORMATION IN SOUTH CENTRAL IDAHO

South-central Idaho is the intersection of the Centennial Tectonic Belt, Idaho Batholith, and Snake River Plane physiographic provinces. The Centennial Tectonic Belt (CTB) is defined by long basin and range-style normal faults, many of which have either been the site of historic earthquakes or show ample evidence for Quaternary fault activity. There have been few documented observations of Quaternary fault activity at the western margin of the CTB with the exception of the Sawtooth fault and Boulder Frontal fault. Here, we present observations of steep, relatively continuous topographic scarps expressed along the trace of the Sun Valley fault and enigmatic lineaments along the Deer Park and Montezuma faults. The Sun Valley fault lies on the east side of Wood River Valley and forms the eastern edge of the Wood River graben. Along the approximate trace, surficial deposits are displaced vertically ~1.0 ± 0.5 m. The age of the surficial deposits is not known, but assuming they are related to the last glacial cycle (~10-30 ka), this displacement may represent a range of slip rates from 0.01-0.29 mm/yr, with a mean of 0.06 mm/yr. To the west, the Montezuma and Deer Park faults are expressed by discontinuous, linear changes in slope and texture visible in lidar. These features may or may not indicate recent faulting but suggest the CTB may be deforming farther west than the Sawtooth, Boulder Frontal, and Sun Valley faults. The western margin of the CTB may represent nascent faulting in a similar style to the longer, Holocene-active faults of the CTB, or possibly slow slip rate features that have been active for a similar amount of time. Additional observations through field mapping, geochronology, and geophysics are required to parse between these hypotheses.