PRELIMINARY EVALUATION OF A NEWLY DISCOVERED HOLOCENE SCARP ON THE SAWTOOTH FAULT, CENTRAL IDAHO

The Sawtooth fault, a major normal fault in central Idaho, has experienced at least two post-glacial offset events along a previously unrecognized fault scarp. Analysis of LiDAR imagery and aerial photos clearly indicates the existence of a previously unmapped, throughgoing topographic lineament along the Sawtooth rangefront. While difficult to discern on the ground because of tree cover and the complex glaciated landscape, the fault scarp is obvious on LiDAR-derived images as a prominent lineament cutting across the general topographic grain of the landscape. In two areas with LiDAR coverage, we identified a continuous scarp in several areas along 10 km of range front length, and a discontinuous scarp along an additional ca. 5 km of range front length. Fault scarp profiles surveyed in the field are nearly identical in offset height and scarp shape to those derived from LiDAR data.

In areas lacking LiDAR imagery, the fault scarp is represented on aerial photographs as lineaments of vegetation, watercourses, and topography, which can be mapped at varying levels of confidence for a total length of 50 km along the Sawtooth rangefront. Field surveys at 12 locations demonstrate that the Holocene scarp exists, continuously or discontinuously, along at least 30 km of range front length. However, we have not yet identified a Holocene fault scarp in the field in the southernmost 20 km of the aerial-photo derived fault length.

Late Pleistocene glacial landforms are offset 4-8 m, while uncommon Holocene fluvial surfaces are offset 2.5-3 m. These relationships potentially document two to three fault ruptures since deglaciation ca. 11-14 ka. Severe disturbance of Holocene lake sediments in Redfish Lake, which overlies the fault, suggest that the fault has generated at least two large Holocene earthquakes. These findings indicate an average recurrence interval of ca. 3.7-7 ka, depending on the age of deglacial landforms and the number of postglacial ruptures.

Recognition of Holocene slip on the Sawtooth range-front fault expands the region of known Holocene faulting in central Idaho, and demonstrates the great utility of LiDAR data for identifying and evaluating previously unrecognized Holocene fault scarps.