PALEOSEISMIC EVIDENCE OF LATE QUATERNARY MOVEMENT ON THE DEADWOOD FAULT, WEST-CENTRAL IDAHO, USA

The Deadwood fault is a north-south trending zone of complex fault traces in the Salmon River Range in west-central Idaho. The fault was assumed to be active based on offsets observed in granitic rocks of the Idaho Batholith, but conflicting interpretations of scarps and other Quaternary geomorphic features mapped along the fault trace have left open the question of whether the fault is active. Geomorphic mapping, enabled by lidar-based, high-resolution digital terrain models combined with observations of two new paleoseismic trenches suggests that the fault is Quaternary active and provides new insights into fault characteristics and activity rates since the Late Pleistocene. New mapping supports previous observations that most scarps are oriented generally north-south parallel to the fault trace along a distance of >100 km, with scarp heights varying from tens of centimeters to over 5 m. Geomorphic observations and analyses suggest that the Deadwood fault accommodates strike slip and oblique motion. The tallest scarps are situated along the southern portion of the fault zone where two paleoseismic trenches were excavated. Stratigraphy of these trenches features evidence of multiple earthquakes, and a seismic reflection profile along one trench reveals complex faulting of Quaternary sediments from the surface to a depth of 80 m. Luminescence and radiocarbon ages indicate surficial sediments are younger than 200 ka and limit the timing of at least two surface-rupturing earthquakes since 22 ka, with the most recent earthquake occurring after 7 ka. Analyses of age limits on recent fault movements from one trench yield a mean recurrence interval of 11.5 ± 5.3 kyr. These results are consistent with characteristics and inferred recurrence intervals of the nearby Sawtooth and Long Valley faults and, with additional study, will help improve the understanding of seismic hazards in west-central Idaho.