NEW INSIGHTS INTO QUATERNARY ACTIVITY ON THE DEADWOOD FAULT FROM LIDAR AND GEOMORPHIC MAPPING, WEST-CENTRAL IDAHO
New lidar data and reconnaissance along the mapped trace of the Deadwood fault provide a new perspective from which to evaluate Quaternary fault activity. The topographic expression of the fault suggests that it may be strike-slip or oblique, in contrast to its previous characterization as a normal fault. From north to south along its ~100 km length, the fault is characterized by incised valleys and two basins. Landmark basin is located within a topographic high, which could represent a restraining bend in the fault, and Deadwood basin is located within a topographic low, which could represent a releasing bend. The relative homogeneity of batholith rock implies that differential erosion is an unlikely explanation for the sustained post-glacial topography of the basins.
Lidar imagery also shows fault-parallel scarps across Quaternary surfaces in Deadwood basin. Towards the north end of the basin, an uphill-facing scarp cuts across Quaternary deposits of multiple ages and beheads drainages. Measured scarp heights range from 0.34 m across what appears to be Holocene alluvium up to 4.2 m across a glacial outwash surface. A downhill-facing scarp on a separate strand of the fault at the southern end of the basin has a scarp height of 5.4 m across a glacial outwash surface, which is higher-elevation and appears to be older than the outwash surface at the northern end of the basin.