NEW INSIGHTS INTO QUATERNARY ACTIVITY ON THE DEADWOOD FAULT FROM LIDAR AND GEOMORPHIC MAPPING, WEST-CENTRAL IDAHO

The Deadwood fault is north-striking fault within the Idaho batholith. It is recognized as a Class A fault in the U.S. Geological Survey Quaternary Fault and Fold database, but the fault location, style of deformation, and recency of activity are poorly constrained, and some previous workers have questioned whether the fault is truly Quaternary-active. These uncertainties stem largely from a geologic setting that is not conducive to evaluating Quaternary fault activity. The plutonic rocks of the Idaho batholith provide limited stratigraphy from which to measure long-term fault displacement and extensive glaciation has scoured the landscape and left behind complex glacial landforms parallel to the fault trace. The Deadwood fault bounds narrow valleys and basins, which contain limited Quaternary deposits that could preserve evidence of fault activity. Heavy vegetation and steep terrain provide additional challenges to field reconnaissance of the fault.

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.