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Evidence for Quaternary Activity on the Duchesne-Pleasant Valley Fault, Uinta Basin, Utah

Session: Cryptic Faults: Assessing Seismic Hazard on Slow Slipping, Blind or Distributed Fault Systems I

Type: Oral

Date: 4/21/2021

Presentation Time: 10:15 AM Pacific

Description: 

The Duchesne-Pleasant Valley fault is an east-west oriented fault zone in eastern Utah near the boundary between the Basin and Range and the northern Colorado Plateau; an area where the style and magnitude of active deformation is not well-understood. The fault zone is ~70 km in length and is composed of two primary fault strands separated by an ~4 km-wide right step about midway along the fault zone. Although the fault has strong geomorphic expression in aerial imagery and prominent scarps, previous workers have disputed whether it is Quaternary-active. An alternative hypothesis suggested that differential erosion along faulted bedrock contacts, related to faulting of the Eocene Green River and Uinta Formations, has formed these prominent scarps along the fault.

In a preliminary study, we mapped fault-related lineaments using a publicly available lidar dataset encompassing the Uinta basin and identified multiple previously unmapped fault scarps across Quaternary deposits. Field reconnaissance was limited to an area of distributed faulting within the fault step-over, where scarps with greater vertical displacement are coincident with more abundant Quaternary deposits. Within the step-over, there are low amplitude scarps (up to ~1 m) that cross a mapped gravel pediment surface (estimated ~800-250 ka). Fault scarps varying in height from ~0.5 m-1m also cross surfaces (latest Quaternary?), adjacent to and topographically lower than the pediment surface, that grade to the active drainage. Although bedrock scarps are prevalent along the fault, we observed a nonresistant mudstone unit on the up-thrown side of the fault in various locations, which is inconsistent with the idea that scarps are formed by differential erosion of bedrock. These new lines of evidence suggest that the Duchesne-Pleasant Valley fault is Quaternary-active.

Presenting Author: Julia Howe

Student Presenter: No

Authors