Active deformation of the South Granite Mountain Fault System : reactivated compressional faults vs. extensional overprinting

Abstract

Quaternary faulting in and around Wyoming's Wind River Basin may pose a moderate earthquake risk for dams and other infrastructure within the region. The South Granite Mountains Fault Zone, located adjacent to the Wind River Basin, is one of several Quaternary faults in the region. The South Granite Mountains formed during the Late Cretaceous Laramide orogeny (75-45 Ma). Subsequently, during the Eocene, the extensive downfaulting and downfolding caused the Precambrian Mountain core to collapse forming the Sweetwater Graben. Recent Quaternary deformation is expressed as fault scarps along the north side of the Granite Mountains (2-4 meters). These features were originally studied in the 1980s but have received little attention since. In particular, the nature and style of faulting of this recent deformation was previously undocumented, and prior age estimates were broad. This study applies new methodologies to assess the active tectonics and earthquake potential of the South Granite Mountains Fault System. Low-altitude aerial surveying using drones facilitate the measurement and analysis of fault scarp morphology. From this scarp degradation modeling is applied to faulted surfaces with an age range of [greater than] 201 to 16 ka. Results indicate long term dip slip rates between 0.01-0.05 mm/yr, and Horizontal extension rates between 0.01 - 0.03 mm/yr along the Ferris Mountains front. Shallow seismic reflection profiling is used to image the fault geometry indicating normal faulting at high angles (83 degrees). Offset of the unconformity between the Split Rock Formation and the Quaternary sediments is estimated at 10-13 meters. Using modern rates of faulting suggest quaternary faults initiated at 80-104 ka.