LATE QUATERNARY ACTIVITY OF THE SOUTH GRANITE MOUNTAINS FAULT SYSTEM IN CENTRAL WYOMING: IMPLICATIONS FOR CRYPTIC REACTIVATION OF LARAMIDE FEATURES

The South Granite Mountains fault (SGMF) is a northwest trending, 135-km-long, Quaternary-active extensional feature in central Wyoming, coincident with the southern extent of the Laramide, basement-cored Sweetwater Arch. Bounding the northern extent of the Seminoe Mountains, Ferris Mountains, Muddy Gap, Green Mountain, and Crooks Mountain, the SGMF is split into five major segments, named accordingly. Though poorly characterized, the SGMF is a significant contributor to the seismic hazard at multiple Bureau of Reclamation (BOR) facilities along the North Platte River. Obscured by preexisting structural features (e.g., anisotropic shear fabric in basement rock and Laramide thrusts), evidence of the down-to-the-north high-angle fault system consists of linear but discontinuous fault scarps, vegetation lineaments, springs, apparent offset drainages, hillside benches, and topographic slope breaks. To better characterize the SGMF for hazard analysis, we excavated three paleoseismic trenches along the two most proximal segments to BOR facilities; two trenches along the Seminoe Mountains segment (Camper trench and The-Real-Snake trench) and one trench along the Ferris Mountains segment (Pete Creek trench). Previous paleoseismic studies have recognized only the Ferris Mountains and Green Mountain segments to have late Quaternary activity. However, our data show possible Quaternary faulting on the Seminoe Mountains segment; youngest faulting coincident with older (early-Cretaceous to Eocene) structures at the Camper trench and spring-accentuated scarp morphology at the The-Real-Snake trench. The Pete Creek trench along the Ferris Mountains segment shows supporting evidence for late Pleistocene and early Holocene ground rupturing earthquakes through the record of multiple scarp-derived colluvial deposits. Late Quaternary activity along the SGMF suggests continued reactivation of inherited structural weaknesses. Therefore, reactivation of Laramide features forming cryptic surficial evidence of active faulting should be considered for seismic hazard analyses.