Topographic lineaments and expression of fracture arrays in the Edelman and North Paint Rock Creek lineaments, Bighorn Mountains, Wyoming

The Bighorn Mountains of north central Wyoming are a basement-cored arch formed during the Laramide Orogeny. Working in association with the Bighorn Arch Seismic Experiment, the Keck Bighorns project seeks to understand the structures within the crystalline basement and sedimentary cover rocks during formation of the arch. Little is known of the origin of the brittle structures within the crystalline rocks at the core of the arch, and distinguishing Laramide-related faults and folds from Precambrian structures is not an easy task. Erosion of these structures has produced topographic lineaments throughout the range.

Combined field measurements and GIS-based topographic lineament analysis of outcrop-scale brittle fractures in the crystalline basement and the Edelman Creek and North Paint Rock Creek regional-scale lineaments in the Bighorn Mountains shows that the Edelman and North Paint Rock Creek zones were reactivated as a conjugate strike slip pair of faults under Laramide principal stress. Other topographic lineaments correspond to fracture orientations that may have functioned as subsidiary arrays for these major fault zones. Field data and kinematic and microstructural analyses show that both the Edelman and North Paint Rock Creek zones, and many m- to km-scale faults, are Precambrian structures that were reactivated by Laramide compression. Many Precambrian events that have created these fractures, including the Trans-Hudsonian, Big Sky, and Medicine Bow Orogenies of the Wyoming Province, all of which occurred at about 1.8 Ga.