STRUCTURAL ANALYSIS OF THE CASPER MOUNTAIN FAULT ZONE AND SURROUNDING AREA, WYOMING: IMPLICATIONS FOR LARAMIDE KINEMATICS AND STRUCTURAL INHERITANCE

Casper Mountain is an E–W trending anticlinal structure that is bound on the north by the oblique-slip Casper Mountain fault. The fault is postulated to reflect preexisting Precambrian fabrics that were reactivated during the Laramide orogeny. A structural analysis of the fault zone and surrounding area was conducted to confirm this hypothesis, and to garner insight into both Precambrian origins and Laramide kinematics. Surface and subsurface data for structural analysis was collected and synthesized from numerous published sources along the proposed deformation corridor that roughly coincides with the Oregon Trail Structural Belt of central Wyoming.

The Casper Mountain fault zone is characterized by an E–W rectilinear zone of en échelon, steeply inclined faults. The Casper Mountain fault strikes E–W with smaller faults in the zone striking N65°E. Folds trend to the WNW and are left-stepping. Foliations in Precambrian rocks of Casper Mountain are oriented subparallel to the Casper Mountain fault. The North Granite Mountains fault zone is located due west of Casper Mountain and is similarly oriented E–W with subsidiary faults striking NW/SE and ENE/WSW, off the dominant master fault. Curvilinear, left stepping, en échelon folds trend to the northwest and are truncated on the south by the North Granite Mountains fault. Faults in basement rocks of the Popo Agie Primitive Area of the central Wind River Mountains are characterized by moderate to high-angle faults striking E–W, NNW, and NE.

Fabric data suggest that Laramide deformation along the Casper Mountain fault was guided by Precambrian anisotropies. Surface and subsurface mapping of the fault zone and the deformation corridor to the west indicate that the Casper Mountain and North Granite Mountains faults are part of a basement-rooted system that likely extends westward into the Popo Agie Primitive Area. Here, the steeply inclined (75–90°) master fault is exposed within a WNW-striking corridor of faults that sinistrally offset steeply dipping, NE-striking diabase dikes. The dikes likely intruded major faults that are antithetic to the WNW faults. Other major faults strike to the NNW and have shallower dips of 45–65°. These three directions of anisotropy likely formed from SW–NE-directed subduction along a long-lived, Neoarchean, active continental margin.