The role of fractures in the structural interpretation of Sheep Mountain Anticline, Wyoming

The development of fractures in the sedimentary layers of Sheep Mountain Anticline, a Laramide asymmetric fault-cored fold of the Bighorn Basin, is documented and interpreted to constrain the kinematic evolution of the fold. The fracture pattern is interpreted to identify five main fracture sets. A fracture set striking 110° (Set I), oblique to the future fold trend, is interpreted as a regional fracture set that predates Laramide compression. A joint set (Set II), striking 045° and present in the hinge and backlimb, is related to the NE-oriented Laramide compression just prior to and during initial anticline growth. Joints striking 135° (Set III), parallel to the fold trend, are found within the hinge and are interpreted to have developed in response to the bending of layers. The two youngest fracture sets are attributed to a late stage of fold growth: a joint set (Set IV) in the backlimb striking parallel to the Set I fractures, but vertical; and a fracture set in the forelimb consisting of Set I fractures reactivated as reverse faults. The relative chronology, mode of formation (opening vs. shearing), and structural locations of these fractures suggest the following kinematics of folding at Sheep Mountain Anticline: little or no lateral fold propagation; no hinge migration; and limb rotation with only minor limb flexure and stretching.