THE ROLE OF FRACTURES IN THE STRUCTURAL INTERPRETATION OF SHEEP MOUNTAIN ANTICLINE, WYOMING

Sheep Mountain anticline is a Laramide asymmetric basement fault-cored fold. This well-known NW-SE trending anticline is located in the Bighorn basin, Wyoming. Field measurements of fracture strikes and dips document the development of fractures in the sedimentary layers. In this contribution, the fracture data are used to constrain the fold kinematics. The field data suggest the existence of a regional fracture set (set I) predating the Laramide compression, and striking N110°E, oblique to the future fold. A set of joints (fold perpendicular, set II), striking N45°E, is present in the hinge, the backlimb and the nose and is associated with the NE oriented Laramide compression. The spatial distribution of this joint set, with its absence in the forelimb, suggests that slip along an early underlying thrust fault created localized stress perturbations that influenced joint development. This notion is supported by the geometry of these joints in the nose of the fold. Joints striking N135°E (fold parallel, set III) are found within the hinge only and are interpreted to have developed in response to bending. In the nose, their geometry varies, with some joint orientations oblique to the fold hinge. Two final fracture sets are attributed to a late stage of fold growth: a new joint set (set IV) in the backlimb striking parallel to the set I fractures, but vertical in orientation; and a set of fractures in the forelimb interpreted as reactivated set I fractures in reverse fault motion. These last two fracture sets highlight the role of pre-existing fractures during folding and their influence on the initiation of new fractures and reactivation of old fractures. The kinematic interpretation of the fractures suggests that there was little lateral propagation of the fold during its growth in amplitude. Apparently the fold acquired its length early, perhaps constrained by the underlying thrust fault. Moreover, there was little hinge migration during fold evolution, an interpretation that is compatible with other studies of basement fault-cored anticlines in the Rocky Mountains. In addition, in some structural locations limb rotation replaced the flexing of limbs as a folding mechanism.