Fracture Pattern as a Constraint on Fold Growth at Sheep Mountain Anticline, Wyoming, U.S.A.

Bellahsen, Nicolas, Patricia Fiore, David D. Pollard, Stanford University, Stanford, CA

The development of fractures in the sedimentary layers of Sheep Mountain anticline, a Laramide asymmetric fault-cored fold of the Bighorn Basin, is documented using field data. The interpretation of this fracture evolution, in conjunction with 3D elastic models that inves­tigate the influence of the underlying basement fault on fracturing, constrains the structural evolution of the fold. We show that during the initial layer parallel shortening stage relating to early Laramide compression, slip along the (reactivated) basement fault perturbed the surrounding stress field, inhibiting jointing in some areas and enhancing jointing in an ori­entation oblique to the far-field compression in other areas. After this initial stage of com­pression, the fold grew in amplitude without propagating laterally. The pattern of fractures observed in the hinge is consistent with a fixed hinge and rotating limbs kinematic style. The limbs did not deform much during fold growth until the very late stage, when fault propaga­tion became locked. At this point, an antithetic thrust fault cut through and uplifted the area, generating new vertical joints. The results of this study allow us to define a conceptual model of fold growth that may be applicable to other basement thrust fault-related folds. And in turn, it may be useful in the prediction of fracture geometry and fold shape evolution in these environments.