as a Constraint on Fold Growth at Sheep
Mountain Anticline, Wyoming,
Bellahsen, Nicolas, Patricia Fiore, David D.
Pollard, Stanford University, Stanford,
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.