FRACTURE PATTERNS ASSOCIATED WITH LARAMIDE ANTICLINES

Outcrop and core fracture data from formations that have been folded to different degrees above Laramide thrust structures in Wyoming suggests that idealized fracture models can be constructed but that local variations are common. Inherited F₀ fracture patterns unrelated to folding are present in some structures. Fold-related extension fractures trend approximately normal (F₁) and parallel (F₂) to the axis of folding. Hinge-normal F₁ extension fractures typically formed in response to horizontal stress prior to uplift and prior to the hinge-parallel F₂ fractures that formed during folding. Areas on anticlines that were not significantly folded, i.e., relatively planar backlimbs, contain only the early, F₁ fractures. This fracture set can strike oblique to the hinge in anticlines where the stress orientation that caused both folding and fracturing was oblique to the inherited basement structure that dictated the hinge trend of the anticline.

Outcrop data from tightly folded structures; such as Beer Mug anticline, with a near-vertical forelimb, and backlimb dips up to 50 degrees, indicate that the entire exposed sedimentary package is cut by numerous intersecting fractures. Fractures of all three sets (F₀, F₁, and F₂) were locally reactivated in both shear and extension as folding intensified. Some fractures also formed originally as shear fractures, F_S. Small, complex thrust faults and shear fractures in the Alcova Limestone on the backlimb and in front of the forelimb record a horizontal compressive stress that exceeded the weight of the overburden. Knowledge of the actual fracture patterns were used to reconstruct tectonic evolution, and knowledge of likely fracture patterns can be used to model fluid flow.