FOLD AND FAULT INTERACTION IN THREE-DIMENSIONS: A SIMULATION OF NORTHEASTERN BIG HORN BASIN, WYOMING

Fault-cored folds, recognized both at the Earth’s surface and within the subsurface, have important implications in studies of both hydrocarbon potential and seismic hazard assessment. Because the geometry of the faults is at times unclear and unresolvable, the shapes of folds associated with slip on these faults can be the only available data for inferring fault geometry. Aspects of fold shape such as amplitude, width, length and tightness can be directly traced to aspects of individual fault geometry including length, width, dip and depth along with fault slip. However, many aspects of fault systems are still not well understood, particularly fault interaction. The magnitude and direction of fault slip can be influenced by surrounding faults so that the slip pattern does not resemble that of an equivalent isolated fault. The associated pattern of folding is similarly influenced.

The Big Horn Basin, WY, has been the location of much geologic study, due to the excellent rock exposure and hydrocarbon potential. One of the largest fault-cored anticlines flanking the basin is Sheep Mountain Anticline. The northern termination of Sheep Mountain Anticline is surrounded by several smaller fault-cored folds, creating an intricate fold pattern; however, the faults associated with these folds are unexposed. Using the Boundary Element Method code, Poly3D, the first-order fault geometry is estimated from correlating model results within observed fold pattern. Sensitivity analyses reveal that some fault parameters greatly contribute to observed fold pattern while other fault parameters are unconstrained from fold pattern alone. Multiple episodes of faulting under different stress fields likely produced the observed folding. The pattern of earliest fracturing in the region can provide clues for the overall sequence of folding and faulting. Additionally, a mechanical efficiency analysis can evaluate the overall behavior of the composite, interacting fault system and may serve to validate the proposed fault model for Sheep Mountain and vicinity.