FEM AND DEM MODELING OF BASEMENT-INVOLVED FOLDS AND THRUSTS, WITH APPLICATION TO SHEEP MOUNTAIN-GREYBULL AREA, WYOMING

Basement-involved fold and thrust systems have been widely studied through field and seismic investigations. However, our understanding of the geometries of the faults that drive folding, and the mechanical conditions within the fold, is still limited. We carried out comparative finite element method (FEM) and discrete element method (DEM) simulations to explore the structural evolution of a representative basement-involved structure, the Laramide-age Sheep Mountain fold, located in the eastern Bighorn Basin of Wyoming. Both numerical systems are initially 26.8 km long and 3.5 km high, comprising the Cambrian to Cretaceous sedimentary cover. The cover sediments were divided into 9 mechanical layers with properties constrained by experimental and well log data. The numerical strata were deformed by prescribed basement boundary displacements, representing the deforming Precambrian basement. The results of the two models are geometrically similar, and match previous field-based interpretations of the Sheep Mountain fold to first order, as required. However, the models differ in detail, such as fault dips and secondary structures within the sedimentary section. For example, the FEM simulation shows distributed stratal thinning and thickening, whereas the DEM results exhibit extensional faulting at the crest of the fold, and discrete thrusting at depth. Both models, however, provide important constraints on the kinematics and mechanics of basement-involved folds and thrusts. Uplift along basement thrust faults is consumed by a combination of basal fault propagation and interlayer flexural slip. Mechanical stratigraphy plays an important role in the tendency for faulting vs. flexural slip, and thereby the fold geometry; strong layers favor flexural interlayer slip rather than fault propagation, whereas weak layers tend to favor faulting. Thus, the local stress conditions within and between the stratigraphic layers ultimately govern the final fold geometry.