Numerical Modeling of the Formation and Evolution of Basement-Involved Structures in Wyoming

Abstract

The Wyoming foreland is composed of basement-involved structures and intermontane basins formed during the Laramide Orogeny. Based on their sizes, structures in this area can be categorized into primary uplifts and secondary folds. Tectonic models suggest the primary uplifts form by sliding the crustal slabs along a deep-seated, large-scale regional detachment in the lower crust, and rotating the basement wedges along listric primary faults. The secondary folds are located close to and trend sub-parallel to the adjacent primary structures, suggesting a causative or correlative relationship between the two, although this connection has not been firmly established through field and seismic investigations. I carry out numerical simulations using both the finite element method (FEM) and discrete element method (DEM) to explore the structural evolution of these secondary basement-involved structures. The first study investigates the Laramide-age Sheep Mountain anticline, located in the eastern Bighorn Basin of Wyoming, using comparative FEM and DEM simulations. The kinematic and mechanical results of the two simulations are similar, thus verifying the methodological comparison. Differences in the geometric details, however, provide important perspectives on the capabilities of the two methods. The mechanical properties defined through this comparative study are then employed in DEM simulations that investigate the relationships between primary and secondary structures during the displacement of large crustal slabs along primary thrust faults. My results show that the displacements and geometries of the primary faults have great impact on the distributions and throw values of the secondary faults. For shallow primary faults with limited regional shortening, the numbers and the displacements of secondary faults are evenly distributed across the basin, with no preference in dip direction. For steep primary faults with significant regional shortening, conjugate faults form early and subsequently cluster into groups. I also explore the influences of initial sedimentary thickness, sedimentary mechanical stratigraphy, and syn-tectonic sedimentation on the distribution of secondary faults. Thicker Pre-Laramide deposits allow more secondary faults to form early during deformation, absorbing the horizontal shortening within the sedimentary layer. The presence of weak shale layers in the sedimentary section allows numerous small faults to form, and limits the depth of all the faults. Syn-tectonic sedimentation reduces the number of secondary faults that form in the basinal area, and displacements along those faults are very small. In this case, most of the deformation is accommodated by the faults located above the ramp take-off location, at the edge of the syn-tectonic deposits.