Mechanical models of fracture reactivation and slip on bedding surfaces during folding of the asymmetric anticline at Sheep Mountain, Wyoming

We use finite element methods to investigate the reactivation of fractures (opening and shearing) and the development of bedding-surface slip during the deformation of the asymmetric anticline at Sheep Mountain, Wyoming. A series of numerical simulations were run to show the effect of mechanical stratigraphy, layer thickness, slip on bedding surfaces, and tectonic shortening of the fold on the response of a representative three-layer (ductile, brittle, ductile) two-dimensional system. The model uses large deformation frictional contact mechanics to capture the response of existing fractures and slip along bedding surfaces, and considers both elastic and elastoplastic layer properties. The computational results demonstrate the relationships among overall configuration of the multilayer, slip on bedding surfaces, and the sequence and mode of deformation (opening versus shearing) of bed-perpendicular fractures. We show that fractures located in the hinge are mainly reactivated as joints and that those in the forelimb are predominantly reactivated as thrust faults. A flexural-slip mechanism develops during folding when the layers bounded by frictional bedding surfaces have similar stiffnesses. In contrast, when the difference in the layer stiffnesses is significant (softer outer layers) the deformation is accommodated within the softer units without exceeding the frictional strength of the bedding surfaces. A reduction of the middle layer thickness from 100 m to 10 m has a minor quantitative effect on the slip along the bedding surfaces. We compare the numerical results with fracture data collected at Sheep Mountain Anticline, and discuss the similarities and differences between the field observations and the model results.