FAULT-RELATED FOLDING IN A VISCOELASTIC MULTILAYER WITH APPLICATION TO PITCHFORK ANTICLINE, WYOMING

We use a boundary element method to develop a model of fault-related folding in a viscoelastic multilayer. The purpose of this study is to understand the fold form of the Pitchfork Anticline located near Cody, Wyoming on the west flank of the Bighorn Basin. We have constructed deep cross sections of the anticline from over 100 borehole records and a seismic profile. The fold formed in strata overlying a deep-seated reverse fault in the pre-Cambrian basement. The fold is characterized by an asymmetric bulge over the fault tip that becomes increasingly more symmetric towards the ground surface. The fold appears to be a product of folding related to movement on the basement fault and buckling of the layered strata. The forelimb and backlimb of the anticline resembles a chevron fold with a nearly uniform wavelength through the section. We show that the fold shape cannot be reproduced by a fault in an isotropic homogeneous elastic half space or in a homogeneous viscous layer overlying displaced rigid blocks. We show that the combination of slip on a buried reverse fault and folding in a multilayer with interlayer frictional resistance in response to uniform shortening can reproduce the major features of the Pitchfork Anticline. Furthermore, folding in a frictionless multilayer does not reproduce the fold form. We therefore argue that the Pitchfork Anticline formed much like a kink-band in a multilayer. It formed as a buckle fold over a reverse fault with the localized frictional slip between layers.