A Numerical and Field-Data Evaluation of the Critical Taper Model for Orogenic Wedge Stability

Abstract

Understanding the mechanics and formation of orogenic belts in contractional regimes is important due to the presence of oil deposits and the potential for catastrophic seismic or slope stability events originating in these environments. Since their earliest observation in the 19th century, the existence of massive overthrusted sheets in fold and thrust belts had been inadequately explained until the rise of the Critical Taper Wedge (CTW) model. The CTW model presents an analytical solution to the orogenic wedge mechanical stability question using the Coulomb failure criteria for brittle materials, and manages to successfully explain the geometries of fold and thrust belts and accretionary wedges in a number of locations worldwide. In this thesis, the exact "Wedge 3" solution of the CTW model is evaluated against a large number of real wedge taper morphologies, and also against wedge tapers produced by Discrete Element Method (DEM) numerical simulations. We find that the Wedge 3 solution describes the numerical results well, but particularly so for small basal angles \(\beta\)= 0 to 6\(^{\circ}\) , and for lower basal frictions. From field data and DEM simulation comparisons we notice that both natural and simulated wedges exhibit "convexity", namely the wedge toes are steeper than the rest of the wedge, and have different portions which are at the critical point of extensional and compressional failure. Despite the utility of the Wedge 3 solution and the CTW model in general, we also show that the assumption that the entirety of the wedge is at shear failure, upon which the CTW model is built, is not necessarily true, as seen from the state of stress within critical wedges in DEM simulations.