Kinematics of basement-involved compressive structures

Abstract

Low-temperature, basement-involved compressive folds are confined largely to the hanging walls of thrust faults and appear to be produced in response to both propagation and slip on non-planar faults. In this paper we develop a simple, two-dimensional, kinematic theory of basement-involved structures capable of predicting much of their geometric complexity and diversity. The theory is tested by applying it in the construction of retrodeformable cross sections and sequential kinematic models of three structures-Willow Creek anticline, Big Thompson anticline, and a small monocline on Casper Mountain-as well as less rigorous comparison with several other well-constrained structures. Thrust faults commonly propagate through the brittle upper crust along non-planar paths due to interaction with inhomogeneities such as preexisting faults or other zones of weakness, rock anisotropy, and/or effects of spacial or temporal stress field variability. The folding associated with displacement on such a complex system of basement faults reduces conceptually to the behavior of a system of fault-fault-fold triple junctions. A second key aspect of basement-involved structures is the response of the stratified cover sequence in the evolving basement structure. The main monocline in the stratified cover in many cases forms as a drapefold over a triple junction in the basement. The model proposed here presents a complex kinematic history for drape fold development. Layer-parallel shortening is predicted for the cover sequence of many triple junctions during early stages of deformation, whereas at later stages the cover experiences layer-parallel extension, especially in the steep limb and beneath the propagating fault. The cover strata in the steep limbs of monoclines we studied exhibit different stages in such a kinematic sequence: (1) layer-parallel shortening with smaller-scale compressional folds (Rattlesnake Mountain anticline, Bighorn Mountains, and Casper Mountain); (2) wedging (Willow Creek anticline); and (3) layer-parallel extension (Big Thompson anticline, Rattlesnake Mountain anticline, and Banner Mountain).