KINEMATICS OF BASEMENT-INVOLVED COMPRESSIVE STRUCTURES

Low-temperature, basement-involved compressive folds are confined larg ely 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 p aper we develop a simple, two-dimensional, kinematic theory of basemen t-involved structures capable of predicting much of their geometric co mplexity and diversity. The theory is tested by applying it in the con struction of retrodeformable cross sections and sequential kinematic m odels of three structures-Willow Creek anticline, Big Thompson anticli ne, and a small monocline on Casper Mountain-as well as less rigorous comparison with several other well-constrained structures. Thrust faul ts commonly propagate through the brittle upper crust along non-planar paths due to interaction with inhomogeneities such as preexisting fau lts or other zones of weakness, rock anisotropy, and/or effects of spa cial or temporal stress field variability. The folding associated with displacement on such a complex system of basement faults reduces conc eptually to the behavior of a system of fault-fault-fold triple juncti ons. A second key aspect of basement-involved structures is the respon se 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 h ere presents a complex kinematic history for drape fold development. L ayer-parallel shortening is predicted for the cover sequence of many t riple junctions during early stages of deformation, whereas at later s tages the cover experiences layer-parallel extension, especially in th e steep limb and beneath the propagating fault. The cover strata in th e steep limbs of monoclines we studied exhibit different stages in suc h a kinematic sequence: (1) layer-parallel shortening with smaller-sca le compressional folds (Rattlesnake Mountain anticline, Bighorn Mounta ins, and Casper Mountain); (2) wedging (Willow Creek anticline); and ( 3) layer-parallel extension (Big Thompson anticline, Rattlesnake Mount ain anticline, and Banner Mountain).