A mechanical model of keystone structures and reverse faulting for the southern Sangre de Cristo Mountains
— R. J. Bridwell

In the southern Sangre de Cristo Mountains near Las Vegas, New Mexico, tilted basement controlled blocks are overlain by drape-folded sedimentary rocks which are cored by three reverse and imbricate thrust fault systems (Prucha et al., 1965; Baltz, 1972). The analytical stress solution for prediction of high-angle faults (Sanford, 1959) is extended to a finite element model containing an initial fault and subsequent failure history for a keystone structure. A keystone structure has a hanging wall, initially broken by a conjugate normal fault into an uplifted triangular butt and toe, and a footwall. A subsequent secondary conjugate normal fault dips into the antithetic fault in the triangular butt, more antithetic faults occur in the hanging wall, and imbricate thrusts occur at the triangular toe. Numerical stress orientations correlate with experimentally induced drape folds (Friedman et al., 1976). With field, experimental and numerical evidence corroborating an integrated keystone model, the concept of keystone structures is extended to genesis of the Rio Grande rift and Sangre de Cristo Mountains. Three tilted blocks are a sequence of high-angle reverse fault structures. Rift bounding faults may consist of the western antithetic conjugate normal fault and the eastern secondary normal fault. The uplifted triangular block forms the highlands of the Sangre de Cristo Mountains.