Deformation related to the Late Cretaceous-Early Eocene Laramide orogeny in north-central New Mexico resulted in the formation of the Nacimiento uplift, Gallina uplift, and corresponding faults. In the past thirty years the nature of Laramide deformation has remained highly debated with kinematic models including single-stage NE-SW to E-W-directed shortening, multiphase-multidirectional shortening, and NNE-SSW-directed transpressive deformation.
However, in recent years a growing body of evidence indicates that the Laramide orogeny in northern New Mexico was dominated by ~E-W horizontal shortening and compression. One key in resolving local structural models is to understand along strike variations in the magnitude of shortening along the Nacimiento-Gallina uplifts. Based on gravity modeling, Pollock et al. (2004) interpreted the high-angle E-dipping Nacimiento fault to root into a gently dipping master fault and calculated an E-W component of shortening of ~7 km. In this study, I used formation boundary elevations from well data in the adjacent San Juan basin and geologic maps of the Nacimiento and Gallina uplifts to construct a 3D geologic model of formation surfaces across the structures. I then modeled their formation as fault propagation folds in the program Move, to make estimates of the fault geometry and the E-W shortening across the uplifts. Based on forward models, the subsurface geometry of the structures responsible for the uplifts includes a gently (~25°) E-dipping master fault and a high-angle (~70°) E-dipping fault beneath the Gallina uplift, similar to the structures documented along the central Nacimiento uplift. My modeling suggests the initial tip of the high-angle fault is on the master fault, indicating it branches from it. At the northern end of the Nacimiento uplift, the total shortening accommodated by slip along the master fault and Nacimiento fault is ~5.3 km. To the north the total shortening decreases to ~4.3 km where the high-angle Nacimiento fault is blind under the Gallina uplift. This study does not support the prediction of Hamilton (2009) that significant E-W shortening was transferred from the northern end of the Nacimiento uplift to the Tusas uplift to the east via a diffuse transpressional zone.



Nacimiento Uplift, Gallina Uplift, E-W shortening, Forward modeling, Trishear Model