LARAMIDE DEFORMATION ON THE COLORADO PLATEAU ANALYZED THROUGH INTEGRATED STRUCTURAL TECHNIQUES

Laramide-style deformation on the Colorado Plateau is evidenced by ten major basement cored uplifts and their associated monoclines. The Colorado Plateau is underlain by Precambrian crystalline basement marked with a series of shear zones formed during Proterozoic intracratonic rifting. Basement shear zones were subsequently reactivated during the Laramide orogenic event via generally NE-SW compression, forming large, asymmetric, doubly plunging uplifts that are hundreds of kilometers in length with up to two kilometers of structural relief. These variably-oriented uplifts generally root into a blind basement shear zone resulting in limited exposures that provide hard constraints on the location and orientation of basement faults. Because of their diverse strike directions and oblique slip motion, previous studies have not converged on whether reactivation of pre-existing basement fabrics or a temporally or spatially variable shortening direction principally controlled formation of the uplifts.

Employing a variety of different structural modeling approaches can help increase confidence in interpreting Laramide structural evolution on the plateau. The fold geometry in the front-limbs of these structures is well approximated by trishear fault-propagation folding. By applying this method to transects across each of the structures with detailed field observations, we are able to construct a 3D model of the faults that underlie the uplifts. The trajectories of the faults at depth may be further constrained through generalized area-depth relationships (Eichelberger et al., 2017). Furthermore, we have evaluated the compatibility of the uplift geometries with different far-field stress conditions by applying boundary-element dislocation modeling to the previously-constrained fault geometries and comparing the modeled to observed structural relief patterns. Various orientations and relative principal stress magnitudes were tested in order to constrain the best-fitting state of stress to explain the overall uplift patterns on the plateau. By integrating these various modeling approaches, we can leverage the limited structural data available into a cohesive structural framework that is capable of testing the previously unresolved questions around these iconic structures.