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Testing Laramide Hypotheses for the Colorado Front Range Arch Using Minor Faults
Controversies regarding vertical and strike-slip Laramide tectonics have served to illuminate the complexities of basement-involved foreland thrust tectonics. The Colorado Front Range is centered on a Laramide basement-cored foreland arch which uplifted the resistant Precambrian rocks that underpin these mountains. The Front Range has been the locus of vigorous debates between advocates of vertical, thrust and strike-slip Laramide tectonics because of its location between northwest-trending arches in Wyoming, which are commonly attributed to thrust deformation, and north-trending arches in New Mexico, which are commonly attributed to right-lateral transpression.
In the center of the Front Range arch, exposures of Precambrian crystalline rocks show little evidence of consistent strike-slip separations on north-striking faults. This indicates that major right-lateral displacements, if present, must be located on arch margins. Along the flanks of the northern Front Range, major fault and fold geometries are asymmetric, with more shortening on the large thrust faults to the west than on higher-angle, out-of-the-basin reverse faults to the east. South of Denver, the asymmetry is less dramatic, with major thrusts dipping toward the arch axis in both directions suggesting formation as a thrust-bounded pop-up. The Front Range arch dies southward into the Cañon City Embayment where slip was transferred to the frontal and Ilse thrust faults of the Wet Mountains.
Laramide models for the Colorado Front Range were tested using the kinematics of 7837 slickensided minor faults. Average slip and compression axes typically indicate subhorizontal, unimodal shortening and show little evidence of regionally consistent, multi-directional Laramide shortening and compression.
These minor faults indicate an overwhelming predominance of horizontal shortening during the Laramide Orogeny. Vertical and normal dip-slip faults are largely restricted to anticlinal hinges, highly rotated strata, and the western arch margin where they can be attributed to post-Laramide extension. The generally high intersection angles of average slickenline and compression orientations with major fault traces indicate dominantly thrust motion on map-scale Laramide faults bounding the Front Range arch. Locally, NW-striking reverse faults can show more vertical than horizontal slip, consistent with their secondary origin as back-limb tightening structures.
Average slip and maximum compression axes have average trends of N77E-S77W, with low-angle plunges indicating that arch-perpendicular shortening was more important than arch-parallel strike-slip faulting during the Laramide. These orientations are not consistent with more N-S shortening and compression directions that might be predicted from models invoking large northward right-lateral transport of the Colorado Plateau. Similar numbers of strike-slip and thrust faults do, however, suggest a component of transpressive deformation. A component of right-lateral transpression is indicated by the obliquity between average minor fault shortening directions and the arch axis as well as by the en echelon geometries of both the northeastern flank of the Front Range and the entire Front Range relative to the Wet Mountains and Sangre de Cristo Mountains. This component of transpression may have allowed the front of Laramide deformation to migrate eastward as it moved northward.
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