NEW CONSTRAINTS ON MAGNITUDE AND TIMING OF STRIKE-SLIP MOTION ALONG THE EASTERN MARGIN OF THE COLORADO PLATEAU, SIERRA NACIMIENTO, NEW MEXICO

The Colorado Plateau behaved as an essentially rigid plate within the North American continental lithosphere throughout the Laramide orogeny, however, the sense of motion of the Plateau, the magnitude of the displacement, and the timing of the motion have been difficult to constrain. A part of the Plateau that has received a lot of recent attention is the eastern margin in north-central New Mexico. This margin corresponds to the Nacimiento fault system, a structure that records both Laramide dextral strike-slip motion as well as east-west shortening. Although the amount of Laramide shortening is generally thought to be on the order of 10 km or less, estimates of Laramide strike-slip displacement range from ~2-35 km. Our recent discovery of a faulted Proterozoic pluton on both sides of the Nacimiento fault shows that the dextral offset is between 3-15 km, although the timing of motion can only be constrained to post-intrusion. We have continued mapping in this area in an attempt to provide tighter constraints on both magnitude and timing of strike-slip motion. Baltz (1967) interpreted a set of en-echelon folds adjacent to the Nacimiento fault as evidence for Eocene onset of strike-slip faulting. Newly recognized syndepostional deformation features (slumps, faults) in the Cretaceous Mesaverde Group, which are associated with the growth of the en echelon folds, show that strike-slip motion began much earlier than Baltz proposed. At the northern end of the Sierra Nacimiento, the Nacimiento fault makes a sharp 5-km right step. This geometry should have produced a pull-apart basin that would have resulted in abrupt thickening of Mesaverde and post-Mesaverde units. We observe little change in the thickness of these units at the bend. We believe that the amount of Laramide strike-slip motion along the Nacimiento fault system was on the order of several kilometers, not tens of kilometers, and began during the Cretaceous.