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1 Department of Earth Resources, Colorado State University, Fort Collins, Colorado 80523, USA
The nature of Laramide deformation in the southern Rocky Mountains remains highly debated. Advocates of north-south strike-slip faulting favor transpression and northward displacement of the Colorado Plateau, whereas advocates of east-west shortening by thrust faulting contest evidence for large right-lateral displacements.
Minor faults (n = 2552) were measured to determine the Laramide to Holocene structural evolution of north-central New Mexico. Multimodal slickenline and ideal 
1 orientations, as well as consistent crosscutting relationships, indicate multiphase, multidirectional faulting. The oldest set of thrust and strike-slip faults indicates east-northeast– to east-trending shortening and compression. The faults cut rocks as young as the upper Paleocene–lower Eocene Diamond Tail Formation. Strike-slip faults from a second phase of northeast- to north-northeast–trending shortening and compression cut the Eocene Galisteo Formation but do not cut the 27 Ma Galisteo dike. Elsewhere, later north-striking strike-slip faults cut 24 Ma igneous units, indicating north-northeast– to north-trending shortening and compression of mid-Tertiary age. Subsequent north-striking normal faults related to Rio Grande rifting commonly reactivate the mid-Tertiary strike-slip fault planes.
These results show the validity of both strike-slip and thrust hypotheses in north-central New Mexico, although neither is adequate in exclusion of the other. Early Laramide east-west thrusting probably formed the north-trending Laramide arches of the region. Later counterclockwise rotation of regional shortening and compression directions may have caused transpression, opening some late Laramide axial basins and causing limited northward displacement of the Colorado Plateau. Subsequent mid-Tertiary strike-slip deformation may be a missing link between Paleogene Laramide shortening and Neogene Rio Grande rifting.
Key Words: Laramide orogeny • faulting • New Mexico • Rocky Mountains • Rio Grande rift
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