Implications of Jurassic, Cretaceous, and Proterozoic piercing lines for Laramide oblique-slip faulting in New Mexico and rotation of the Colorado Plateau
Regional isopach patterns and pinch-out data for nine stratigraphic units of Jurassic and Cretaceous age are either permissive or supportive of significant dextral slip of Laramide age along the eastern Colorado Plateau boundary in New Mexico. The best-constrained dextral offset estimates are for the Sand Hill–Nacimiento fault system (20–33 km) and the previously published 13 km estimate for the Defiance monocline, which together yield a cumulative offset of 33–46 km. Mesozoic stratigraphic constraints for other Laramide fault systems are less precise, and typically provide only maximum limits for possible dextral offsets because of widely spaced control points and broad areas of Tertiary erosion. These less precise constraints allow as much as 40–60 km of Laramide dextral slip along what is now the Rio Grande rift and as much as 110 km across the entire breadth of the Laramide deformed zone in central and northern New Mexico. Well-documented dextral offsets of Proterozoic lithologies and structures across the Tusas-Picuris fault system (15 km) and Picuris-Pecos fault (37 km) probably represent minimum Laramide displacements because of the need to account for sinistral components related to other deformations. These displacements, when combined with dextral offsets along Sand Hill–Nacimiento and Defiance structures, yield a minimum dextral offset of ∼85 km for Laramide structures in northern New Mexico. This minimum dextral offset is approximately equivalent to the amount of Laramide crustal shortening on and northward of the Colorado Plateau, a result that argues against nearby Euler pole locations for Laramide rotation of the Colorado Plateau relative to cratonic North America. Geological constraints allow ∼0° to 3° of Laramide clockwise rotation of the Colorado Plateau. Additional clockwise plateau rotation during late Cenozoic development of the Rio Grande rift was 1° to 1.5°. Geological constraints thus indicate that clockwise rotation of the Colorado Plateau from combined Laramide and Rio Grande rift deformations was between about 1° and 4.5°.