PALEOMAGNETIC EVIDENCE FOR STRIKE-SLIP ACCOMMODATION WITHIN THE MONOCLINES OF THE COLORADO PLATEAU

The axial trends of basement-cored uplifts in the Colorado Plateau have often been used as proxies for the Laramide shortening direction through the assumption that these folds' trends are orthogonal to the shortening direction. However, difficulty arises because fold trends are sinuous and thus inconsistent with one shortening direction. These nonlinear fold trends have been explained by (1) local stress rotations above blind reverse-slip faults, (2) multiple, non-coaxial episodes of shortening, (3) laterally varying compression directions, or (4) shortening oblique to the fold trend. We determined paleomagnetic vertical-axis rotations in several monoclines to test whether these folds accommodate transpressional deformation and to constrain the Laramide shortening direction. We sampled pre-Laramide formations in the forelimbs of the Nacimiento, East Kaibab, San Rafael, and Grand Hogback monoclines. The site mean declinations from the Nacimiento and East Kaibab monoclines do not yield statistically significant vertical-axis rotations. Sites in the forelimb of the San Rafael Monocline exhibit clockwise rotations of 9-10°. Clockwise rotations of 24° are found in the north-south trending limb of the Grand Hogback Monocline, and 15-23° of counterclockwise rotations in the east-west trending limb. The oblique-slip motion accommodated within these monoclines can be best explained by a northeast shortening direction during the Laramide orogeny. Assuming uniform rotations across the forelimb, these rotations would be consistent with about 5 km of right-lateral strike-slip on the north-south trending limb of the San Rafael Monocline and less than 20 km of right-lateral slip across the north-south limb of the Grand Hogback and less than 14 km of left-lateral slip across the fold's east-west limb. Requiring geometric compatibility of slip along both limbs of the Grand Hogback Monocline limits strike-slip motion to 1-4 km on each limb. The absence of vertical-axis rotations within the forelimbs of the Nacimiento and East Kaibab monoclines indicates that either strike-slip displacement is absorbed entirely in unrotated en echelon folds and minor strike-slip faults cutting the folded strata or that the strike-slip displacement is too minor (less than 1 km displacement) to produce any rotations.