PALEOMAGNETIC CONSTRAINTS ON OBLIQUE DEFORMATION WITHIN FOLDS, COLORADO PLATEAU, WESTERN USA

The Rocky Mountains rose in the Laramide in response to shortening, but the shortening direction remains controversial.  Lying between the Rockies and the plate margin, shortening directions across the Colorado Plateau are important to determining the cause of the Laramide Orogeny.  The curvilinear axial trends and varying dip angles of the monoclines of the Colorado Plateau would suggest that oblique slip has occurred during fold development in the Laramide Orogeny.  Recent structural work suggests that oblique deformation has occurred along some of the monoclines of the Colorado Plateau, which can be used to constrain the shortening direction on the Plateau (Tinsdall and Davis, 1999; Bump and Davis, 2003).  If these folds are localizing oblique deformation, then lateral strain should occur within these folds.  As discrete faults with large slip are not generally observed along monoclines, a more distributed deformation seems likely.  Such distributed deformation might be expected to produce paleomagnetic rotations about a near vertical axis. Reconnaissance paleomagnetic sampling of the Cretaceous Mesaverde Formation in the Nacimiento Uplift, where dextral slip has been documented by several authors (e.g. Cather, 1999), produced insignificant rotations of only -7.0 ± 13.0°.  The East Kaibab and San Rafael monoclines show -1.3 ± 6.0° and 5.5 ± 9.9° of rotation, respectively, from sampling of the Chinle, Kayenta, and Moenave formations.  The absence of convincing rotations on these folds indicates that either (1) rotation is focused in a different part of the fold, or (2) these particular folds lacked oblique slip on their underlying faults, or (3) oblique slip is absorbed by an irrotational mechanism.  Preliminary results from ongoing work on the Defiance, Hogback, and the Grand Hogback monoclines will be presented; these are expected to help resolve between these possibilities.   Sampling of these folds will extend to the fold hinges as well as the forelimb.