Dextral Shear Along the Eastern Margin of the Colorado Plateau—
a Kinematic Link Between Laramide Contraction and Rio Grande Rifting
(ca. 75 Ma to 13 Ma)

Dr. Timothy F. Wawrzyniec
Bureau of Economic Geology

ABSTRACT
Kinematic data associated with both Laramide-age and style and Rio Grande rift-related structures show that the latest Cretaceous to Neogene interaction between the Colorado Plateau and the North American craton was dominantly coupled with a component of dextral shear. Consistent with earlier studies, minor-fault data collected during this study yielded results of varied kinematics. Inverted to a common northeast-oriented hemisphere, the mean trend of kinematic shortening associated with Laramide age structures is 056°+/-6°. Inverted to a common west-oriented hemisphere, the mean trend of kinematic extension associated with Neogene rifting is 300°+/-34°. The observed dispersion in these directions could be suggestive of multiphase deformation, particularly during rifting, along the margin of the plateau since the latest Cretaceous.

Using a new approach, these data are evaluated using a simple 2-D transcurrent kinematic model; assuming a minimal importance of strain partitioning, a mean trend of convergence between the Colorado Plateau and the North American craton is estimated to be 055°+/-5°. Subsequent Rio Grande rifting, which separated the plateau from the craton, is associated with a mean divergence trend of 307°+/-5.8°. To test our strain-partitioning assumption we analyzed paleomagnetic data obtained from Pennsylvanian to Triassic red-beds exposed in uplifts along the eastern margin of the plateau and from rocks within the rift. The localities examined contain a primary to early-acquired secondary magnetization, and we interpret declination discordancies with respect to appropriate reference directions to indicate clockwise rotations of uplifted blocks. Given the lack of regional strike-slip and dip-slip faults of common trends, the consistent clockwise rotations are interpreted to support an absence of strain partitioning. Correspondingly, for the north-south-trending eastern margin of the plateau, the apparently clockwise-rotated paleomagnetic data are consistent with dextral transpressive shear between the plateau and the craton.

Previous data interpreted to demonstrate counterclockwise rotations of crust within parts of the Española rift basin are, if reliable, consistent with dextral transtensive shear. Overall, it is plausible that the transition from latest Cretaceous/early Cenozoic shortening to Cenozoic extension was characterized by a quasi-continuous change from dextral transpressive to dextral transtensive deformation. This interpretation for the kinematic history of the eastern margin of the plateau demonstrates the importance of a dextral shear coupling between the craton and the Farallon plate system—a conclusion rarely implied by previously proposed models of Cenozoic multi-stress field tectonics during deformation of the Cordilleran foreland.