Paleomagnetic dating of fault slip in the Southern Rocky Mountains, USA, and its importance to an integrated Laramide foreland strain field

The Laramide orogen of the U.S. Cordillera formed in the latest Cretaceous, and deformation lasted into the earliest Oligocene. Along and proximal to the eastern and northern margins of the Colorado Plateau, deformation associated with this event mainly took place along reactivated structures. Related tectonic models invoke some role for the plateau either as a stress guide transmitting compression to the foreland or as a freely rotating microplate. Models dominated by northward displacements of the Colorado Plateau also require covariance between timing and magnitude of dextral strike-slip deformation in the eastern domain and thrust deformation in the northern domain. Here we show that fault-zone materials that are exposed in a major, large-magnitude-displacement strike-slip fault zone east of the plateau contain a well-defined magnetization of late Paleozoic age, suggesting that the fault zone has not been strongly modified since the late Paleozoic. Given that these fault-zone materials include indurated metagranitic crush breccias that must have been at or near the surface at the onset of Carboniferous sedimentation, it is likely that the observed large-magnitude displacements are the result of a poorly understood Precambrian tectonic event. Large-magnitude dextral-slip estimates along this and similar structures may be incorrectly assigned to younger tectonic events. In this context, Laramide strain estimates north of the plateau should not be linked with these older displacements and may instead have resulted from a complex combination of Laramide plateau rotation and general east-directed shortening associated with the formation of the Sevier fold-and-thrust belt salient.