LARAMIDE DEFORMATION OF THE COLORADO PLATEAU AND THE EVOLUTION OF ANCIENT FAULT ARRAYS

The structure of the Colorado Plateau is dominated by 10 major monoclines and a large number of smaller ones. In cross-section, these monoclines consist of a thin section of folded Paleozoic and Mesozoic sedimentary rocks overlying a basement reverse fault that may cut much or all of the crust. Where exposed, these faults are recognized as reactivated Precambrian normal faults. In map view, the monoclines are irregularly distributed with widely varying sizes and orientations. Some, such as the Zuni and Defiance uplifts appear to stand entirely alone while others, such as the San Rafael and Circle Cliffs are relatively close and possibly connected along strike. Examination of structure contour maps reveals two systems, each consisting of a number of individual uplifts that appear to have some geometrical connection. in some cases the tips overlap along strike and in others, a small monocline sits close behind a large, opposite-facing large one, appearing to represent a backthrust off a larger forethrust. On the west side of the Plateau, the Balanced Rock, Paiute Mesa, Beaver Creek, Waterpocket, Cainville, Teasdale, San Rafael and Woodside monoclines form one system and in the central Plateau, the Organ Rock, Capitan, South Comb Ridge, Raplee, Cedar Mesa, and North Comb Ridge monoclines form the second.

Using structure contour maps of the Cretaceous Dakota Sandstone, we constructed strike parallel throw profiles for each of these monoclines. Assuming that the underlying faults do not exhibit wild changes in dip along strike, these throw profiles reflect strike-parallel variations in reverse displacement on the underlying basement faults. Individual profiles exhibit varying degrees of sawtooth character but each tapers from a maximum in the middle to a minimum at each end, much like a typical fault. Aggregate throw profiles for the two systems display a similar shape, grading from near a minimum at each end to a maximum in the middle. By analogy with modern fault systems, we suggest that these monoclines are the surface expression of kinematically linked basement faults. Thus, while ancient weaknesses may have provided the initial locus for Laramide deformation, they are not static features. Indeed, these faults appear to have evolved, growing laterally and forming new linkages during the Laramide orogeny.