FOLDING VERSUS THRUSTING IN THE ROCKY MOUNTAIN FOLD-THRUST BELT

Typical features of fold and thrust belts include a wedge-shaped geometry, forward propagation of thrusts, piggyback style, a basal décollement, and decreasing intensity of deformation towards the foreland. These features are well explained in general by the critical wedge theory of thrust development. We compare structures in two cross sections of part of the Rocky Mountain cordillera of western North America that exhibit all these features, and that also are of similar age, developed in rocks of similar lithology, and show similar total amounts of tectonic shortening, but which nonetheless differ markedly in internal structure. The two sections are of the southern Canadian Rocky Mountains and the Mexican Fold and Thrust Belt of Central Mexico.

In the Canadian Rocky section, the dominant tectonic style consists of large imbricate thrust sheets stacked from west to east with relatively little internal deformation of the individual slices. In the Mexican section, the individual thrust sheets exhibit much more internal deformation, as manifest by meter-scale buckle folds, and displacements on thrusts are not as great. One difference between the two sections is the lateral variation of facies. In Mexico this involves massive limestones associated with two carbonate platforms separated by thinly bedded limestones with chert ribbons corresponding to deeper basins in the Central Mexico section. In Mexico, strain is concentrated toward the margins of the basinal sequences where they are in transitional contact with the platform limestones. A single significant facies change occurs at the western edge of the carbonate sequence in the Canadian section, with argillaceous strata continuing to the west. Key factors we suggest as being responsible for the differences between the two sections include the friction along the basal detachment, which influences the angle of taper of the wedge, and the degree of anisotropy of the rocks, which in the case of Mexico promoted internal buckling within individual stratigraphic units.