THE INFLUENCE OF CRUSTAL STRUCTURE AND WEAKNESSES ON THE SPATIAL PATTERN, STRUCTURAL STYLE, AND DISTRIBUTION OF LARAMIDE UPLIFTS

The accommodation of orogenic shortening in foreland retro-arc systems is often modeled as a wedge of deforming sedimentary cover that propagates toward the continental interior by thrust translation, folding, and internal strain over a margin-ward dipping detachment. However, some foreland systems propagate far into the continental interior, do not include a classic pre-deformation wedge, and accommodate localized deformation associated with pre-existing crustal weaknesses. The North America Laramide is the archetypical example of this type of system and is defined by an anastomosing array of basement-cored arches bound by variably verging crustal-scale reverse faults. The basement segments of these fault zones partly follow pre-existing weaknesses including shear zones, dike sets, plutons, and crustal boundaries that ultimately had an important influence on the orientation and spatial distribution of Laramide arches. Today, these arches are separated by intervening basins that overall lack a consistent topographic slope and thus did not form a tapered wedge during shortening.

Regional patterns of Laramide shortening modeled using minor structures from our work and data from the literature are overall oriented SW-NE parallel to the estimated plate motion between the ancient Farallon and North American plates during the Late Cretaceous. Locally, however, shortening directions refract and are widely variable, trending from overall SSW-NNE for more W trending arches to W-E for more N to NE trending arches. Complexities occur where there are multiple fold trends, and along the steeper forelimbs of more W- and N-trending arches where wrenching can occur. Overall, shortening directions are oriented oblique to subperpendicular to the variably trending arches with only limited secondary vertical axis rotation. This relationship is broadly consistent with a model of plate-scale SW-NE shortening with local refraction of shortening directions associated with variably trending regional crustal weaknesses. Though the initiation of Laramide uplifts likely took advantage of these weaknesses, continued uplift may have increased local mean stress resulting in the suppression of additional uplift. As a consequence, other, less favorable weaknesses may have been activated, giving rise to the complex system of variably trending Laramide ranges observed today.