POST-GREAT UNCONFORMITY EVOLUTION OF BASINS, DOMES, AND FAULTS IN NORTH AMERICA'S CRATONIC PLATFORM: CONTRASTS BETWEEN THE TECTONICS OF CONTINENTAL INTERIORS AND CONTINENTAL MARGINS

The continental interior of the USA encompasses the cratonic platform between the Rocky Mountain front on the west, the Appalachian-Ouachita front on the east and south, and the Canadian Shield on the north. Its crust consists of Precambrian basement (assembled by accretionary and collisional orogenies between 1.9 and 1.0 Ga) separated by the Great Unconformity from a cover of Phanerozoic sedimentary strata. Formation of the Great Unconformity, which involved significant crustal exhumation, marked a turning point in the evolution of the continental interior. In post-Great Unconformity time, the region became a locus of sedimentary accumulation. However, because extreme stretching characteristic of passive-margin basin formation did not happen in the cratonic platform, the continental interior did not undergo later extreme orogenic shortening of the Wilson cycle, so it did not develop fabrics associated with the penetrative deformation or dynamothermal metamorphism that characterizes plate-boundary tectonics along continental margins. Instead, the continental interior displays the consequences of relatively slow epeirogeny resulting in regional-scale domes, basins, and arches. Although these structures produce dips of less than 2°, they have yielded as much as 5 to 8 km of structural relief, as manifested by depth variations of the Great Unconformity. Throughout the continental interior, Phanerozoic reactivation of Proterozoic faults resulted in the formation of numerous transpressional and transtensional midcontinent fault-and-fold zones. Some of these die out up-dip in flower structures and monoclinal folds. Continental-interior tectonics also triggered basin-scale fluid-flow events that resulted in pulses of regional diagenesis. Overall, therefore, the tectonic signature of post-Great Unconformity continental-interior tectonics in North America contrasts with that of marginal orogenic belts, or of the softer lithosphere north of the Himalayan collision in Asia, where relatively rapid plate-boundary displacements produced large strains, and where higher temperature gradients promote plastic deformation. The record of tectonism in the USA continental interior highlights the role of far-field stress transmission (documented by calcite twinning studies) in cratonic lithosphere, triggering both long-wavelength vertical movements and fault reactivation (which accommodates jostling of crustal blocks), and provides key constraints on the evolution of Laurentian lithosphere over time.