Abstract

North-south changes in thickness of the prethrust stratigraphy of the North American Cordillera within the United States controlled kinematic development of the late Mesozoic (Sevier) fold and thrust belt. Variation in stratigraphic thickness determined the initial slope (βo) of the basal decollement beneath the thrust wedge. As predicted by critical-wedge theory, segments of the Sevier thrust belt with initially thick Precambrian and Paleozoic stratigraphic sections are structurally simple and are composed of few thrust sheets; segments with thin prethrust stratigraphy and comparably low values of βo are intensely imbricated. The difference in structural style resulted because wedge segments with higher values of βo required less internal deformation to build and maintain the critical taper required for thrust-wedge advance. Transverse structural zones, across which kinematic style differed, formed at the terminations of thrust-belt segments and controlled the locations of long-lived antecedent drainage systems that transported gravel to the foreland. Therefore, along-strike changes in prethrust stratigraphic thickness resulted in an observed disjunct distribution of conglomerate in synorogenic deposits of the thrust wedge and foreland basin.

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