Early Mesozoic southward migration of Cordilleran Transpressional Terranes

Oblique plate convergence can cause forearc, arc, and back arc regions of active margin orogenic belts to behave as semi-independent terranes relative to bordering continental and oceanic lithospheric plates. These terranes, called here "transpressional terranes," are bound by the subduction zone on one side, by arc-parallel strike-slip faults in the forearc, in the volcanic arc, or in the back arc region on the other and are underlain by throughgoing basal decollements. These terranes undergo shortening in a direction perpendicular to the subduction zone and migrate as coherent structural bodies parallel to the subduction zone along bounding strike-slip faults. Many terranes of the North American Cordillera may have behaved in this manner and perhaps did not travel across the Pacific basin but rather were always peripheral to North America. We propose that during the Triassic to mid-Cretaceous, portions of the Cordillera borderland generally were displaced southward with respect to the North American craton due to left-oblique convergence. Since the mid-Cretaceous, they moved northward due to right-oblique subduction. The proposed displacements are consistent with structural, paleomagnetic, and paleontologic data and with the relative and absolute motions of North America and the Pacific basin plates based on hotspot reference frame models.