CONSOLIDATION OF THE AMERICAN CORDILLERAS

The continental margin orogenic systems of the western Americas are en ormous features that formed along the Pacific margins of the North and South American plates during late Mesozoic through Cenozoic time. The re has been considerable debate concerning their origin, and they are often compared with intra-oceanic fringing arc-trench systems more typ ical of the Australasian margins of the Pacific Ocean, in that both in volve the subduction of oceanic lithosphere, often with similar conver gent relative motion vectors. The onset of orogenesis in the two Cordi lleras, as shown in reversal of sedimentary polarity from sources gene rally on the continent to sources along the Pacific margin, seems to d ate from shortly after emplacement of the oldest oceanic crust in that part of the Atlantic Ocean east of each continent - i.e., about 170 M a, or Middle Jurassic, in the case of the Central Atlantic, and about 135 to 100 Ma, or Early to mid-Cretaceous, in the case of the South At lantic. These ages also seem to mark the onset of westward motion of t he two continents over the Pacific Ocean basin and subsequent crustal thickening and uplift, with development of thrust belts, foreland basi ns, and foredeeps. Prior to this prolonged westward drift, both margin s had been convergent for at least several hundred million years, but no massive mountain building had taken place. Instead, the margins wer e tectonically ''neutral,'' with typically submarine fringing are-tren ch systems or shallow marine to continental margin arcs which stood '' outboard'' of shallow marine platformal shelves or basins whose main s edimentary polarity was from the continent. Although accretion of ''su spect'' terranes, high rates of convergence, and age of subducting lit hosphere all may have influenced particularly local tectonic response and/or phases of orogenic activity in the two chains, the absolute mot ion of the two continental margins over the Pacific Ocean basin is con sidered to have been the dominant factor in Cordilleran tectonic evolu tion.