Consolidation of the American Cordilleras

Abstract

The continental margin orogenic systems of the western Americas are enormous features that formed along the Pacific margins of the North and South American plates during late Mesozoic through Cenozoic time. There has been considerable debate concerning their origin, and they are often compared with intra-oceanic fringing arc-trench systems more typical of the Australasian margins of the Pacific Ocean, in that both involve the subduction of oceanic lithosphere, often with similar convergent relative motion vectors. The onset of orogenesis in the two Cordilleras, as shown in reversal of sedimentary polarity from sources generally on the continent to sources along the Pacific margin, seems to date from shortly after emplacement of the oldest oceanic crust in that part of the Atlantic Ocaen east of each continent — i.e., about 170 Ma, 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 Atlantic. These ages also seem to mark the onset of westward motion of the two continents over the Pacific Ocean basin and subsequent crustal thickening and uplift, with development of thrust belts, foreland basins, and foredeeps. Prior to this prolonged westward drift, both margins had been convergent for at least several hundred million years, but no massive mountain building had taken place. Instead, the margins were tectonically “neutral”, with typically submarine fringing arc-trench systems or shallow marine to continental margin arcs which stood “outboard” of shallow marine platformal shelves or basins whose main sedimentary polarity was from the continent. Although accretion of “suspect” terranes, high rates of convergence, and age of subducting lithosphere all may have influenced particularly local tectonic response and/or phases of orogenic activity in the two chains, the absolute motion of the two continental margins over the Pacific Ocean basin is considered to have been the dominant factor in Cordilleran tectonic evolution.