TECTONIC EVOLUTION OF WESTERN SOUTH AMERICA

The Pacific margin of South America has been the locus of subduction of oceanic crust under the continent during most of the Phanerozoic. Two main stages can be recognized: a first one related to the protomargin of western Gondwana when several allochthons have been accreted during the Paleozoic after Rodinia break-up, and a second stage, related to Andean-type subduction that forms the present Andes. These allochthonous terranes comprise: 1) para-autochthonous accreted blocks, riftted away from Gondwana during late Proterozoic-early Paleozoic times, and colliding from the Ordovician to early Permian times; 2) exotic terranes derived from Laurentia and colliding against Gondwana during Ordovician and Devonian times; and 3) Gondwanian blocks rifted away in the Ordovician that docked in North America during late Paleozoic times. After Pangea break-up the Pacific margin was controlled again by subduction. The northern Andes had the accretion of oceanic plateaux with strong Caribbean affinities, responsible of obduction of oceanic crust, overthrusting, and metamorphism during Cretaceous and middle Miocene times. The central Andes, south of Guayaquil Gulf, evolved from the interaction of different Pacific oceanic plates and the continent, punctuated by different periods of flat-slab subduction. The reconstruction in time and space of the flat-slab segments shows waves of shallowing and steepening subduction systems, which controlled timing of shortening and fold and thrust belt formation, changes and migration of the magmatism, and in some cases, delamination of the lower crust. The southern Andes are characterized by closure of oceanic back-arc basins, ophiolite obduction, and localized metamorphism during Cretaceous and Cenozoic times. The southernmost Andes of Tierra del Fuego are presently deformed by transform faulting related to the Scotia-South America plate boundary.