Sea-floor spreading and continental drift seem to imply, for the Pacific segment of South America, upthrust of the continent and compression along its margin. Geologic evidence, however, indicates extension in this area since at least the Miocene. Andean folding has migrated east, away from the Pacific and toward the continent, whereas the tectonic development near the continental margin, which is backland of Andean orogeny, is retrograde and has resulted in a breakdown of the crust mainly along old lineations. There extensional stress dominates the upper crust over an area 300-400 km wide.

The observed extensional deformation is not necessarily incompatible with the forces and stress conditions as postulated for an active continental margin. Compression farther inland away from this margin, and continentward migration of fold belts appear to be related directly to the destructive processes that occur in the continental border area.