LATE CRETACEOUS-EARLY TERTIARY COLLISION OF THE CORDILLERAN RIBBON CONTINENT AND THE TRANSITION FROM PANTHALASSIC TO PACIFIC SUBDUCTION

Ever since land-based geologists began to understand the implications of the plate tectonic revolution they have invoked the South American Andes as the model for Cordilleran-type margins. In that model a long-lived arc complex formed on the continental lip above an eastward-dipping subduction zone and variable stresses within the margin, generally attributed to changes in slab dip and obliquity, led to arc migration and shut-down, as well as periods of extension, which formed basins, and periods of compression, which created fold-thrust belts. However, modern field studies, along with advances in geochronology, suggest that the type Cordilleran orogen, the Andes, might instead be dominantly collisional.

The available data for western South America suggest that the Late Cretaceous-Early Tertiary deformational event, long called the Andean orogeny, was likely caused by the accretion of a composite arc-bearing ribbon continent. In the north the event resulted from the attempted subduction of the South American margin beneath the Great Caribbean arc and attached oceanic plateau at about 75 Ma. To the south the Casma arc and Coastal batholith of Peru with their Mesoproterozoic crystalline basement collided with South America at about the same time to form the eastward-vergent Marañon-central Andean fold-thrust belt. A foreland basin, extending from the Caribbean Sea at least as far south as Bolivia formed in response to the thrust loading. In Chile and Argentina the accretion of the Ocoite arc in the Late Cretaceous-Early Tertiary led to formation of the Neuquén foreland basin and Chos Malal, Agrio, and Aluminé fold-thrust belts. To the south Late Cretaceous-Early Tertiary accretion of the Patagonian arc above a west-dipping subduction zone led to the development of the Magallenic foredeep on southern South America.

Overall, the deformation in South America was essentially synchronous with that in western North America. Based on polarity of thrusting and development of foreland basins we infer that both Americas were the lower plate in a collision with a more or less continuous ribbon continent. The ribbon continent was amalgamated above a long-lived zone of mantle downwelling located between Panthalassic and Pacific oceanic plates. Eastward subduction beneath the Americas started after collision during the Eocene.