DO CORDILLERAN OROGENS EXIST?

Traditional models for the tectonic evolution of western North America (NOAM) involve simple eastward subduction beneath the margin. However, recent high-resolution mantle tomography beneath NOAM appears to confirm our geological models of west-dipping subduction of the margin during the Cretaceous.

Like western NOAM, the Andes have long been considered the product of eastward subduction beneath South America (SOAM). As with older NOAM models, eastward thrusting is attributed to strong coupling between the subducting oceanic and overriding cratonic plates. Here we explore an alternative model that prompts the question whether the current models for Cordilleran orogens are far too simple and in many ways holdovers from the early days of plate tectonics.

The NW Andes resulted from the attempted subduction of the SOAM margin beneath the Great Caribbean arc and attached oceanic plateau at about 75 Ma. To the south the allochthonous Mesoproterozoic Arequipa block sits west of the Cretaceous-Tertiary central Andean fold-thrust belt with its 300 km of east-vergent shortening. Within the eastern Cordillera of Bolivia, a Maastrichtian-Paleocene foredeep developed, whereas to the west, the mid-late Cretaceous Salar de Atacama foredeep formed in front of the east-vergent Cordillera de Domeyko thrust belt. In the south-central Andes, oceanic-arc volcanic rocks in the western Cordillera of Chile, the east-vergent Late Cretaceous Agrio fold-thrust belt, associated foredeep rocks of the Neuquén group, and the ~75Ma attempted subduction of the SOAM western margin to the west beneath an offshore arc represented by the Patagonian batholith, provide additional evidence of a major late Cretaceous collisional event. South of SOAM, the east-vergent Scotia arc came out the Pacific realm to migrate eastward into the Atlantic Ocean above a west-dipping subduction zone.

The available geologic evidence indicates that both the NOAM and SOAM Pacific margins were partly subducted to the west beneath a linear complex of arcs and microcontinents during late Cretaceous-early Tertiary time. This leads us to propose that classic models of simple subduction beneath a continental margin–the classic Cordilleran models–are not actualistic; but instead, reality is closer to models for Alpine-Himalayan belts involving bipolar subduction.