The Assembly of Southern South America in the Late Proterozoic and Paleozoic: Some Paleomagnetic Clues

The accretion of southern South America mainly occurred in latest Proterozoic-Early Paleozoic as part of a major plate reorganization that led to the assembly of Gondwana. Several independent crustal blocks took part in the process. Paleomagnetic data, essential to reconstruct it, is still very scarce. Paleomagnetic data from the Rio de la Plata craton or La Plata plate (LP)suggest that LP was already part of Gondwana by late Vendian times (ca. 550Ma). A 595 Ma paleopole coincident with coeval poles from the West Nile block in Northern Africa opens the possibility of an older proto-Western Gondwana. Collision with the Kalahari craton would probably be a younger event, although it is not paleomagnetically constrained, and may have produced the accretion of the allochthonous Rocha terrane on the eastern side of LP. To the West, LP limits with Pampia (PA), a block that apparently collided with LP in the Early Cambrian, but no paleopoles are available to constrain its paleoposition. To the West of PA is the Laurentian terrane of Precordillera, its origin being proved by biogeographic, stratigraphic, isotopic and paleomagnetic data. Time and mechanism of its accretion to SW Gondwana is still under debate, although docking in the Late Ordovician is likely. Its relation to the Chilenia allochthonous block on the west is not constrained paleomagnetically. Consumption of parts of the Southern Iapetus along the southwestern Gondwanan margin in the Early Ordovician is recorded in the short-lived Famatina-Eastern Puna (FP) peri-Gondwanic magmatic arc. Paleomagnetic data suggest a large clockwise rotation of the arc previous to its accretion in the Middle Ordovician. To the W and NW of FP and PA is the Arequipa-Antofalla massif with Proterozoic basement rocks. Its paleomagnetic data consist in five Late Cambrian to Silurian paleopoles from the western Puna in Argentina and Chile, the oldest of which suggests a long displacement previous to its accretion in the Late Ordovician. This is supported by some geological evidence but contradicts other. The paleomagnetic data only apply to the Antofalla block which may be a separate terrane from the older Arequipa massif. The Late Proterozoic-Early Paleozoic evolution of Patagonia is virtually unknown. Paleomagnetic data suggests a common evolution with Gondwana since the Devonian.