TECTONIC EVOLUTION OF SOUTH-AMERICA DURING THE LATE PROTEROZOIC

This paper discusses the evolution and the Late Proterozoic assembly of the South American continent (i.e. the West Gondwana) under the light of global tectonics. The northern portion of the continent-the Amazonian craton-is a "Pre-Brasiliano" domain; it may be a fragment derived from Laurentia. This domain behaved as a large continental plate which was accreted to the central and southeastern portions of the continent ("Brasiliano domain"). The Brasiliano domain had a more complex history and composition during the Brasiliano cycle (Middle Proterozoic to Early Paleozoic). In the Brasiliano domain, several Late Proterozoic continental plates have been identified (Sao Luiz-West Africa, Congo-Kasai/Sao Francisco, Rio de La Plata, Arequipa-Antofalla) which acted as forelands to the Brasiliano orogenic belts that surround and amalgamate these plates. Smaller fragments of Pre-Brasiliano continental lithosphere are common in the interior of the Brasiliano orogenic belts and played diversified roles (microplates, microcontinents, "structural highs"). Some of them have acted as true backlands (hinterlands) flanked by arc magmatism. Two principal types of Brasiliano orogenic belts are recognized between the continental plates: (a) marginal orogenies to the Late Proterozoic plates, with pelitic-carbonatic rocks and discrete volcanism, which were thrust by collision onto the margin of the plates, and which seem to represent final products of previous passive margin sequences; (b) distal orogenies (internal belts within the continental masses, sometimes a branching system of orogenies), that contain varied clastic and minor carbonate sedimentary rocks, accompanied by important bimodal volcanism and calc-alkaline plutonism. For these orogenies a greater variety of basinal scenarios and tectonic settings may be visualized. The amalgamation of the continent during the Late Proterozoic, i.e. the formation of the Brasiliano structures, indicates the existence of practically synchronous multilateral stresses within lithospheric plates. This can be produced by complementary adjustment of the continental plates as a response to major global events elsewhere (analogous to modern continental collisions). Two large and important tectonic zones may be candidates to such a scenario: the Hijaz-Mozambique zone (site of connection of East and West Gondwana) and the Transbrasiliano lineament, which cuts across South America and extends to North Africa following the zone of connection between Pre-Brasiliano and Brasiliano domains. Other small ocean-openings and -closings inside the various Brasiliano areas (Dom Feliciano-Gariep, for instance) were minor events and complementary to the other two major tectonic zones mentioned above.