Photographic book

The Brasilides


Rio Sao Francisco
Rio Sao Francisco

Rocks of the Brazilian cycle today are manifested in a series of orogenic belts—developed mainly on previously deformed continental crust—that were formed during the amalgamation of the Precambrian cratons into the first supercontinent in late Proterozoic time (1 billion to 540 million years ago). Most of present-day South America, encompassing the platforms of Brazil, Guyana, and southern Venezuela, was accreted at this time—together with Africa—to form the western part of the huge southern supercontinent of Gondwana; Precambrian blocks that were not part of Gondwana—notably the Santa Marta Massif in Colombia, the Arequipa block in Peru, and Patagonia in Argentina—were accreted later during Paleozoic times.

The Brasilides in the southern Brazilian state of Matto Grosso represent the type locality of the Brazilian orogenic cycle. There, important sequences of green schists, platform limestones, and quartzites, as well as red bed molasse formations (associated with granitoids), permit a reconstruction of the collision between the Amazonia craton’s passive (i.e., without active volcanoes) margin and the Alto Paraguay craton’s active margin (now partially covered by the Paraná basin).

The interpreted suture zone between these two cratons corresponds to the Paraguay-Araguaia line, along which mafic and ultramafic rocks are found today. Several other Brazilian belts are known, such as the structurally complex Borborema belt and the Dom Feliciano belt in southern Brazil and Uruguay, which resulted from the collision between the Río de la Plata craton and the Kalahari craton of present-day Africa.

The Dom Feliciano belt represents a complex suture zone where rocks typical of a late Proterozoic arc system were trapped between the two cratons; these rocks were then covered by plateaus of rhyolites during the early Cambrian Period (about 540 million years ago). A striking coincidence exists between this suture, which is known as the Brazilian–Pan-African suture, and the inception of the future rift system that opened the Atlantic Ocean.

The Pampean Sierras in Argentina are a good example of a Brazilian belt formed by accretion of an island-arc system and several small continental plates. "South America" © Emmanuel BUCHOT, Encarta, Wikipedia

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