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The continent’s early Paleozoic rocks


Ecuador sunset
Ecuador sunset

The continent’s early Paleozoic rocks depict the breakup of the first supercontinent, an event probably related to the separation of eastern North America from the pre-Andean basement rocks of western South America. As a result of this separation, a series of passive continental margins developed along the western side of the continent from Venezuela and Colombia to central Argentina; essentially, the Precambrian platform amalgamated during the Brazilian cycle. These rifted margins today are represented mainly by clastic rocksfrom the Cambrian Period (i.e., roughly 500 million years old) bearing numerous trilobites and graptolites, as in the Cordillera Oriental of Bolivia. The early Paleozoic rift that produced these margins also initiated the development of several large intracratonic basins within the continent (e.g., the Amazonas, Parnaíba, Paraná, and Chaco basins). Thick deposits of sedimentary rocks have since accumulated in these basins.

The passive margins of the early Paleozoic were partially activated by subduction of oceanic crust (i.e., the forced descent of oceanic crust beneath the leading edge of an overriding continental plate) during late Cambrian to Ordovician times (about 500 to 470 million years ago). When the oceanic crust was totally consumed, subduction ceased and a series of small continental blocks collided against the western side of the continent. Allochthonous (transported) continental blocks thus were emplaced in the Cordillera Oriental of Ecuador, Colombia, and Venezuela at the end of the Silurian Period (about 415 million years ago). Rock ages corresponding to those of the North American Grenville orogenic belt (c. 1.3 to 1.2 billion years old), as well as affinities to North American fauna of the Devonian Period (about 415 to 360 million years ago), suggest that these blocks were once part of North America. Farther south, another series of blocks collided against the continent.

These included the Arequipa block in southern Peru and Bolivia, the Precordillera region of west-central Argentina, and Patagonia in southern Argentina. At the same time, some minor blocks consisting of rocks exhibiting a marine affinity were accreted to the continent in the southern Patagonian archipelago of Chile. In the course of the subduction process that preceded these collisions, a series of north–south-trending belts of plutonic and volcanic rock formed offshore of the continent and parallel to the coast. Because of the later accretions of continental crust to the coastal margin, these belts were shifted more than 250 miles westward and today form prominent outcrops in northern Patagonia, the western Pampean Sierras, the Cordillera Oriental of Bolivia and northern Argentina, and the Cordillera Oriental of Colombia and Venezuela.

The collision of these blocks


The collision of these blocks also produced a series of peripheral foreland basins, which were the result of crustal deformation and the stacking of slices of basement rocks in the orogenic areas. Examples of basins of the early Paleozoic age are the Beni basin in Bolivia and the Alhuampa and Las Breñas basins in northern Argentina. The late Paleozoic Claromecó foreland basin in northern Patagonia is now occupied by a sedimentary accumulation more than five miles thick that was formed at the same time as the Karoo basin in southern Africa, both basins resulting from the collision of the microcontinent of Patagonia against Gondwana. Britannica "South America" © Emmanuel BUCHOT, Encarta, Wikipedia

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