Ensaio sobre o relevo tectônico do Brasil

Resumo

            In the present paper the author, Prof. Rui Osório de Freitas, studies the tectonic relief of Brasil. This tectonic relief resulted from the epeirogenetic deformations which took place from the Devonian period on.

            The results of these tectonic movements is reflected on the topography in three ways: 1) Warping; 2) Basins and Plateaus and rift valleys. There are two main directions of folding on the austro-brazilian nucleus, from ENE-WSW and NNW-SSE, the latter being considered is principal. As to the "sul-riograndense" and "boreo-brasilia" nuclei the directions observed are mainly NNE-SSW and WNW-ESE.

            The warping constitutes a deformation which resulted from the dislocation of the brasilian fore land against the arogenetic belt of the Andes and result also from pure isostasy phenomena. The tectonic plateaus are positive areas subject to continuous erosion; they are composed of ancient rocks of the crystalline shield and feature polyciclic reliefs.

            The basins are depressions of tectonic origin, located either on the continent or an the ocean bottom, filled up with sediments which present a predominantly continental faces, without orogenic deformations.

            In Brasil, one may distinguish the Atlantic, Goiano, Matogrossense and Borborema plateaus.

            The important basins are those of the Paraguai, Paraná, Parnaíba and Amazonas rivers, as well as the cretaceous basin. There are some local basins which are grabens filed by continental sediments, and these are: São Paulo. Curitiba, Gandarela and Fonseca basins.

            The rift valleys - in which the linnear structure, the horst and grabens have a conspicuous topographic expression keep a certain uniformity in their width and have a large length. The bottom of these valleys is generally occupied by continental Cenozoic sediments. The brasilian rift valleys are a bit more modest than their African correspondents, They follow the directions. ENE-WSW to NE-SW and NNE-SSW. They are: 1) The Paraíba valley, 2) Campo Grande - Guanabara Rio Bonito, 3) Campos-Abrolhos-Recôncavo, and 4) the São Francisco valley.

            The grabens are formed by linnear depressing, they are not very wide (6 km medium width), they have a varying length which attains 90 km, they are located in rupture zones of the brasilian shield and are of Cenozoic age. The most important are: 1) Camaquã, 2) Lagoa dos Patos, 3) Cananéia, 4) Itatins, 5) Juqueriqueré, 6) São Sebastião, 7) Itaboraí, 8) Campos and 9) Marajó.

            The principal horsts are those found on the 1) Serra do Mar (and its various local denominations), 2) Serra de Itatins, 3) Serra de Itapitangui, 4) Serra do Dom, 5) Serra dos órgãos, 6) Serra da Madureira, 7) Maciço da Ilha Grande, 8) Serra da Boa Esperança, 9) Maciço de Cabo Frio, 10) Serra da Pedra Branca, 11) Mac1ço da Ilha do Cardoso, 12) Serra da Mantiqueira, 13) Serra da Borborema, 14) Maciço do Itatiaia, 15) Maciço de São Sebastião, 16) Maciço de Poços de Caldas, 17) Serra do Espinhaço, 18) Maciço do Caparaó, 19) Serra da Carioca.

            The brasilian rift valleys, as the African ones, show eruptive activity of either alkaline or basic nature, but this fact does not constitute a necessarily associated phenomenon.

            The sedimentation which took place in Brasil between the Devonian and Cretaceous periods was chiefly notable on the tectonic basins; between the Eocene and Recent periods, with the epeirogenic elevation of the country and consequent rupture of the crystalline shield, the sedimentation was limited to the coastal belt, river valleys and grabens, with exception of the amazonian basin which presents one of the largest tertiary deposits in the world.      Nevertheless, this basin has more the character of a graben than of a basin with its typical periclinal aspect.

            The actual configuration of Brazil results exclusively from epeirogenic deformations and this fact explains the intense erosion which takes place on the country and the relative scarcity in mineral deposits, generally formed on the geosyncline belts.