Brazilian structural provinces: An introduction

Abstract

The territory of Brazil coincides almost entirely with the South American Platform, the crystalline core of the continent. Its basement is composed of ancient metamorphic and igneous rocks and it has not suffered any tectonic regeneration since the beginning of the Phanerozoic. Sedimentary rocks with almost horizontal bedding cover this crystalline basement. This latter shows ages as old as Early Precambrian, although ages between 500 and 1000 m.y. are conspicuously frequent. The cratonic areas became consolidated more than 1700 m.y. ago, whereas the fold belts formed essentially between 500 and 1700 m.y. ago. The sedimentary cover accumulated from the Early Silurian in three large intracratonic basins, until the platform became completely stabilized. A Late Jurassic to Early Cretaceous reactivation, caused by the break-up of the ancient Gondwana continent, created another basin sequence chiefly along the Atlantic continental margin.

Based on the nature of the crystalline basement rocks and the sedimentary cover, ten structural provinces can be distinguished:

• Rio Branco Province, in the north of the country, occupied by the Guyana Shield and still only poorly known, with an important fold belt developed during the so-called Transamazonian Cycle (2000 ± 200m.y.) with high-grade metamorphic rocks and only slight influence of later events.

• Tapajós Province, in central Brazil, corresponding to the Amazonas or GuaporéCraton, showing chiefly Precambrian crystalline rocks and behaving as an cratonic area during the Phanerozoic.

• Sa˜o Francisco Province, located on the Atlantic Shield, with its basement covered by rocks of different ages, chiefly affected by the Brasiliano Cycle (between 1000 and 500 m.y.) and constituting another cratonic area.

• Tocantins Province, between the Amazonas and Sa˜o Francisco Cratons, with the oldest rocks in its centre (ages over 2600 m.y.), and at the eastern and western borders metamorphic sequences of various fold belts, and almost no Phanerozoic deposits.

• Mantiqueira Province, located along the southern part of the Atlantic coast, affected chiefly by the Brasiliano folding cycle.

• Borborema Province, in the northeast Brazilian fold belt, affected by the Brasiliano Cycle in a very complex way, and with important faulted zones. It was reactivated in Phanerozoic times, when also sedimentary covers accumulated.

• Amazonas Province, represented by the Amazonas sedimentary basin (syneclise), and subdivided into four parts by three important arcs.

• Parnaiba Province, coinciding with the Piauf — Maranha˜o syneclise, and filled with a rather thick sedimentary sequence.

• ParanáProvince, the sedimentary basin of southern Brazil, in which the well-known Late Paleozoic glaciation features are found, and which possesses an extensive cover of basaltic rocks of Late Jurassic to Early Cretaceous age.

• Coastal Province and Continental Margin, the youngest structural unit, developed during the separation of the continent and represented by rift-valleys and coastal basins filled with Mesozoic-Cenozoic deposits of various kinds.