THE PIRINEUS SYNTAXIS: AN EXAMPLE OF THE INTERSECTION OF TWO BRASILIANO FOLD-THRUST BELTS IN CENTRAL BRAZIL AND ITS IMPLICATIONS FOR THE TECTONIC EVOLUTION OF WESTERN GONDWANA

Abstract

The Pirineus Syntaxis (PSX) is a concave to the foreland (toward the craton) curve in the otherwise NS-trending structural grain of the erogenic province bordering the west side of the Sao Francisco craton. Its geometry reflects the intersection between two non-parallel foldthrust belts of different ages and style, formed during two separate non-coaxial deformation episodes during the Brasiliano Orogenic period of Neoproterozoic (probably around 900-600 Ma). These ages are not well constrained, since very little radiometric dating is available. The two limbs of the syntaxis are here interpreted as the younger and narrower Northern Brasilia belt (NBB), and the older and more complete Southern Brasilia belt (SBB). Rocks involved in the deformation that formed the PSX include the Paranoa, Canastra and Araxa Groups, generally accepted as passive margin environment and more restricted to the SBB. The Rio do Peixe Group is a volcanic-sedimentary sequence that prevalently occurs in the SBB, while the Intrusive Rock Suite and Related Metasedimentary Rocks predominantly occur in the NBB. The Granite-gneiss complex of the Goias Massif (GCM) to the west, comprises the metamorphic core of the syntaxis. The collision of the Rio de la Plata (Paranapanema?) craton with the Sao Francisco craton formed the SBB, that preceded the final collision and accretion of the GCM with the SFC that formed the NBB. A weak collision of GCM with the southern border of the Amazonian craton determined the last deformation phase in the syntaxis. This is the first attempt to subdivide the Brasiliano of central Brazil and thus contributed to a better understanding of the evolution of western Gondwana. The conclusions of this paper were primarily based on field structural analysis.