Structural Characteristics and Tectonic Evolution of the Pirineus Syntaxis, Central Brazil
The Pirineus Syntaxis is a pronounced concave-to-the-foreland curve in the otherwise north-south trending structural grain of the Brasilia orogenic belt, along the western margin of the Sao Francisco craton in central Brazil. Deformation in the Brasilia belt has been attributed broadly to the Neoproterozoic Brasiliano (=Pan African) orogeny. My 1:50,000-scale mapping of a 12,000 km2 region encompassing the Pirineus Syntaxis suggests that the curve represents the overlap of two distinct fold-thrust belts. The southern limb of syntaxis consists of an east-verging fold-thrust belt in which spoon-shaped thrust sheets, bordered by lateral ramps, are arrayed in an imbricate fan. These thrust sheets consist of continental-margin strata of the Araxa, Canastra and Paranoa Groups. An east to west traverse across this belt passes through Barrovian-type metamorphic facies, from lower greenschist in the east to upper amphibolite in the west. Granulite-grade basement slices, derived from the Goias Central Massif, border the belt at its western edge. The northern limb of the syntaxis consists of a southeast-verging fold-thrust belt. This belt consists of retrograded basement slices interleaved with basement-derived psammitic/pelitic schist. The Goianesia mafic/ultramafic massif borders this belt to the north. Thrust faults parallel to those of the syntaxis's northern limb occur in the southern limb as well, where they cross-cut the east-verging structures of the southern limb. Because the two limbs of the Pirineus Syntaxis have different structural and metamorphic two limbs of the Pirineus Syntaxis have different structural and metamorphic histories, and because structures of the northern limb cross-cut those of the southern limb, I conclude that the two limbs formed during separate, non-coaxial deformation events. This interpretation implies that the Brasilia belt itself formed as a result of at least two non-coaxial episodes. These episodes may reflect sequential collision of microcontinents with the western margin of the Sao Francisco craton during the Neoproterozoic assembly of Gondwana. Subsequent to this history, the region was cut by NW and NE-strike slip faults, perhaps during a late phase of the Brasiliano, and finally by N-S strike Cretaceous dikes.
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