UnB - Instituto de Geociências - Produção Científica
RESUMOS/ABSTRACTS
WORKING HYPOTHESES FOR THE GENERATION OF THE CURVE OF THE PIRINEUS SYNTAXIS IN THE BRASILIA BELT OF GOIÁS
José Oswaldo de Araújo Filho
In: Congresso Brasileiro de Geologia, 41, 2002, João Pessoa, SBG Nucleo Nordeste. Anais: - .
The Pirineus Syntaxis (PSX), is a pronounced concave to the foreland curve in the otherwise north-south trending structural grain of the Brasília orogenic belt in the State of Goiás. The study of the origin and evolution of the PSX bears implications to the evolution of western Gondwana. Deformation in the Brasília belt has been attributed broadly to the Neoproterozoic Brasiliano orogeny, which is currently being reinterpreted as a possible collage of terranes of Paleo- to Neoproterozoic ages, with peaks of metamorphism ranging from 670 to 630 Ma. The geometry of the syntaxis displays two limbs, the southern limb consisting of an east-verging fold-thrust belt and arrayed in an imbricate fan. These thrust sheets possibly consist of continental margin strata of the Araxá, Canastra and Paranoá Groups. The Rio do Peixe Group of probable Paleoproterozoic age and convergent margin environment (back-arc ?) is tectonically imbricated with rocks of the Araxá Group. An east-west traverse across the southern limb crosses Barrovian-type metamorphic facies, from lower greenschist in the east to upper amphibolite in the west. Granulite-grade basement slices, locally retrograded to the amphibolite facies, derived from the Goiás Central Massif, border the belt at its western margin. The northern limb of the syntaxis consists of a narrow south-southeast-verging fold-thrust belt. This belt consists of retrograded basement slices interleaved with basement-derived psammitic-pelitic schist. The Goianésia-Barro Alto mafic-ultramafic massif borders this belt to the north. Various hypotheses could have led to the formation of the PSX in terms of its geometry, but the most probable one rely on the combination of geometric features and results of field structural analysis. Basically, there are two models: the single belt and the double belt models, with four hypotheses altogether. The single belt model considers the possibility of a collision between a deforming fold-thrust belt where it reaches a point obstacle (the domic structure inherited from a basement high in the surroundings of the city of Brasília). The double belt hypothesis considers the folowing possibilities: i. the intersection between a fold-thrust belt and a regional strike slip fault, the latter being the northern mafic-ultramafic massifs of Goiás; ii. the intersection between two separate non-parallel fold-thrust belts of diffrent ages; iii. the intersection between two belts, one being formed by a series of east-verging arcuate thrusts connected by lateral ramps.
The testing of the hypotheses was done by field structural analysis, resulting in a strutural map of an area of ~12.000km2 at the sacle of 1:50.000. Hypothesis iii was the most plausible one, by the testing of both field and lab data analysis and also by geometric considerations. A coherent model resulted for the orign and evolution of the PSX. Therefore, the PSX is a curved, concave toward the foreland (the São Francisco craton) belt, forming a regional recess within the Brasília belt. The curve results from the intersection of two separate limbs of the PSX, a northern younger limb whose structures cross-cut the southern older limb. The southern limb form a more complete and zoned belt with characteristics of a sheath belt, displaying a series of shallow-scoop-like thrusts or scallops, that terminate at lateral ramps, constituting a belt with numerous shear zones, displaying contrary sense of shear. It displays a broad salient with convexity toward the NE. Transport direction is toward the E. The northern limb, defines a shaper salient with convexity toward the SE, and is a narrower, less complete belt than the southern one. Transport direction is toward SSE in a series of parallel fronts.
Convergence of the Paranapanema, Amazonian/Goiás Massif and São Francisco cratons, concurring to the assemble of western Godwana, were the ultimate cause for the PSX origin.