TERRENOS PROTEROZÓICOS NA PROVÍNCIA BORBOREMA E A MARGEM NORTE DO GRÁTON SÃO FRANCISCO

Abstract

The northern inargm of the São Francisco craton is defined by major Brasiliano south verging thrust tod nappe structures. In the inner part of the Sergipano belt, recognition of allocthonous blocks and arc-type petronic assemblages allow us to envisage possible Brasiliano cycle suture zones. Indie Riacho do Ponud belt, reappraisal of field relations and new geochnological data confirm a Neoproterozoic age for the nappe structures, with the most external ones overlying the autocthonous Mesoproterozoic; cover rocks of the craton. In the northern part of tins belt, early Brasiliano accretion of a Mesoproterozoicterrane is postulated to have occurred along a transcunent suture. Further north, well inside the Boiborema Province, the Brasiliano Cycle is characterized by transcurreat/transpresstve deformation associated with crast-and mantle-derived phttonism, besides an ensialic, monocyclic supracrustal belt at Orós. In the Seridó belt (and in few other places) these structures rework a flat-lying Transamazonian fabric related to a southward thrusting regime. On the other hand, Mesoproterozoic units (and pre-Brasiliano thrusts ?) are present south of the Patos shear zone. Evidence so far available allows the recognition of major blocks with contrasting tectonic evolution, possibly bounded by suture zone. The prominence of Brasiliano strike-slip kinematics leads us to speculate that transcurrent/transform terrane accretion was important during that event Alternatively, me presence of older thrusting regimes may record different periods of amalgamation of continental blocks, building up a heterogeneous litbosphere in die Borborema Province during Proterozoic times.