THE DORSAL DE CANGUÇU STRIKE-SLIP SHEAR ZONE OF THE UPPER PROTEROZOIC DOM FELICIANO BELT OF SOUTHERN BRAZIL: MAIN CHARACTERISTICS AND TECTONICIMPORTANCE. Crustal-scale strike-slip shear zones of the inner part of Dom Feliciano Belt show evidence of a long history of left-lateral movement. They started as transtensional structures along which porphyritic granitoids with mantle-derived signature and a well-developed magmatic fabric were emplaced, followed by the intrusion of a suite of crustal-derived peraluminous leucogranites. Several stages of movement of these shear zones where recognized and characterized at different scales. The eldest is typified by a welldeveloped magmatic planar and linear fabric with NE-NS trends in syntectonic granites. Transitional magmatic to solid state deformational microstructures include undeformed quartz grains within feldspars showing high temperature deformation. Abundant myrmekites along contacts between feldspar porphyroclasts and matrix are suggestive of stress induced diffusion. Mechanical twining, kink bands and interpenetrating grain boundaríes in plagioclase crystals indicates that oriented growth was active. Other high-T structures include grain boundary migration in quartz and grain-size comminution of K-feldspars through ubiquitous development of myrmekites producing a granoblastic polygonal texture. Quartz occurs as mono or polycrystalline ribbons with large polygonal grains with lobate boundaries while biotite shows grain-size reduction keeping cleavage planes parallel to the main foliation. Lovv-T microstructures are abundant in thick sequences of phyllonites and quartz mylonites and nucleation of new grains was the main grain-size reduction process during this stage. Extensive fracturing of feldspar crystals and their replacement by white mica and biotite suggests abundance of fluids. Quartz shows basal slip during this stage. Mylonites of these lOs of km-thick shear zones are affected by several sets of syngenetic and late folds The former show variable geometry becoming cylindrical towards the high strain zones. These strike-slip shear zones which were formerly interpreted as boundaries between allochthonous terranes and collisional sutures are better interpreted as intracontinental structures responsible for the accomodation of late-orogenic belt-parallel movement.
Malmesbury fold belts and the transverso Oubanguide-Sergipe and Damara belts elongated in a roughly E-W direction. Most of them result from a complex and multistage collision that occured in relation with the closure of a large submeridional ocean named Adamastor ocean in its southern extremity. Accurate delimitation of the different plates and unraveling of their interaction are importam to better understand orogenic processes that resulted in building the brasiliano belts. This is especially well examplified by the Brasiliano Ribeira belt. Its main structural characteristics are induced by the sudden termination of the São Francisco cratonic block along the 20S parallel. A model of continental collision with SW lateral escape, south of the cratonic block, is suggested. It accounts satisfactorily for the proposed kinematic reconstruction.
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