Tectonic evolution of the southern part of the sergipano fold belt, northeastern Brazil

This thesis is a stratigraphic and structural analysis of the Itabaiana Dome Area in the southern part of the Sergipano Fold Belt, northeastern Brasil. The ESE-WNW trending Sergipano Fold Belt is a subgreenschist to amphibolite grade metavolcanosedimemary wedge polydeformed during the 700-600Ma Brasiliano-Pan-African orogeny. Crystalline basement gneiss domes mantled by metasediments are found within the fold belt which lies between the Sao Francisco Craton in the south and the Pernambuco-Alagoas Massif to the north. An area of 4000km surrounding the Itabaiana and Simao Dias gneiss domes was mapped at 1:50,000 scale. A new stratigraphy has been established for this part of the Sergipano Fold Belt. Two major Middle to Late Proterozoic sedimentary cycles have been recognised, each with a basal siliciclastic megasequence overlain by a carbonate megasequence. Four main stratigraphic groups have been established. The =330m thick Estancia Group consisting of metasandstones, pelites, and metacarbonates unconformably overlies the crystalline rocks of the craton and is laterally equivalent to the =200-1100m thick Miaba Group which consists of the Itabaiana quartzites; the Ribeiropolis phyllites, pebbly phyllites, diamictites and metavolcanics; and the Jacoca metacarbonates. The Miaba Group is unconformably deposited around the gneiss domes. The =700m thick Lagarto Group has been identified as a new lithotectonic element in the southern part of the Sergipano Fold Belt. This group is a coarsening-upward siliciclastic sequence that overlies the older rocks of both the craton and the fold belt. It consists of the LagartoPalmares mud stones, silts tones, sandstones and lithic wackes together with the Jacare metasiltites and the Frei Paulo phyllites, metarhythmites, metagreywackes, minor metacarbonates and metavolcanics. The =200-2000m thick Vaza Barns Group, consisting of the Palestina diamictites and the Olhos D'água metacarbonates, unconformably overlies the older rocks to the north of the Itaporanga fault. The thickest Palestina diamictites occur in a WNW-ESE trending, fault-bounded depocentre which also received relatively deep water accumulations of the other formations. The siliciclastics generally thin towards the basement gneiss domes, whereas the carbonates thin away from the domes into the diamictite trough. The thickness and facies distributions of these units indicate tectonically controlled sedimentation, with basement highs supplying sediments from both the southern and northern margins of an asymmetric basin. The structure of the Itabaiana Dome Area is dominated by shallowly plunging, SSW vergent Fj and F2 recumbent to inclined folds and steep D2 thrust faults. D1 is characterised by WNW-ESE trending, SW vergent, nappe-like folds associated with a penetrative layer-parallel foliation. D2 is characterised by co-axial, up-right, tight and WNW-ESE trending folds, associated with penetrative axial surface foliations, lineations and high-angle, SSW vergent, oblique slip thrust faults. These regional thrust faults are probably inverted extensional faults. Late-stage, orogen-parallel movement generated transverse F3 kink-style folding and rotation of fault bounded domains. Regional lithostructural correlations between the southern and northern parts of the Sergipano Fold Belt indicate that the Sergipano Basin may be interpreted to have