Humberto Luis Siqueira Reis

The Araçuaí Belt

The Aracuai belts extends along the curved southeastern margin of the Sao Francisco craton between the Brazilian coast and Lat 21°S, where it merges with the Ribeira belt. It represents the external, basement-involved fold-thrust belt of the Aracuai-West Congo confined orogen (AWCO), which formed due to the closure of the terminal branch of the Adamastor ocean during the amalgamation of West Gondwana in the Ediacaran and beginning of the Cambrian. Bounded to the east and southeast by the high grade and granitic core of the AWCO, the Aracuai belt involves a basement assemblage older than 1.8 Ga, the 1.7–0.9 Ga rift to rift-sag successions of the Espinhaco Supergroup, the Tonian-Edicaran rift-passive margin Macaubas Group, as well as the syn-orogenic Salinas Formation and crustal derived granitic intrusions. The Macaubas Group, the type unit of the belt, contains a glaciomarine sequence made up of thick diamictites, sandstones and Rapitan-type banded iron formations. The units exposed along the belt were metamorphosed under greenschist to amphibolite facies conditions and affected by thrusts, reverse faults and cratonward verging folds, developed between 575 and 530 Ma. The Aracuai orogenic front propagates into the craton interior and interacts with preexistent rift structures. This chapter describes the stratigraphic framework and overall structure of the Aracuai belt, emphasizing the paleogeographic and tectonic significance of its sedimentary and volcanic assemblages.

The São Francisco Basin

The intracratonic Sao Francisco basin covers almost the whole NS-trending lobe of the Sao Francisco craton, encompassing multiple and superimposed basin-cycles younger than 1.8 Ga. Underlain by a relatively thick and cold lithosphere, the basin contains three major Precambrian first-order sequences. The Mesoproterozoic to Early Neoproterozoic Paranoa-Upper Espinhaco sequence consists of a sand-dominated rift-sag succession that grades laterally into the sediments of a rift-passive margin basin developed along the western Sao Francisco plate between 1.3 and 0.9 Ga. The main occurrence of this sequence is associated with the NW-trending Pirapora aulacogen, a prominent graben nucleated in the early stages of Sao Francisco basin evolution (Paleoproterozoic?). The Neoproterozoic Macaubas sequence and its correlatives record extensional events that affected the Sao Francisco-Congo in same time period of the dispersal of Rodinia. The Ediacaran Bambui sequence covers large areas of the basin and marks the onset of a foreland basin-cycle triggered by the successive Brasiliano orogenies that involved the cratonic margins during the West Gondwana assembly. Diamictite-bearing successions of both Macaubas and Bambui sequences record important glacial ages that might have covered low latitudes during the Neoproterozoic. The Precambrian fill units of the basin are involved in foreland fold-thrust belts of opposite vergences, the Brasilia, on west, Rio Preto on the north, and the Aracuai, on the east. The Proterozoic assemblages are unconformably overlain by the Paleozoic Santa Fe Group, as well as the Cretaceous Areado, Mata da Corda and Urucuia groups. The glaciogenic Santa Fe Group is exposed in a few portions of the central and northern Sao Francisco basin and records the passage of the Gondwana through polar latitudes in the Late Carboniferous to Early Permian. The Lower Cretaceous Areado Group contains a package of sand-dominated strata deposited under arid to semi-arid conditions. They are overlain by Upper Cretaceous volcanic and epiclastic successions, generated during a magmatic event that affected large areas of the central and southeastern Brazil. This event is probably coeval with the deposition of the widespread Urucuia desertic successions and marks an important uplift phase of the Alto Paranaiba arch, a 350 km long and 80 km wide structure that separates the Parana and Sao Francisco hydrographic and sedimentary basins. The Cretaceous cover assemblages are contemporaneous to the South Atlantic opening and the consequent separation of the Sao Francisco and Congo cratons.

Tectonic Genealogy of a Miniature Continent

The Sao Francisco craton (SFC), although small in size, exhibits elements of some of the Earth principal evolutionary phases. Repeated cycles of granite-greenstone terrain formation recorded in the craton basement attest the high-heat tectonic regime that characterize the Archean Earth. Like in many other places of the world, these terrains amalgamated to form a coherent and stable continental mass in the Late Archean. In the course of the Paleoproterozoic, subduction-driven accretionary orogens, which incorporated island arcs and continental terranes, were added to the Archean continental nuclei. Typical of the Proterozoic plate tectonic regime, these events took place around the important 2.1 Ga age peak of juvenile crustal production. The collage of several blocks concurred to form the Sao Francisco paleocontinent, which always united to the Congo landmass, experienced a series of intraplate processes, such as rifting and associated bimodal magmatism during the following 1300 Ma-long period. Very likely, the Sao Francisco-Congo did not take part of the Columbia and Rodinia supercontinents. Later, in Ediacaran and Cambrian times, it was involved in the collage that resulted in the formation of the Gondwana Supercontinent. Driven by slab-pull subduction and collisional processes, typical of modern-type plate tectonics, the Rio Preto, Riacho do Pontal, Sergipano, Aracuai-Ribeira and Brasilia belts surrounded and shaped the present configuration of the SFC. Finally, the SFC was separated from the Congo craton following the Brazil–Africa continental drift and the formation of the South Atlantic Ocean. This chapter explores the significance of rock assemblages and tectonic features exhibited by the miniature Sao Francisco continent in terms of Earth global processes.

O cinturão epidérmico de antepaís da Bacia de Irecê, Cráton do São Francisco: principais elementos estruturais e modelagem física analógica

Located in the central portion of Bahia state, Irece Basin displays the best exposures of neoproterozoic sedimentary cover at Northern Sao Francisco Craton. Despite of the large amount of geological studies performed there, some questions remain unsolved, especially concerning the tectonic evolution of the thin-skinned fold-and-thrust belt that involves the rocks of the basin. In order to contribute to the understanding of such evolution, the present study reviews the main structural elements of the basin and surroundings, and present new data acquired through sandbox physical analog modeling. The Thin-skinned Fold-and-thrust Belt of Irece Basin is a great curved feature, confined in the homonymous syncline, whose genesis is related to the development of orogenic belts north of Sao Francisco Craton. Its evolution was conditioned by a N-S tectonic vector, responsible by the nucleation of E-W folds and thrusts. At basin boundaries, the deformation is accommodated by strike-slip faults, which locally rotated early structures. Towards south, the belt gradually loses its expression, only remaining structures related to the Chapada Diamantina thrust-and-fold system. The sandbox analog model successfully simulated the development of the Thin-skinned Fold-and-thrust Belt of Irece Basin, and indicates that its map-view curve results from the interaction with the syncline borders, as well as substrate geometry of the foreland belt. The propagation was made through a low-friction detachment, probably conditioned by the rheological contrast between the Una Group carbonates and the underlying Espinhaco Supergroup siliciclastic rocks.