Benjamin Bley de Brito Neves

Tectonic evolution of South America during the late proterozoic

Abstract This paper discusses the evolution and the Late Proterozoic assembly of the South American continent (i.e. the West Gondwana) under the light of global tectonics. The northern portion of the continent—the Amazonian craton—is a “Pre-Brasiliano” domain; it may be a fragment derived from Laurentia. This domain behaved as a large continental plate which was accreted to the central and southeastern portions of the continent (“Brasiliano domain”). The Brasiliano domain had a more complex history and composition during the Brasiliano cycle (Middle Proterozoic to Early Paleozoic). In the Brasiliano domain, several Late Proterozoic continental plates have been identified (Sao Luiz-West Africa, Congo-Kasai/Sao Francisco, Rio de La Plata, Arequipa-Antofalla) which acted as forelands to the Brasiliano orogenic belts that surround and amalgamate these plates. Smaller fragments of Pre-Brasiliano continental lithosphere are common in the interior of the Brasiliano orogenic belts and played diversified roles (microplates, microcontinents, “structural highs”). Some of them have acted as true backlands (hinterlands) flanked by arc magmatism. Two principal types of Brasiliano orogenic belts are recognized between the continental plates: (a) marginal orogenies to the Late Proterozoic plates, with pelitic-carbonatic rocks and discrete volcanism, which were thrust by collision onto the margin of the plates, and which seem to represent final products of previous passive margin sequences; (b) distal orogenics (internal belts within the continental masses, sometimes a branching system of orogenics), that contain varied clastic and minor carbonate sedimentary rocks, accompanied by important bimodal volcanism and calc-alkaline plutonism. For these orogenies a greater variety of basinal scenarios and tectonic settings may be visualized. The amalgamation of the continent during the Late Proterozoic, i.e. the formation of the Brasiliano structures, indicates the existence of practically synchronous multilateral stresses within lithospheric plates. This can be produced by complementary adjustment of the continental plates as a response to major global events elsewhere (analogous to modern continental collisions). Two large and important tectonic zones may be candidates to such a scenario: the Hijaz-Mozambique zone (site of connection of East and West Gondwana) and the Transbrasiliano lineament, which cuts across South America and extends to North Africa following the zone of connection between Pre-Brasiliano and Brasiliano domains. Other small ocean-openings and -closings inside the various Brasiliano areas (Dom Feliciano-Gariep, for instance) were minor events and complementary to the other two major tectonic zones mentioned above.

Neoproterozoic oceanic remnants in eastern Brazil: Further evidence and refutation of an exclusively ensialic evolution for the Araçuaí–West Congo orogen

The Aracuai (eastern Brazil) and West Congo (southwestern Africa) belts are counterparts of the same Neoproterozoic orogen located between the Sao Francisco and Congo cratons. The Macaubas Group represents a major passive margin sequence and is a key unit for interpreting the evolution of that orogen. The Salinas Formation is the distal rock assemblage of the Macaubas Group and consists of a deep-sea sand-mud sequence, and a volcanic-sedimentary unit called the Ribeirao da Folha facies. The latter includes metamorphosed volcanic-exhalative sediments associated with ocean-floor basalts (amphibolites). The magmatic protoliths of these amphibolites crystallized at about 816 ± 72 Ma (Sm-Nd whole-rock isochron, ϵ Nd(t) =+3.8 ± 0.2). Regional metamorphism reached the amphibolite facies at about 630 Ma (Rb-Sr whole-rock isochron), when slabs of ultramafic rocks were tectonically emplaced over the Ribeirao da Folha facies. We consider this volcanic-sedimentary facies and the coeval slabs of ultramafic rocks to be remnants of a branch of the Adamastor-Brazilide ocean. The extensive occurrence of syntectonic to late tectonic calc-alkalic granitoids along the internal domain of the Aracuai belt implies that a reasonably large amount of ocean crust was consumed, via an east-dipping subduction zone, during formation of the Aracuai–West Congo orogen.

Evidence for Neoproterozoic Continental Arc Magmatism in the Santa Quitéria Batholith of Ceará State, NW Borborema Province, NE Brazil: Implications for the Assembly of West Gondwana

Abstract Recent field investigations and geochronological studies of Neoproterozoic rocks in the northwestern part of the Borborema Province, Ceara State, NE Brazil provide important clues pertaining to the nature of convergence between the Borborema Province and the West African-Sao Luis craton during the assembly of West Gondwana. U-Pb zircon data indicate that the earliest evidence of convergent magmatism along the northwest margin of the Borborema Province occurred around 777 Ma, and was followed by the development of a large continental arc batholith (Santa Quiteria batholith) between ca. 665 and 591 Ma within the central part of Ceara State. These findings, along with supporting geophysical data, suggest that convergence between the Borborema Province and the West African-Sao Luis craton involved closure of an oceanic realm with subduction polarity to the southeast beneath the northwestern part of the province. Consequently, it seems likely that the Pharusian Ocean was continuous from the Hoggar Province in West Africa into South America during the late Neoproterozoic and additional data suggests that it may have even been connected with the Goianides Ocean of the Brasilia Belt farther to the southwest.

North-western Africa–North-eastern Brazil. Major tectonic links and correlation problems

Abstract The establishment of a geological correlation between northwest Africa and northeast Brazil faces a series of problems of both a virtual and a real nature. Several aspects are summarised in this work that include pre-Mesozoic and Mesozoic features on both continental sides.

Chapter 7.1 The Paranapanema Lithospheric Block: Its Nature and Role in the Accretion of Gondwana ☆

Abstract The SW Gondwanan Parana Basin in Brazil is developed on a gneissic-granitic terrane differing from the other basement rocks. Its geologic and geophysical characteristics indicate the existence of a distinct continental lithosphere segment, the ‘Paranapanema Block’. Deep boreholes show a predominantly granitic composition. It acted as an upper plate during Neoproterozoic subduction processes. Zones of plate interaction mark the external boundaries of this portion of the basin, characterised by continental magmatic arcs and post-collision structures. Gravimetric data corroborate the geological observations. Well-defined gradients delineate its contour. Presence and location of its northern portion were confirmed by an MT survey. Small differences in seismic velocity suggested its cratonic nature. According to geochemistry, Sr and Pb isotopes, two major sources, divide the area into northern and southern segments. Recent studies point to the existence of two lithospheric blocks, one of which is the Paranapanema.

The inventory of geological heritage of the state of São Paulo, Brazil: Methodological basis, results and perspectives

An inventory of geological sites based on solid and clear criteria is a first step for any geoconservation strategy. This paper describes the method used in the geoheritage inventory of the State of São Paulo, Brazil, and presents its main results. This inventory developed by the geoscientific community aimed to identify geosites with scientific value in the whole state, using a systematic approach. All 142 geosites representative of 11 geological frameworks were characterised and quantitatively evaluated according to their scientific value and risk of degradation, in order to establish priorities for their future management. An online database of the inventory is under construction, which will be available to be easily consulted and updated by the geoscientific community. All data were made available to the State Geological Institute as the backbone for the implementation of a future state geoconservation strategy.

A CONTRIBUTION TO THE SUBDIVISION OF THE PRECAMBRIAN IN SOUTH AMERICA

On the shield areas of the South America continent (about 5,000,000 km large) there are about 17,000 geochronological determinations available. More than 75% of this amount has been carried out in the Centro de Pesquisas Geocronológicas da Universidade de São Paulo, CPGeo-USP, in the last 25 years. The majority of these data were accomplished through Rb-Sr (ca. 60%) and K-Ar (ca. 40%) methods The authors tried to work this amount of data on a coherent way according to the progress observed in the geologic and geotectonic knowledge during the last two decades. One of the fundamental goals was to outline an up-to-date subdivision for the Precambrian of this continent, having in mind a contribution to the International Commission on Stratigraphy (ICS), Subcommission of Precambrian Stratigraphy (SPS) of the IUGS. A threefold subdivision for the Proterozoic is adopted and discussed. Such a subdivision differs, in terms of geochronometric boundaries between eras, from that proposed by SPS and published by ICS during the last International Geological Congress in Washington. The geological reasons for these differences are extensively discussed in this article.