Benjamim Bley de Brito Neves

The origin and evolution of the South American Platform

Abstract The South American Platform is defined as the stable continental portion of the South American plate not affected by the Phanerozoic — Caribbean and Andean — orogenic zones. It is surrounded by these orogenic zones and extends to the marginal Atlantic coast. The basement of the platform consists of Archean and Proterozoic continental crusts arranged during three main sets of orogenic events: (1) Trans-Amazonian (Paleoproterozoic), (2) Late Mesoproterozoic and (3) Brasiliano/Pan African. The latter resulted in the consolidation of the youngest mobile belts of the platform basement. It is, by far, the main phenomenon responsible for the overall pattern of tectonic components (cratonic nuclei and fold belts) and the formation of the general structural framework at the time when the platform was a portion of the Gondwana supercontinent. During the Phanerozoic Eon, different cover stages were developed through six main sedimentary cratonic sequences, of which the last one is exclusive to the South American continent. The final individualization stages and their respective post-Paleozoic sequences were accompanied by a series of specific intracratonic processes, both tectonic (rift basins, overprint of new structural styles in previous basins) and magmatic (basaltic and alkaline). The activation processes have generally been attributed to the opening of the Atlantic Ocean on the east and the Andean orogeneses on the north and west. Nevertheless, a minor part of these events may have been caused by sublithospheric actions (mantle-activated processes) beneath the interior of the platform.

The Brasiliano collage in South America: a review

Analysis of recent geological and geochronological data from the basement of the South American platform indicates that the Brasiliano orogenic collage took place in four distinct pulses: a) Early Cryogenian (ca. 800 - 740Ma); b) Late Cryogenian-Early Ediacaran (ca. 660 - 610 Ma); c) Early-Middle Ediacaran (c. 590 - 560 Ma); and d) Late Cambrian (520 - 500 Ma). The first three pulses are well represented in most Neoproterozoic structural provinces in West Gondwana. The youngest orogenic phase/pulse, however, is only seen in Argentina (Pampean Orogeny) and Brazil, in eastern Rio de Janeiro State (Buzios Orogeny). The period between ca. 750 and 500 Ma is comparable to that reported for the amalgamation of various continental fragments in East (Arabian-Nubian, Mozambique, Kuunga) and North Gondwana (Cadomian). However, important differences in the nature and ages of events are recognized, which can be expected in view of the magnitude of Gondwana agglutination and the diversity of paleogeographic and tectonic scenarios. West Gondwana shows an interesting peculiarity: lithologically and tectonically diversified Tonian terranes underlie Brasiliano orogenic buildups. They were strongly reworked during most of the orogenic pulses. The Tonian terranes (1000 - 900 Ma) and their relation with Rodinia or with the processes of Gondwana fusion remains an open question. Indications of their presence in East Gondwana are still poorly documented.

Main Stages of the Development of the Sedimentary Basins of South America and their Relationship with the Tectonics of Supercontinents

Abstract Twelve main stages of sedimentary basins of the South American Platform have been identified as follows: one Neoarchean, seven Proterozoic and four Phanerozoic. Basin-forming tectonics are related to the accretion/fusion and fission of the major continental landmasses of the Earths history, such as Atlantica, Nena, Rodinia, Gondwana/Pannotia and Pangea. The first proposed stage of basin development precedes the formation of these major landmasses and was developed on Mesoarchean microcontinents. The last stage succeeds the fission of Pangea and is still in progress.

Idades SHRIMP U-Pb do Complexo Sertânia: implicações sobre a evolução tectônica da Zona Transversal, Província Borborema

The Sertânia complex in the Alto Moxoto Terrane, Zona Transversal of the Borborema Province, Northeast Brazil, is a metasedimentary sequence with a subordinate metavolcanic component, metamorphosed under upper amphibolite facies conditions. Several previous regional geological surveys have included this rock assemblage in an undivided gneissic-migmatitic complex, due to the lack of a tectonostratigraphic model for this unit. This paper presents the first geochronological zircon U-Pb ages (SHRIMP U-Pb ages) and the first Sm-Nd isotopic data for this complex. Field and microscopic research reveals the existence of a volcanoclastic fraction of andesitic and dacitic composition, besides the dominant clastic components in these supracrustals. Furthermore, cathodoluminescence images of zircons show well formed little rounded crystals, thus suggesting reduced sediment transport prior to deposition. The calculated ages means with a 95% confidence level, based on a group of zircons of the same generation plot around 2.0 Ga, representing the episode of primary zircon crystallization. Some observed overgrowths testify to an additional Brasiliano overprint (ca. 0.6 Ga). Geochronological data on the supposed meta-plutonic rocks of the same complex point to the same Paleoproterozoic time interval, thus indicating contemporaneity with the sedimentation of the supracrustals. The obtained Nd model ages and µNd(t) parameters also confirm a contribution of Archean and Paleoproterozoic sources to these supracrustals rocks. These data clearly demonstrate contrast between the supracrustal rock assemblages of the Alto Moxoto Terrane and the supracrustals of other terranes of the Zona Transversal, which display Meso to Neoproterozoic ages, and imply that this crustal segment represents an independent lithotectonic domain.

Idades U-Pb em zircão de alguns granitos clássicos da Província Borborema

A evolucao neoproterozoica da Provincia Borborema, no Nordeste Oriental da plataforma Sul Americana, exibe como caracteristica marcante o plutonismo granitico, objeto de diversas teses e outras publicacoes em revistas especializadas. Ja em 1967, F. F. de Almeida e colaboradores identificaram uma serie de tipologias graniticas, Conceicao, Itaporanga etc, com base em feicoes de geologia regional e estudos petrograficos. Estas identificacoes persistiram praticamente em todas as publicacoes posteriores, mesmo quando um numero significativamente maior de granitos foi observado, com o respaldo de estudos geoquimicos, tectonicos e isotopicos. Neste trabalho, estes tipos classicos foram analisados pelo metodo U-Pb em zircao, para sanar uma carencia de ha muito reclamada pela comunidade, posto que para todos eles preexistiam apenas determinacoes isotopicas de menor poder de resolucao. Estes dados novos foram compatibilizados com outros dados existentes na bibliografia da ultima decada e permitiram definir tres etapas principais de magmatismo granitico na Borborema: 650 - 625 Ma; 580 - 570 Ma e 545 - 520 Ma, estes ultimos com intima relacao com os eventos de extrusao tectonica da colagem orogenica Brasiliano-Pan Africana, que antecedeu a consolidacao de uma massa supercontinental (Gondwana Ocidental).

Novos Dados Geológicos e Geofísicos para a Caracterização Geométrica e Estratigráfica da Sub-bacia de Alhandra (sudeste da Paraíba)

Some recent geological and geophysical reconnaissance studies carried out in the Alhandra Sub-basin (southern segment of the Paraiba Basin) have revealed very interesting structural and stratigraphic behaviors that have not yet been described. Four different structural compartments with NNE-SSW trends were identified, each characterized by a particular stratigraphic pile, as a result of vertical tectonic displacements, probably post-Pliocene in age: the Alto Rio Mumbaba High (with widespread basement outcrops), the Rio Mamuaba Graben (the well-exposed Beberibe Formation with thicknesses from 100 m up to 300 m), the Rio Gramame High (a horst zone, with some basement outcrops, underlying the Beberibe Formation) and the Alhandra-Guruji- Conde-Caapora monoclinal zone, east of the BR-101, limited to the west by a fault line. The stratigraphic sequence of the Paraiba basin represented by the Paraiba Group is complete only along the eastern monoclinal zone, where the Barreiras Group is also preserved. Along the Rio Mamuaba graben, occurrences of the Barreiras Group were not found as would be expected, and this seems to be an indication that more than one phase of vertical movement occurred, the second of which had probably removed the Barreiras Group. All these observations are preliminary and demand further geological and geophysical studies, especially at scales greater than 1/50.000

Evolução petrogenética e tectônica do evento Cariris Velhos na região de Afogados da Ingazeira (PE), terreno Alto Pajeú, Província Borborema

O evento Cariris Velhos foi investigado numa area em torno da cidade de Afogados da Ingazeira, Pernambuco, Nordeste do Brasil, situada no terreno Alto Pajeu, um dominio composto toniano-ediacarano da Provincia Borborema. Nesta area as rochas metassupracrustais do Complexo Sao Caetano sao formadas por biotita-muscovita paragnaisses e gnaisses quartzo-feldspaticos, de protolito arcosiano ou vulcânico felsico, com intercalacoes de marmores e quartzitos. Sua historia deformacional comecou com um episodio tangencial D1/D2, cujo metamorfismo variou do facies xisto verde ao anfibolito, sendo marcado pela intrusao de sheets de ortognaisses. O episodio subsequente D3, de carater extensional foi marcado pela injecao de diques de metadioritos, os quais foram fortemente transpostos pelo episodio posterior D4, de natureza transcorrente. A principal estrutura deste episodio transcorrente e a zona de cisalhamento de Afogados da Ingazeira (ZCAI), cuja idade ediacarana e bem calibrada pela intrusao contemporânea do granito de Solidao, datado em 574 ± 74 Ma. Um estudo geoquimico das rochas metassedimentares indica que os protolitos do bloco SE da ZCAI sao grauvacas, enquanto que os do bloco NW representam grauvacas, litoarenitos e arcoseos, mas o padrao de terras raras normalizado para o NASC (North American Shale Composite) nao mostra grandes diferencas composicionais entre elas. Possuem assinatura geoquimica de sedimentacao sinorogenica, transicionando entre sedimentos derivados de arcos de ilhas oceânicos a continentais. Os padroes geoquimicos dos ortognaisses tambem sao indicativos de ambiente de arco magmatico, confirmando conclusoes encontradas por outros autores no estudo das rochas metavulcânicas do terreno Alto Pajeu. A descoberta do evento extensional D3 pre-transcorrencia sugere que a deformacao tangencial D1/D2 e o metamorfismo associado devem estar relacionados a evento orogenico, o que confirmaria a existencia da orogenese Cariris Velhos.

A discordância angular e erosiva entre os grupos Chapada Diamantina e Bambuí (Una) na folha Mirangaba-Bahia

The stratigraphic relationships between the Chapada Diamantina and Bambui (or Una) groups were practically established with the papers of J. C. Branner, a century before. Sometimes that proposed lithostratigraphy was questioned, but we do know now that these arguments were based up incomplete or inadvertent field observations. At the sheet of Mirangaba, central-northern part of Bahia, both the angular and erosional unconformity between the above-mentioned groups are exposed of a conspicuous way, according to their three dimensions, what deserves the special mention here consigned. The quartzites and conglomerates of the Chapada Diamantina group are cropping out as folded and re-fold rock units (general strike ENE-WSW) and they present features of intense superposed erosional processes. Two paleosurfaces were formed and are being exhibited, with complementary features of erosional scarpments, as well as amphitheatres, suppression of some lithostratigraphic units, testimonies-hills (quartzitic islands), etc. The lower pediplan then formed (today with summits >; 500 m) was completely covered by the limestones (Salitre Fm.), where calcilutites are predominating, presenting many algalic edifications. We are not able to affirm that the proterozoic limestone progradation have covered the higher pediplan (today with summits ca. 1,000 m), but there are some testimonies-hills of limestone presenting tops up to 600 m, clearly above the present and predominating surface (Neogene, Velhas Surface), so reiterating that an intense erosional dissecation has occurred (Cenozoic in age) of the limestones. These limestones present monotonous subhorizontal structural attitudes, with the presence of some open folding (ondulations) when they are near of the contacts with the subjacent quartzites. Quite often these limestones were submitted to local ruptural displacements. The polyphasic deformation of the subjacent Chapada Diamantina group is being hypothetically attributed to the Brazilian cycle (as foreland domain of the Riacho do Pontal fold system), but this proposed condition needs to be better constrained, as well as the northern limit of the Sao Francisco Craton (to the north) demands to be reviewed. The basal diamictites of the Bambui Group (Bebedouro Fm.) is only locally cropping out, at the southern and at the northwestern part of the area. The time interval between the deformation of the quartzites and the beginning of the limestones deposition was necessarily very long, some millions of years, and this is an obligatory target for future researches.

Análise Estrutural e de Deformação de Clastos das Unidades Sedimentares da Formação Camarinha - PR

Sedimentary units of Camarinha Formation (Parana State, Southern Brazil) are affected by the Lancinha Transcurrent Shear Zone, and show brittle and brittle-ductile features such as folds, fractures, tilted and truncated layers, cataclastic foliation, fracture cleavage, and stretched clasts. Analysis of these features revealed their compatibility with the Riedel Model and confirms the right-lateral movement of the Lancinha Fault. Low values of the X-axis plunge are typical of transcurrent zones