A B S T R A C TThis study focuses on the analysis of high-resolution seismic profiles, integrated with sedimentological, echosounder, SRTM and satellite image datasets, of the Brazilian continental shelf adjacent to the Rio Grande do Norte State, NE Brazil. Located in the northeast of Brazil, the State of Rio Grande do Norte is bounded by two main coastal and shelf systems: the eastern coastal-shelf, from the Sagi River to the Touros High, and the northern coastal-shelf, extending from Touros High to Tibau. This shelf represents a modern, highly dynamic mixed carbonate-siliciclastic system characterized by reduced width and shallow depths as compared with other parts of the Brazilian shelf. It has an average width of 40 km, the shelf-break lying at a depth of ~ 60 m. This shelf is subject to the full strength of the westerly South Equatorial current combined with high winds and moderate to high tides and waves. A sharply defined stratigraphic boundary, probably between the Pleistocene and Holocene deposits, is clearly to be observed in the seismic record. Incised-valleys extending from the main river mouths (e.g.the Potengi, Açu, and Apodi) to the shelf break dominate the area investigated and may indicate periods of lower sea level. R E S U M OEste estudo está direcionado ao conhecimento da plataforma continental brasileira adjacente ao Estado do Rio Grande do Norte, NE do Brasil, através da analise de perfis sismicos de alta resolução integrados a dados sedimentológicos, batimétricos, SRTM e imagens de satélites. O Estado do Rio Grande do Norte, localizado no nordeste do Brasil, apresenta dois sistemas costeiros-plataformais: Setor Este, do Rio Sagi (divisa PB-RN) ao Alto de Touros e Setor Norte, do Alto de Touros a Tibau (divisa RN-CE). Esta plataforma representa um sistema plataformal moderno misto (carbonático-siliciclástico), altamente dinâmico. É caracterizado por sua reduzida largura e águas rasas, quando comparado com outras partes da plataforma brasileira. A plataforma apresenta largura média de 40 km e a sua quebra encontra-se a profundidades médias de 60 m. A plataforma é sujeita a ação da corrente sul equatorial, combinada a ventos fortes e variação de maré moderada a alta e/ou ondas. Um limite estratigráfico fortemente definido, provavelmente entre os depósitos Pleistocênicos e Holocênicos, pode ser bem observado e vales incisos, estendendo-se a partir dos principais rios (como por ex., Potengi, Açu e Apodi) dominam a área investigada.

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ABSTRACT. Areas with gas accumulations and gas seeps, where gas escapes from the river-bottom to the water column, have been mapped in the fill of a fluvial valley, the Potengi estuary. The various gas features have been classified into four types according to their specific seismic signatures: (1) acoustic blanket, (2) acoustic curtains, (3) acoustic columns, and (4) acoustic turbidity. At the same time, two types of gas escapes features have been distinguished: (1) acoustic plumes and (2) pockmarks. The neotectonic activity in the semigraben of the Potengi river may be one of the responsible factors for the migration of gas coming from the seismofacies located below the present river level where it is possible to recognize the connection with the level of the gas source through the acoustic column type seismic signature.It is concluded that sedimentary facies is the main factor determining whether gas accumulates or seeps, and also determines the specific type of accumulation or seep at each location.Keywords: acoustic records, shallow gas accumulations, gas seeps, Potengi estuary. RESUMO.Áreas com acumulações e escape de gás foram identificadas no estuário Potengi. As várias ocorrências de gás foram classificadas em quatro tipos, deacordo com suas assinaturas sísmicas específicas: (1) blankets ou coberturas acústicas, (2) cortinas acústicas, (3) colunas acústicas e (4) turbidez acústica. Ao mesmo tempo, dois tipos diferentes de escape de gás foram identificados: (1) plumas acústicas e (2) pockmarks . A atividade neotectônica na porção fluvial do sistema Potengi pode ser um dos responsáveis pela migração de gases provenientes das sismofácies localizadas abaixo do atual nível do rio, ondeé possível reconhecer a conexão com o nível da fonte do gás, através da assinatura sísmica do tipo coluna acústica. Conclui-se que a fácies sedimentaré provavelmente o fator determinante da acumulação e escape de gás, como também do tipo específico de acumulação ou escape em cada local.Palavras-chave: registros acústicos, acumulações de gás raso, escape de gás, estuário Potengi.Universidade Federal do Rio Grande do Norte -UFRN, Cx.

Crystalline continental rocks and associated crust-contaminated basaltic rocks were unexpectedly dredged on the crest and at seamounts of the Rio Grande Rise, South Atlantic. Zircon U-Pb ages of one gabbro (ca. 2,200 Ma) and four granitoids (between ca. 1,430-480 Ma) indicate that the breakup of SW Gondwana left behind continental fragments of dominantly African age. These rocks may have been incorporated into the oceanic lithosphere by complex processes including rifting and interaction of the Tristan-Gough mantle plume with hyperextended continental margins. Until ca. 80-70 Ma, the Rio Grande Rise and an old portion of the Walvis Ridge formed a conjugate pair of aseismic ridges, and the Tristan-Gough plume was positioned at the Mid-Atlantic Ridge. The finding of continental rock fragments in one of these conjugate pairs opens new perspectives on the mechanisms of continental break-up, the nature of this conjugate pair, and the geodynamic evolution of rifted Gondwana margins in the South Atlantic.

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