Eliane da Costa Alves

Geophysical and geochemical evidence for cold upper mantle beneath the Equatorial Atlantic Ocean

This paper presents geophysical and geochemical evidence for the possible existence of cold, subducted lithosphere beneath the Saint Paul Fracture Zone of the Equatorial Atlantic Ocean. The ocean floor along the fracture zone is characterized by a high percentage of abyssal peridotites. The abyssal peridotites were emplaced by tectonic uplift of the oceanic lithosphere. The top of the ridge is exposed at Saint Peter and Saint Paul islets. The Os isotopic composition of these abyssal peridotites indicate the presence of old depleted mantle material in this region with Re-depletion model ages (TRD) from 0.32 to 1.1Ga. Melt inclusions in plagioclase phenocrysts of the MORB close to this location have boninitic composition, suggesting that some basalts originated from low-degree mantle melting. The global tomography models show fast seismic velocities in the upper and lower mantle of the Equatorial Atlantic Ocean, consistent with the presence of cold subducted lithosphere. The range of Re-depletion model ages are consistent with paleo-reconstructions of plate motion, suggesting that the fossil subducted slab was formed during the closure of both the Iapetus and the Rheic oceans.

Zona de fratura de Vitória-Trindade no Oceano Atlântico sudeste e suas implicações tectônicas

The integration of geological and geophysical data allowed a better understanding of the inter-relation between the oceanic and continental structure. Tectonic and magmatic phenomena of the Meso-Cenozoic were probably related to the prolongation of oceanic fracture zones into the Brazilian Southeast continental margin. This work will focus on the behaviour of the Vitoria-Trindade Fracture Zone (VTFZ). The Vitoria-Trindade Fracture Zone (VTFZ), located at 18o40S in the Mid-Atlantic Ridge (MAR), was defined by seismic, gravity and magnetic isochrones. At the MAR axis the VTFZ corresponds to a second-order discontinuity, with a 6 km axial offset. Away from the ridge axis, potential field data shows a gravimetric depression in E-W direction, which represents the asseismic portion of the VTFZ. Towards the Brazilian Continental Margin there is a series of topographic highs defining the Vitoria-Trindade Chain, which is divided into distinct sedimentary thickness and tectonic domains. This area is cut by sills and dikes, indicating tectonic instability, remobilization of faults and strike-slip tectonics which has uplifted the entire sedimentary column, including the oceanic bottom. All of these observations suggest recent oceanic crust reactivation with alternations of compressive and extensional stress associated with volcanism and normal faulting. Geological and geophysical studies, in addition to mapping the earthquakes epicenters, suggest that the continuation of the VTFZ that occurs onland is related to the Vitoria High and to tectonic and volcanic activities, which are expressed by the Trindade-Paxoreu /Alto do Paranaiba Alkalic Magmatic and Seismic Lineaments. We propose that the VTFZ acts as a pathway for the magma generated by the Trindade Mantle Plume.

Architecture and stratigraphic framework of shelf sedimentary systems off Rio de Janeiro state, Northern Santos Basin-Brazil

A analise sismica de dados sparker das Operacoes GEOMAR permitiu a elaboracao de um primeiro arcabouco estratigrafico da secao rasa (~300 msec) da plataforma continental do Estado do Rio de Janeiro, norte da bacia de Santos. Dois conjuntos de sequencias sismicas foram interpretados como sequencias deposicionais induzidas por oscilacoes glacio-eustaticas. O Conjunto I (SqA-SqC), composto por sequencias dominantemente sigmoidais, reflete condicoes de geracao de espaco de acomodacao sedimentar capaz de preservar seus componentes agradacionais-progradacionais; o Conjunto II (Sq1-Sq5), composto principalmente por prismas de regressao forcada, indica diminuicao relativa de espaco de acomodacao. Dados cronoestratigraficos de pocos permitiram posicionar a secao sismica investigada na janela plio-quaternaria: a maior parte do Conjunto I (SqA e parte inferior da SqB) foi atribuida uma idade Plioceno (indiferenciado); a secao estratigrafica que se estende da porcao superior da SqC ate o Conjunto II foi atribuida uma idade quaternaria. A correlacao entre a base de dados e curvas globais de variacoes isotopicas de δ18O permitiu ainda sugerir que as sequencias Sq1-Sq4 registram sequencias regressivas de quarta ordem (ciclos glacio-eustaticos de cerca de 100-120 ka) durante os ultimos 440-500 ka. A sequencia Sq5 representaria a deposicao holocenica, constituida por depositos transgressivos e de sistemas de mar alto.