Emanuel Ferraz Jardim de Sá

Microstructural evolution of the SeridÓ Belt, NE-Brazil: the effect of two tectonic events on development of c-axis preferred orientation in quartz

The polyphase evolution of the Serido Belt (NE-Brazil) includes D, crust formation at 2.3-2.1 Ga, D-2 thrust tectonics at 1.9 Ga and crustal reworking by D-3 strike-slip shear zones at 600 Ma. Microstructural investigations within mylonites associated with D-2 and D-3 events were used to constrain the tectono-thermal evolution of the belt. D-2 shear zones commenced at deeper crustal levels and high amphibolite facies conditions (600-650 degreesC) through grain boundary migration, subgrain rotation and operation of quartz Q-prism slip. Continued shearing and exhumation of the terrain forced the re-equilibration of high-T fabrics and the switching of slip systems from (c)-prism to positive and negative (a)-rhombs. During D-3, enhancement of ductility by dissipation of heat that came from syn-D-3 granites developed wide belts of amphibolite facies mylonites. Continued shearing, uplift and cooling of the region induced D-3 shear zones to act in ductile-brittle regimes, marked by fracturing and development of thinner belts of greenschist facies mylonites. During this event, switching from (a)-prism to a basal slip indicates a thermal path from 600 to 350 degreesC. Therefore, microstructures and quartz c-axis fabrics in polydeformed rocks from the Serido Belt preserve the record of two major events, which includes contrasting deformation mechanisms and thermal paths

Caracterização meso e microscópica de bandas de deformação em arenitos porosos: um exemplo nas tectonossequências Paleozoica, Pré- e Sin-rifte da Bacia do Araripe, Nordeste do Brasil

Bandas de deformacao sao estreitos volumes tabulares desenvolvidos em arenitos porosos. Apesar de produzidas em condicoes de deformacao fragil, internamente podem apresentar gradiente de deslocamento continuo. Quando o mecanismo de deformacao dominante durante a nucleacao dessas bandas e a cataclase granular, podem ser alteradas significativamente as propriedades originais de suas rochas hospedeiras, tais como porosidade e permeabilidade. Bandas de deformacao, presentes nos arenitos pre- e sin-rifte da Bacia do Araripe, foram estudadas em meso e microescala com o intuito de classifica-las e de entender os mecanismos deformacionais envolvidos durante sua nucleacao e desenvolvimento. Criterios geometrico-espaciais, cinematicos e reologicos permitiram estabelecer as relacoes entre a genese das bandas de deformacao e a litificacao dos arenitos protolitos. A luz desses dados, discutiu-se o impacto dessas estruturas no fluxo de fluido, na escala de reservatorio. Alem disso, o estudo das bandas de deformacao auxiliou no entendimento da evolucao geotectonica da bacia sedimentar estudada. Dessa maneira, o estudo das bandas de deformacao pode subsidiar pesquisas sobre a evolucao tectonossedimentar local e regional de bacias sedimentares.

Desenvolvimento simultâneo de semigrabens ortogonais e oblíquos à direção de distensão: modelagem física de análogo natural no Nordeste brasileiro

The physical modeling tool is being increasingly used in geology to provide information about the evolutionary stages (nucleation, growth and geometry) of geological structures at various scales. In the simulations of extensional tectonics, modeling provides a better understanding of fault geometry and evolution of the tectonic-stratigraphic architecture of rift basins. In this study a sandbox type apparatus was used to study the nucleation and development of basins influenced by previous structures in the basement, variably oriented as regards to the main extensional axis. Two types of experiments were conducted in order to: (i) simulate the individual (independent) development of half-grabens oriented orthogonal or oblique to the extension direction; (ii) simulate the simultaneous development of such half-grabens, orthogonal or oblique to the extension direction. In both cases the same materials (sand mixed with gypsum) were used and the same boundary conditions were maintained. The results were compared with a natural analogue represented by the Rio do Peixe Basin (one of the eocretaceous interior basins of Northeast Brazil). The obtained models allowed to observe the development of segmented border faults with listric geometry, often forming relay ramps, and the development of inner basins faults that affect only the basal strata, like the ones observed in the seismic sections of the natural analogue. The results confirm the importance of basement tectonic heritage in the geometry of rift depocenters.