Influência da reativação rúptil de zonas de cisalhamento continentais e da Zona de Fratura Romanche na evolução da Margem Equatorial do Brasil

Abstract

The separation between the South American and African continents, in the Lower Cretaceous, was strongly conditioned by the Precambrian structural fabric of amalgamated crustal blocks along extensive ductile shear zones originated during the Brasiliano Orogeny (720-540 Ma). This dissertation investigates the tectonic role of the brittle reactivation of Precambrian shear zones (especially the Transbrasiliano Lineament) and the Romanche Fracture Zone (RFZ) in forming the central part of the Brazilian Equatorial Margin. To this end, gravimetric, magnetic, elevation, seismic and well data were compiled, processed and integrated. The brittle reactivations of these continental-scale shear zones occurred mainly where there was high mechanical contrast between Precambrian terrains, compartmentalizing the continental margins on both sides of the Equatorial Atlantic. In this context, the RFZ divides this sector of the Equatorial Margin into one E-W-transform and two NW-SE-oblique divergent segments. Gravimetric models along regional seismic sections reveal distinct crustal behaviors for each segment: south of the RFZ, a divergent margin with a crustal stretch of ~90 km; along the RFZ, a transforming margin with little extended continental crust (~30 km); and to the north of the RFZ, another divergent segment shows a hyperextended continental crust up to 160 km long on the continental shelf. The Moho discontinuity and the internal geometry of the sedimentary basins were also modeled, completing the lithospheric characterization of the Equatorial Margin. The basins South of RFZ are thicker than those to the North because this structure acted as a barrier to the deposition of continental sediments in the oceanic regions. The boundary between continental and oceanic crusts identified in the present work is also consistent with previous studies. However, it diverges from these studies north of the RFZ due to the hyperextended continental crust described here. These shear zones arrest at the RFZ, where the continent-ocean crustal boundary occurs. The structures (faults and folds) related to brittle reactivation of the shear zones are compatible with an initially extensional stress field during the rift phase up to the Aptian. This stress field inverted to strike-slip after the main breakup phase, with the direction of maximum compression varying from vertical to horizontal from Aptian to Present.