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Rift Evolution of the Equatorial Margin Basins in the Brazilian South Atlantic


This study investigates the major structural trends of the Equatorial Margin of Brazil, both offshore and onshore. An interpretation of lineaments and a physical modeling of the continental breakup of the conjugated South America and African margins were performed. Quantification of the eroded sedimentary thickness using sonic logs and estimation of crustal thickness through gravimetric inversion was also undertaken. The central sector of the Brazilian Equatorial Margin can be subdivided into a NW-SE segment to the west (Barreirinhas Basin), an E-W segment (western part of the Cearà Basin) and a NW-SE segment comprised by the Mundaù Sub-basin (eastern portion of the Cearà Basin). The structural analysis and kinematic evaluation of major features as the Transbrasiliano lineament, the Mundaù Sub-basin, and the Tutòia High were based on available seismic data. Also, the E-W Romanche fracture zone cuts through the Cearà Basin from the oceanic crust to the Tutòia High in shallow water. The results suggest that the Romanche transfer zone was active since the Aptian, controlling the deposition in en echelon NW-SE half-grabens in the Barreirinhas Basin. That structure was active at least until the early Albian, when compressional structures were created in the region of the Tutòia High. At the same time, the Transbrasiliano lineament (a major NE-SW feature oriented for several thousands of kilometers along the onshore Precambrian basement) was reactivated and underwent a NW-SE shortening, compatible with dextral motion along the Romanche zone. In the Mundaù Sub-basin the tectonic regime characterized in the Aptian-Albian was dextral transtensional. Physical modeling suggests an important control of the transverse lineaments on the erosional domains. Likewise the fracture/ transfer zones conditioned an en echelon arrangement of the uplifted areas and regional depocenters, which are associated with the terminations of the fractures zones. Our interpretation suggests two rift stages: an early rifting phase in the Neocomian controlled by basement trends restricted to some areas, followed by the main phase during the Aptian-Albian. This main rifting phase differs from the Neocomian stage by a distinct trend and an eastward regional tectonic transport, along the strike of the fracture zones. We suggest that the rift framework is a key element in controlling the distribution of the Upper Cretaceous reservoirs.