Crustal architecture of the Almada Basin, NE Brazil: an example of a non-volcanic rift segment of the South Atlantic passive margin

Abstract

Abstract The Almada Basin, located in the southern Bahia State segment of NE Brazil, shares a similar sedimentation history and stress regime with the other eastern Brazilian basins. But when considering the composition of the transitional crust, a remarkably different behaviour is observed between the Almada Basin and the other eastern Brazilian basins. The architectural elements of the basin such as the reflection Moho discontinuity, the oceanic and continental basements, and the sedimentary section, as well as the tectonic style, are discussed in this study using gravity, seismic, regional geology and well-drilling results. The Almada Basin is part of a continental rift system that developed during the Early Cretaceous, heralding the Gondwana break-up and subsequently evolving into a passive divergent margin. Deep seismic profiles show the progressive thinning of the continental crust but there is no clear evidence of mantle exhumation. The Almada Basin is the conjugate margin of the South Gabon Basin in West Africa. Although both basins display similar stratigraphic record and geological evolution, the rifting mechanism resulted in a considerable asymmetric break-up. The Almada Basin is floored by the Itabunas–Salvador–Curaçá Belt of the São Francisco Craton and is characterized by Palaeoproterozoic granulites in a high-angle east-dipping thrust fault regime. However, the South Gabon Basin lies on the Neoproterozoic West Congolian Belt that is made up of low- to medium-grade metamorphic rocks in a low-angle thrust tectonic regime. The Precambrian basement fabric of both the Almada and South Gabon basins is marked by lithospheric-scale discontinuities that controlled the implantation of the rift zone and the extension style during the Mesozoic. The strong basement structural inheritance can be recognized in both the geological and geophysical records. The key architectural elements of a volcanic margin, such as large igneous provinces, seaward-dipping reflectors and the basinal syn-rift magmatism, are not recognized in the Almada Basin. Although the South Atlantic margin is mostly volcanic in the southernmost segment, the presence of these two non-volcanic segments – the southern Bahia State and the southern Gabon – are important as a record of the differences in the geotectonic process that governed the formation of the South Atlantic divergent margins.