Arcabouço estrutural, geometria e estrutura crustal das margens rifteadas do Oeste Africano SubSaariano

Abstract

The Western African SubSaharian continental margin includes a large region from Cameroon to South Africa reflecting a tectonic environment that comprises distinct margins types magma-rich, magma-poor and transform margins. The integrated study of geological, seismic and potential field data has enabled to establish a structural framework characterized by a set of comparable architectural elements known as proximal, necking, distal, outer and oceanic domains. These elements show a systematic oceanward succession, not necessarily in the same order, taking in account the distinct aspects of the environment that they were generated. These key domains display a strong and consistent continuity along the continental margin. Between different margins types transitional regions normally occur in a gradual way, but interactions between structural domains may also occur with the possible occurrence of spatially and temporally overprint. In this context, the Outer Domain, the key feature in the magma-rich margin, tends to disappear northwards where the Distal and Proximal Domains in magma-poor margin performed the main role. Likewise, it is observed the recurrence of the Outer Domain as the Distal Domain in magma-poor margin (north of Gabon Basin) is disturbed by the fracture zones presence. The nature of the basement in the Distal Domain of the magma-poor margin is believed to be constituted by hyperextended continental crust as much as hyperextended continental crust infiltrated by mantle melting, what was validated by gravity profiles. The gravity modeling in crustal scale shows that in the transitional regions, beneath hyperextended crust, the subcrustal mantle appears locally anomalous with density values lower than adjacent regions. The presence of these low density values is probably caused by active small-scale convection cells caused by some changing in the fundamental constrains imposed in the extension process, when rifting reaches Archean cratonic regions and magmatic activity. Eventually, the proposed new crustal boundary, mainly in the region of Namibe Basin (transitional zone between magma- rich margin and magma-poor margins) applied over the tightest possible reconstruction continental blocks before the break-up (~133 Ma) shows a best fit between South Atlantic coast lines implying in drastic reduction of the gap resulting from the extension in this region. In this model, less amount of horizontal movement during the extension of the crust is needed to accommodate the continental portion, which is in agreement with the recent geological observation in this region. Another relevant point addressed by the model is the fact that this new board displays the same inflection verified in the Paraná Igneous Province, in this region of the South American block