Abstract: The Transversal Zone: a Key Feature Between NE Brazil and W Africa

Matos, Renato M. Darros - Petrobras/E&P

Even though rifting architecture in NE Brazil and West Africa were controlled by the Braziliano/Pan African heritage, the site of lithospheric rupturing does not seem to have followed any previous upper crustal weakness, especially northward of the megashear zone of Pernambuco (Brazil) and Ngaunder (Africa). This "Transversal Zone" makes up a roughly orthogonal structure, spreading between the Pernambuco shear zone (roughly the Cameroon Volcanic Line in Africa) and the Benue Trough (Touros High in Brazil), encompassing at least five basins, from south to north: the Cabo basin (Brazil), Rio del Rey (Africa), PE-PB (Pernambuco-Paraíba, Brazil), Benue Trough-Niger Delta and the easternmost portion of the Potiguar basin (Touros High).

Based on detailed rift and pre-drift tectonic frameworks of the Brazilian and African basins and regional potential field data from Petrobras and Geosat data base, paleogeographic reconstructions from Neocomian through Barremian and Aptian time are discussed, helping to establish the limits between the stretched continental crust (SCC) and the first formed oceanic crust. Most of the SCC in the Transversal Zone was preserved in the Brazilian side. In the Cabo basin and the Pernambuco Plateau, the SCC is quite wide when compared with its counterpart in Africa, north of the Kribi Fracture Zone (Cameroon), where the margin is quite narrow, and possible Aptian rift sediments are buried beneath a + 5 Km thick post-rift section of the Niger Delta. However, in the PE-PB basin, beneath a thin post-rift section is seismically imaged as a thin rift section, that filled a series of westward dipping shallow half-grabens. The rigidity of the West-African / São Luiz Craton (to the west) and the Borborema / Nigerian shield prevented the onset of rifting in the far east of Brazil and Equatorial Guinea-Cameroon until the early Aptian. This Transversal Zone had a unique evolution defined by a very fast stretching phase (~5 My) during the Aptian time, responsible for overcoming the last tectonic link between Africa and Brazil. A roughly linear hinge zone denotes localized deformation, induced by a focused lithospheric thinning, resulting in a transient thermal anomaly, and minor localized topographic uplift.

Figure: Paleogeography and the spatial distribution of rift basins during the Aptian, immediately prior to the break-up of the South Atlantic. The large arrows indicate the direction of tectonic transport in the Equatorial Branch. Key: PSZ: Pernambuco Shear Zone, Brazilian basins are: Recôncavo-Tucano-Jatobá (RTJ), J (Jacuípe), Sergipe-Alagoas (SE-AL), Cabo (C), Potiguar (P), Pernambuco Paraíba (PE-PB), Ceará (CE); African basins: Togo-Benin (TB), Benue (B), RR (Rio del Rey), Douala (DB), Rio Muni (RM), and Gabon (G).

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil