Tectonic Control of the Campos-Santos Basins, Offshore Brazil -- A Discussion

By

 F. R. M. Pires¹, L. A. M. da Costa²

(1) UFRJ - Federal University of Rio de Janeiro, N/A, Brazil (2) CPRM - Companhia de Pesquisa de Recursos Minerais, N/A, Brazil

 Several tectonic events were responsible for generations of the oil fields in the Campos-Santos Basins, offshore Brazil. Distensive processes probably started as early as the end of Paleozoic, beginning of Triassic. However, tectonic activity was recorded approximately 130Ma and developed up to nowadays. The brittle regime which prevailed during those events produced ENE-WSW prominent fracture system, which aligned the coastal flat-lying gneisses at Rio de Janeiro. Diabase and phonolite dikes and plugs were emplaced along some of the fractures. Sills occur in the proximities of the joints. Thermal output generated by the magmatic activity was favorable for hydrocarbon formation and/or migration. These system imparted characteristic magnetic anomalies detected by the aeromagnetometric surveys. Almost orthogonally disposed to the fracture system there is another set of fractures, which produced a basculated type of structure, by a rotation along one of the fault lines. This structure has been designaled as resequent tilt-blocks. From onshore, there are sets of splays diverging from the Paraiba do Sul Megashear Zone and related structures. Reactivation of the splays contributed significantly to the opening of the basins and sub-basins during the Cretaceous. A graben-horst system was developed, and several troughs filled by the high sedimentation rates dominated by turbidites. Salt diapirism, probably triggered by tectono-magmatic activities and/or gravitational differences, initially developed in depocenters, have facilitated the formation of the oil traps.