Estrutura e evolução tectono-magmática das margens rifteadas e oceânica, offshore do Sudeste Brasileiro

Abstract

The Santos and Campos segments of the SE Brazilian rifted margin has a complex poly-phase tectono-magmatic evolution. Determining the resulting distribution of thinned continental, hybrid, magmatic and oceanic crust using reflection seismology is difficult and further complicated by extensive thick salt. Consequently, there are many contrasting interpretations of crustal structure, composition and its evolution. This study uses combined geophysical and geodynamic methodologies to investigate the crustal thickness, crustal type, and variation in timing of crustal rifting and magmatic addition of the SE Brazilian margin, revealing the distribution and co-existence of continental, hybrid, and magmatic crust on the São Paulo Plateau (SPP). Reverse post-rift subsidence modelling of base and top salt palaeo-datum on the SPP shows that post-tectonic salt deposition primarily corresponds to continental crust thinned by widespread early Aptian rifting while syn-tectonic salt deposition corresponds to late Aptian magmatic crust and rifting. A schematic kinematic plate reconstruction of the Santos Basin margin segment with its conjugate Namibia margin is presented. In the distal Campos rifted margin, the high-amplitude SEMA magnetic lineament is identified and interpreted as a volcanic outer high, formed before salt deposition and likely predating breakup, highlighting early magmatic additions in the Campos Basin. This method invokes Warner’s “10 second Moho rule” for thermally equilibrated lithosphere and provides estimates of crustal basement thickness from top basement TWTT. It reveals distinct crustal basement thickness patterns in TWTT for magma-normal, magma-poor, and magma-rich margins, independent of sediment thickness. Applying this methodology on the Campos and Southern-Santos margin segments shows the higher magma budget of the Santos margin segment classifying it as magma-rich, while the Campos margin segment is classified between slightly magma-poor and normal, and underwent poly-phase deformation and magmatism. A new map of magnetic anomaly transformation to pseudo-gravity anomaly for the South Atlantic between 15ºS and 45ºS is made and presented. High-amplitude positive pseudo-gravity anomalies over the Rio Grande Rise (RGR) in the oceanic region correspond closely with areas of thick crust determined from gravity inversion, consistent with their magmatic composition. The SPP, Florianopolis Ridge (FR) and the Torres High on the SE Brazilian margin also show high-amplitude positive pseudo-gravity anomalies. The Campos rifted margin also shows positive pseudo-gravity anomalies but with lower amplitudes. On the African plate, strong positive anomalies are found along the Namibian Ridge, northern Walvis Ridge and the African continental margin, forming a distinctive "7". Plate reconstructions show that prior to 85 Ma, high positive anomalies formed a near-continuous west to east arc extending from the Paraná Basin on land Brazil, to the SPP, FR and RGR on the South American Plate, then joining the northern Walvis Ridge and Namibia Ridge on the African plate. Using a multi-disciplinary approach, this PhD thesis contributes to the understanding of the tectono-magmatic structure and evolution offshore SE Brazil.