Event diachronism in the South Atlantic rift

Abstract

The diachronous relationship observed between generations of seaward dipping reflectors (SDR), dike swarms, and magnetic lineations along of the facing Atlantic margins of South America and Africa permitted the interpretation of the initial opening of the South Atlantic in terms of a propagating rift. The pre-rift stage was characterized by gradual subsidence of the northern segment together with the rise of the asthenosphere in the south, caused by the Tristan da Cunha mantle plume that produced the Paraná-Etendeka Province (PEP). Igneous activity in the PEP from 138 to 127 Ma coincided with climatic warming in the Brazilian rifts, the first greenhouse in the Cretaceous. The first break-up extended to outcrops of the PEP at 132 Ma (M11), at a time when continental igneous activity was sharply reduced and the symmetric San Jorge - Pelotas SDR sequence formed over the break-up unconformity. In the north, the rising rates of crustal stretching created the Atlantic hingeline together with an intra-rift unconformity in the African basins. On the South American side the Sergipe Microplate formed and rotated while the Sergipe-Alagoas transform zone replaced the Recôncavo-Tucano rift as the path of continental break-up. The pre-Aratu propagation unconformity at 132+2 Ma formed simultaneously with the unconformity on the African side, marking the Valanginian-Hauterivian boundary. The change in the direction of the dike swarms oriented first (137-130 Ma) orthogonally and then (133-125 Ma) parallel to the South Atlantic rift, the rotation of SDR in the north of the Pelotas Basin, and the appearance of a regional unconformity in the Santos Basin were all produced by the tectonic activity that preceeded the second break-up. This reached the Ponta Grossa Arch and produced the second generation of SDR (magnetic lineation M3; 126 Ma), suggesting a marine environment to the south of the Santos basin already in the Buracica stage. After dislocating along the Capricorn Lineament and coasting the São Paulo Plateau, the third break-up propagated to the north, and was recorded in the Aptian SDR in front of the Jacuípe and Sergipe basins.