Chapter 1 - South Atlantic Ocean: postbreakup configuration and Cenozoic magmatism

Abstract

This work is related to some of the most important indications for tectonic reactivation and magmatism in the South Atlantic Ocean and adjacent continental margins subsequent to the Gondwana breakup and development of the continental margins in the Early Cretaceous. The Precambrian continental crust was affected by major events during the rift phase in the Brazilian and southwestern margins, resulting in fluviolacustrine environments developed in depressions along the margin, which formed a system of grabens and horsts. The next phase of subsidence was associated with transitional environments forming a sag basin and lagoonal environments leading to salt deposition in the Late Aptian. Magmatic episodes are observed during this phase, which are coeval with development of the early oceanic propagators or embryonic spreading centers that advanced from the Pelotas to the Santos Basin. After salt deposition, there are indications of major fault zones in the region north of the oceanic propagator, which resulted in the development of the N–S trending Merluza Graben. Salt deposits also covered the faulted volcanic escarpment of the Cabo Frio fault zone in the central Santos Basin, as indicated by residual salt masses at the center of the fault zone. Lineaments trending NW–SE are observed both on oceanic crust, extending from the Rio Grande Rise toward the Cabo Frio High, forming the Southern Cross Lineament, and in the southern Santos Basin, forming the Capricornio Lineament. These lineaments reach the coastline in the Cabo Frio and São Sebastião Island, respectively, forming plugs of alkaline igneous rocks. Both NW–SE and E–W lineaments formed during the postsalt drift phase-controlled igneous intrusions offshore and onshore. The onshore alkaline igneous plugs are aligned parallel to the continental margin, forming the E–W Poços de Caldas–Cabo Frio magmatic lineament. The NW-trending lineaments in the offshore region are associated with several igneous intrusions forming seamounts, and onshore they separate Cenozoic basin depocenters, as observed in the Taubaté and Resende basins, which are characterized by Paleogene to Neogene subsidence phases. Although considered a passive margin associated with plate divergence, the southeastern South Atlantic continental margins are characterized by significant tectono-magmatic events that indicate plate rearrangements in the Coniacian-Santonian, when the Rio Grande Rise formed an oceanic plateau above the Tristan da Cunha hot spot, and in the transition from Late Cretaceous to Cenozoic, when igneous plugs and tectonic activity are registered both offshore and onshore.