Abstract

Santos Basin, offshore southeastern Brazilian margin, is unusual among the other offshore basins for its almost 700 km width and extensive submarine area covered by the São Paulo Plateau. Within the past 12 years, this basin has also become the site for some of the largest offshore oil fields ever discovered in deep water. Therefore, the nature of the crust under this basin and its role in the opening history of the South Atlantic have been intensely studied recently.

Using gravity, magnetic, and seismic data combined with the depositional history of the basin has allowed us to map a short-lived spreading center in the southern Santos Basin. Various other authors have also interpreted this spreading center, but we believe the 3-D prestack depth migrated data presented here offer the most convincing case for this center published in the scientific literature. This early phase spreading center, named herein the aborted spreading axis (ASA), jumped to the east when the South American plate finally broke through and developed the present-day mid-oceanic ridge around 113 Ma. As a result of the earlier rupture, an important tectonic lineament developed comprising a several-hundred-km broad plateau separated from the Brazilian mainland by an associated trough. This feature was a major factor in defining the early water circulation and the depositional setting of the early Aptian pre-breakup sediments in the restricted Santos Basin. These deposits comprise the main reservoirs and source rocks of the giant pre-salt fields in Santos Basin. The high-standing volcanoes, lava deposits, and associated valleys could have guided early marine incursions into the basin to the north through waterways created by the tectonic trough. Those episodic events have probably controlled the coquina deposits and the heterogeneous lacustrine carbonates in a hypersaline gulf setting, to be overlain by confined evaporitic sediments. Widespread and thick evaporite deposition followed this previous stage along the Brazilian margin when a broader seawater invasion flooded the sinking continental margin.