Rifts of the Campos and Santos Basins, Southeastern Brazil

Abstract

Satellite-derived gravity anomalies were used to map the location and distribution of rift subbasins comprising the Campos and Santos Basins of the southeastern Brazilian continental margin. Free-air and crustal Bouguer gravity anomalies define several features. A negative‐positive gravity gradient along the southeastern Brazilian margin correlates generally with termination of oceanic fracture zones, the boundary of synrift evaporites, and an abrupt change in anomaly trends from east‐west to margin-parallel. The gravity gradient thus defines the location of the ocean‐continent boundary and suggests that much of the Sao Paulo Plateau is underlain by thinned continental crust. Second, a major offshore tectonic hinge zone, consisting of a series of short-segment, en echelon, high-standing blocks subparallel to the Brazilian margin demarcates the western limit of significant continental extension. The Badejo High of the Campos Basin is part of this hinge zone trend. The Serra do Mar and Serra da Mantiqueira mountains represent an onshore hinge zone. Third, a series of major rift subbasins exist seaward of both the Campos and Santos hinge zones. These have limited along-strike continuity, implying that synrift lake communication, water chemistry, and possibly source quality and preservation were restricted to each subbasin. Extension between west Africa and Brazil occurred during the Early Cretaceous as a series of rift pulses that culminated in the initiation of sea floor spreading. The Congo‐Cabinda margin of west Africa and the Camamu‐Almada margin of eastern Brazil are characterized by a common tripartite rift history: Berriasian‐Hauterivian, Hauterivian‐middle Barremian, and late Barremian‐early Aptian. Early depthindependent, broadly distributed, and increasingly focused brittle deformation (rift phases I and II) was replaced by depth-dependent deformation dominated by plastic thinning of the lower crust and lithospheric mantle (rift phase III). The nonmarine part of the Lagoa Feia Formation correlates with rift phase II while the “transitional” part is associated with rift phase III. The intervening pre-Alagoas unconformity is equivalent to the pre-Chela unconformity on the Congo margin. The possibility of a mid-crustal detachment beneath the Brazilian margin active during rift phase III has profound implications for the hydrocarbon maturation history of the margin. Seismic reflection data from the Sao Joao da Barra Low (Campos Basin) have helped define an early synrift depositional package that likely equates with rift phase I.