Abstract Gravity anomalies have been used to map the location and distribution of rift subbasins comprising the Campos and Santos basins of the southeastern Brazilian continental margin. Even though thick post-rift sediment units, evaporites, and diapiric structures have often tended to obscure clear seismic imaging of the basement and syn-rift structures and stratigraphy of the Campos and Santos basins, the free-air and crustal Bouguer gravity anomalies (see figure below) have helped define: 1) a prominent gravity "dipole" anomaly along the southeastern Brazilian margin that correlates spatially with the termination of oceanic fracture zones, the positive component of which corresponds to Aptian-Albian thickened oceanic crust. The gravity dipole gradient thus defines the location of the ocean-continent boundary and as such, requires that the Sao Paulo Plateau is underlain by oceanic crust; 2) a major tectonic hinge zone, consisting of a series of short segment, en-echelon high-standing blocks subparallel to the Brazilian margin that demarcates the western limit of significant continental extension. The Badejo high of the Campos basin is part of this hinge zone trend; and 3) a series of major rift subbasins seaward of both the Campos and Santos hinge zones. These subbasins have limited along-strike continuity, implying that syn-rift lake communication, water chemistry, and possibly source quality and preservation are spatially restricted to each of the subbasins.
Details concerning the distribution and timing of rift events responsible for the structuring of the Campos and Santos basins are poorly known. However, extension between west Africa and Brazil occurred during the early Cretaceous as a series of rift pulses with varying distribution and duration that culminated in the initiation of seafloor spreading. The Congo and Cabinda margins of west Africa and the Camamu-Almada margin of east Brazil are characterised by a common tripartite rift history; Berriasian to Hauterivian, Hauterivian to mid Barremian, and late Barremian to early Aptian. Extension style changed dramatically as a function of space and time. Early, depth-independent, broadly-distributed brittle deformation (rift phases I & II) was progressively replaced by depth-dependent deformation in which rifting was dominated by plastic thinning of the lower crust and lithospheric mantle (rift phase III). The dominance of lower crustal deformation results in both a general paucity of basement-involved normal faulting and regional syn-rift subsidence across the region. By analogy with the west African margin and dating from ostracod zonations, the non-marine component of the Lagoa Feia Formation correlates with rift phase II while the "transitional" component is associated with rift phase III, the plastic thinning of the lower crust and lithospheric mantle. The intervening pre-Alagoas unconformity is time and tectonically equivalent to the pre-Chela unconformity on the Congo margin, being formed during the uplift and truncation of earlier rift units and the development of restrictive depositional environments. 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) has helped define an early syn-rift depositional package that likely correlates with rift phase I.
