The Santos and Campos basins, offshore Brazil, resulted from the Early Cretaceous break-up of Gondwanaland. South Atlantic margin plate reconstruction models propose uniform east-west extension; rift-basin symmetry is thus inferred with the conjugate African margin. This would imply that half-graben development and planar dip-slip faults would characterise the structural style and geometry of the Santos and Campos syn-rift basins. Interpretation of depth-migrated regional 2D seismic data has revealed important structural relationships previously obscured by post-rift Aptian salt cover. Fault geometries and syn-rift isopach maps infer a more complex rift-margin evolution that controlled sediment dispersal patterns for the two basins during rifting. Pre-rift lineaments inherited from Archaean basement had imparted an important mechanical anisotropy to South Atlantic crust rheology. Contrasting basement response to extension differentiates deformation styles in the Santos and Campos basins, with the Santos Basin margin evolution more consistent with oblique extension and asymmetric rifting. Onshore igneous extrusives allow the direction of extension to be constrained temporally, with shallow crustal heat flow a primary influence on brittle versus more ductile rift kinematics.

These observations challenge the simple symmetric rift model and explain the spatial relationship of syn-rift tectonic and ‘sag’ phase thermal subsidence patterns with associated fault styles. Reconstruction of the continent/ocean boundary along the Campos and Santos basin margins and consideration of their conjugate African basins demonstrates clear rift asymmetry. The structural expression of the transition from continental to oceanic crust in the Campos and Santos basins is distinctive and is related to the primary rifting mechanism inferred for each part of the margin. The presence of the ‘outer-horst’ that seismically defines the continental oceanic boundary is well evidenced in the Campos Basin whilst in the Santos Basin this is equivocal. The conspicuous remnant intra-basinal Sao Paolo Plateau/Ridge, however, does suggest that the loci of oceanic crust formation in the Santos Basin may have experienced a dynamic basinward translation as a response to continued asymmetric lithosphere extension. The Cabo-Frio Arch structurally partitions contrasting Campos and Santos syn-rift basin architectures, and the new interpretation has fundamental implications for their respective petroleum systems.

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