Geodynamics of the central South Atlantic conjugate margins: implications for hydrocarbon potential

Using a new margin restoration model for the central South Atlantic we highlight margin-opposed rift basin subsidence characteristics, reconcile important facies relationships and consider the associated processes responsible for contrasting basinward transitions to oceanic crust. We interpret these parameters as diagnostic of strain evolution during rifting. Pre-salt subsidence patterns and sequence isopachs of late syn-rift and early post-rift sequences are symptomatic of these complex extension behaviours, inherited basement fault trends partition extensional strain during stretching with resultant rift-related structural styles recording the heterogeneity of the pre-rift crust. Correspondingly the seismic and gravity expression of the transition from continental crust to oceanic crust differs systematically along the length of the 'conjugate' basins. The presence of a conspicuous coast parallel positive linear gravity anomaly, referred to as the 'terminal horst', defines the basinward extent of attenuated continental crust in the more symmetrical Campos Basin and its interpreted conjugate. In the 'asymmetrical' Santos and Benguela Basins the ocean-continent transition at the present-clay OCB (ocean-continent boundary) becomes equivocal with little gravity expression. Consequently the relationship of primary salt basin edges to the basinward extent of crustal extension appears more complex. Pre-existing crustal and lithospheric mantle scale heterogeneities are considered to impart a first-order control on whole crust deformation and ultimately 'sag' basin development. These structurally defined heterogeneities are believed to partition crustal strain and juxtapose end-member mechanisms of pure-shear and simple shear deformation styles as recorded by the complex distribution of syn-rift subsidence patterns within the pre-salt basins of the central South Atlantic. These conclusions challenge the necessity for invoking dominantly depth-dependent processes as a mechanism to explain apparent pre-salt, syn-rift subsidence anomalies. The complex interplay of structure and stratigraphy with an overlay of palaeoclimatic models provides important insights into hydrocarbon play fairways along the margin. In particular the impact on reservoir and source rock distribution and heat flow are discussed.