Leonardo Muniz Pichel, Frank Peel, Christopher Aiden-Lee Jackson , Mads Huuse

Ramp-syncline basins (RSBs) are characterized by asymmetric depocentres that form by translation above flat-ramp extensional faults or salt detachments having basal steps. Both types of RSBs are composed by landward-dipping and expanding strata defined by basinward-dipping axial traces. 3D seismic data from the São Paulo Plateau, Santos Basin, Brazil, image a series of RSBs formed above thick salt and distributed above and/or basinward of the main base-salt steps that define the boundaries of fault blocks hosting giant and super-giant oil fields in the area. The RSBs trend NNE-NE, being composed of landward-dipping, gently folded sigmoidal strata, and defined by diachronous boundaries, a basal onlap surface and a top unconformity, both of which become progressively younger landward. These systems record a total of 28-32 km of ESE to SE-oriented translation during the Late Cretaceous and Paleocene at an average rate of 0.8 to 0.9 mm/year. In this study, we present the 5 least deformed and thus best imaged examples of RSBs in the area, together with numerical forward models in order to analyse their 3D kinematics, tectono-stratigraphic evolution, and interaction with diapirism and base-salt structures. The results show that layer-parallel shearing and salt flux variations due to changes in salt thickness across base-salt topography produces ramp-syncline basins. The RSBs form not only by translation above base-salt basinward-dipping ramps but also over landward-dipping ramps, having similar stratigraphic architecture but different kinematics. Translation over stepped ramps generates stacked RSB systems and onlap surfaces. Thickness maps show translation is higher at the centre of RSBs and that depocentres become progressively more affected by diapirism as the system evolves. Stratal architecture and terminations vary along dip and strike in these systems. This study presents the first ever analysis of the 3D kinematics of ramp-syncline basins, and the first documentation of their occurrence above thick salt in the Santos Basin, Brazil. It also applies more realistic models that treat the salt detachment as a volume of viscous material, improving our understanding of these systems. The recognition and analysis of RSBs allow quantification of overburden translation above a deforming salt layer and the identification of base salt topography, thus aiding the understanding and exploration of salt basins.