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Tectono-Stratigraphic Development of Ramp Syncline Basins


Classic ramp-syncline basins form above ramp-flat-ramp normal faults and are defined by an asymmetric depocentre defined by a basinward-dipping axial trace. Ramp-syncline basins formed above thin salt detachments can have a similar geometry; but, in this case, the down-to-the-basin trajectory is defined by a basinward-dipping ramp along the base salt. Moreover, above thick salt detachments, salt expulsion and diapirism can cause vertical motions resulting in complex minibasin geometries and stratigraphic architectures. Even though the overall 2D development of ramp basins is relatively well know, nothing has been published about their 3D geometry and strike-variability and very little about their translation rates. 3D seismic data from the São Paulo Plateau, Santos Basin, offshore SE Brazil image a series of ramp-syncline basins developed above thick autochthonous salt. The basins are 10-15 km wide and 15-30 km long in a N-NE direction; they contain stratigraphic successions that are up to 800 m thick. The basins are defined below by an onlap surface and above by a local erosional unconformity. Internal strata dip landward, are gently folded, and contain numerous intra-formational unconformities. Both the basal onlap surface and the capping unconformity are diachronous, getting progressively younger landward. To understand the evolution and distribution of ramp-syncline basins and the development of diachronous unconformities, we use a numerical forward modelling technique to simulate overburden translation and deformation above extensional fault systems and salt-detachments with variable thicknesses. Model results show that diachronous unconformities and onlap surfaces are formed due to the passage of the cover section over a basinward-dipping ramp and due to local uplift caused by upward tilting of the footwall or salt inflation. Models with stepped basinward-dipping ramp systems generate more complex geometries, characterised by the development of double unconformities and stacked ramp-syncline basins. Variable sedimentation rates produce several small-scale onlap and offlap unconformities, and if the rate of sedimentation is higher than the rate of local uplift, the capping unconformities do not form. This study improves our understanding of the stratigraphic architecture of ramp-syncline basins and overburden translation rates, and provides a guide to the interpretation of subsalt structure based on suprasalt stratigraphic architecture.