The Merluza Graben: how a failed spreading centre influenced margin structure, salt deposition and tectonics in the Santos Basin, Brazil

Abstract

The relative timing between crustal extension and salt deposition can vary spatially along passive margin salt basins as continents unzip or the locus of extension shifts through time towards the embryonic spreading centre. Determining the relative timing of salt deposition, rifting and seafloor spreading is often problematic due to the diachronous nature of rifting, the ability of salt fill pre-existing topography and to the post-rift gravity-driven salt tectonics. We use 2D PSDM seismic data and structural restorations to investigate the Merluza Graben, a large rift-related depocentre located at the proximal and southernmost portion of the Santos Basin, Brazil and at the continuation of a failed spreading centre, the Abimael Ridge. The graben presents up to 3.5 km of base-salt relief at its basinward-bounding fault and internal base-salt horsts up to 1 km high. This compartmentalises deformation, producing extensional and contraction salt structures, ramp-syncline basins and expulsion rollovers within the graben, resulting in a remarkably different and variable style of salt tectonics than in the adjacent areas. We also conduct structural restorations to analyse the spatial and temporal evolution of this complex style of deformation and potential prolonged crustal extension in the Merluza Graben by using discrepancies in the present and restored salt area. This approach affords further constraints on local variations in the relative timing of rifting and salt deposition and its impact on salt tectonics along the margin, which can be applied to other hyper-extended rifted margins worldwide.