The Merluza Graben: How a Failed Spreading Center Influenced Margin Structure, and 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 as the locus of extension shifts toward the embryonic ocean spreading center. 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 to fill pre-existing topography, and the subsequent flow and deformation of that salt. We here use 2D PSDM seismic data and structural restorations to investigate the Merluza Graben, a large rift-related depocentre located in the southern, most proximal part of the Santos Basin, Brazil, along-strike of a failed spreading center, the Abimael Ridge. The graben is defined by up to 3.5 km of base-salt relief along its basinward-bounding fault and internal base-salt horsts that are up to 1 km high. This compartmentalizes deformation, producing intra-graben extensional and contraction salt structures, ramp-syncline basins, and expulsion rollovers, resulting in a remarkably different salt-tectonic structural style to that seen in the adjacent areas. We also conduct structural restorations to analyze the spatial and temporal evolution of salt-tectonic structural styles and the relationship this has to potential prolonged crustal extension in the Merluza Graben. This approach further constrains local variations in the relative timing of rifting and salt deposition, and the impact this has on salt tectonics along the margin. The results of our study can be applied to better understand the tectono-stratigraphic development of other salt-bearing rifted margins.