Ductile salt units have a profound influence on the subsequent tectono-stratigraphic evolution of sedimentary basins. The development of thin-skinned, kinematic zones of updip extension and downdip contraction, driven by the interplay between gravity gliding and gravity spreading, is well established. In detail, however, there is great complexity in the range of structural styles observed on salt-influenced passive margins, and their key controls are still not fully understood. I use 3D seismic reflection data from two salt-influenced passive margin settings (the Kwanza Basin, offshore Angola and the Levantine Basin, offshore Lebanon) to interpret their post-salt tectono-stratigraphic evolutions. I assess the relationship between sub-, intra-, and supra-salt structures, with a particular emphasis on the influence of base-salt relief and intra-salt lithological heterogeneity.
A number of different tools are employed to perform this analysis, including structural restorations, strain calculations and translation measurements. Ramp syncline basins develop due to salt flow over base-salt relief and provide a record of horizontal overburden translation during gravity gliding. This record can be used to calculate rates of translation, revealing spatial and temporal variations at the basin scale in both study areas, which are linked to thick- and thin-skinned processes. On the Angolan margin, the interaction of salt flow with base-salt relief is inferred to generate local stress fields that allow synchronous extension and contraction despite closely spaced positions on the margin. On the Lebanese margin, the salt flow over large sub-salt anticlines modulates the rate of basinward translation. In both cases I show that the geometry of the base-salt surface can have an important influence on the orientation and distribution of supra-salt structures.
The seismic-based interpretations are tested using physical analogue models, designed to investigate the effect of salt thickness and heterogeneity on the degree of coupling between sub- and supra-salt structures in experiments with controlled boundary conditions. These results support and integrate the seismic case studies, showing how thinner and more heterogeneous evaporite sequences are more strongly influenced by the base-salt geometry. I conclude that the interaction between salt flow and base-salt relief is a primary control on the structural development of the salt and overburden in gravity-driven systems, and this may explain some of the observed contrast in structural styles between different salt basins.