The South Atlantic and Gulf of Mexico conjugate-margin salt basins display similar relationships between crustal architecture and presalt and salt sequences. 3D and 2D depth-migrated seismic data reveal that: (1) the base salt is mostly smooth but drops into an outer marginal trough just landward of and deeper than oceanic crust; (2) the Moho climbs basinward to shallow depths and is largely absent below the trough; (3) faults are low-angle and asymmetrical beneath the smooth base salt but steeper and symmetrical in the trough; (4) the trough contains triangular highs between faulted lows; and (5) the smooth base salt is underlain by sag sequences that often dip and thicken basinward. The observations suggest that these salt basins shared a common evolution. Crustal faulting gradually shifted basinward. Consequent thermal/loading subsidence plus lower-crustal thinning generated basinward-shifting accommodation for sag sequences, but slow sedimentation relative to subsidence resulted in deep depressions. A switch to symmetrical boudinage of thinned crust created the troughs and possible reactive mantle diapirs. Evaporites formed during this stage, with deposition near sea-level in proximal positions but 2–3 km deep in basin centres. Oceanic spreading separated the salt into conjugate basins, with allochthonous flow out over oceanic crust.