Many passive margins have deep-water, contractional fold belts that formed above salt or shale. Margin failure, accommodated by proximal extension and distal shortening, is caused by some combination of gravity gliding above a basinward-dipping detachment and gravity spreading of a sedimentary wedge with a seaward-dipping bathymetric surface. Gravitational failure is inherently self-limiting, and sedimentation patterns provide fundamental control of deformation. Continued shortening is driven primarily by shelf and upper-slope deposition, which maintains the bathymetric slope and the gravity potential, and by increased basinward tilting. Deformation is retarded or halted by distal thickening of the overburden caused by the folding itself or by lower-slope and abyssal sedimentation. Net shortening amounts and deformation rates are lower than in collisional/accretionary fold belts, because the driving forces are weaker than those induced by lithospheric plate motions.

Structural styles vary but depend largely on the nature of the dcollement layer, not the driving forces. Fold belts detached on shale typically comprise basinward-vergent thrust imbricates and associated folds because of the relative strength and frictional behavior of the plastic shale. Deformation does not occur until there is sufficient overburden, and it is facilitated by high fluid pressures. In contrast, salt is a viscous material with essentially no strength, which leads to symmetrical detachment folds and early deformation beneath only a thin overburden. Moreover, the surface slope can be reduced by proximal subsidence into salt and distal inflation of salt, and much of the shortening can be accommodated by lateral squeezing of diapirs and salt massifs and by extrusion of salt nappes.