Why does salt start to move?

This paper concerns mechanisms of salt (and ductile shale) movement. It investigates salt flow due to differential loading, folding of the overburden during compression and drag by a moving overburden. The approach is to compare the salt flux caused by these processes to that generated by buoyancy. It is demonstrated that overburden folding and drag by the overburden can, under commonly encountered conditions, result in greater amounts of salt movement than that produced by buoyancy or differential loading. These conclusions apply during the early stages of salt anticline, salt pillow and salt roller formation but not during the later stages of salt diapir and salt wall growth when buoyancy dominates. The quantitative significance of these alternatives to buoyancy is determined by considering an elastic plate overlying a viscous fluid. This is the simplest mathematical model that can reproduce the processes considered. The model shows that: (1) Under certain conditions, these mechanisms produce more salt movement than buoyancy. Differential loading dominates when the surface slopes become more than a small fraction of the slope of the salt top. Overburden buckling dominates if the in-plane stress exceeds a critical value. Drag dominates when the salt layer is thinner than a few hundred metres. (2) The strength of the overburden inhibits formation of salt diapirs, even those due to buoyancy, on wavelengths less than about 12 km.