WHY DOES SALT START TO MOVE

This paper concerns mechanisms of salt (and ductile shale) movement. I t investigates salt flow due to differential loading, folding of the o verburden during compression and drag by a moving overburden. The appr oach is to compare the salt flux caused by these processes to that gen erated by buoyancy. It is demonstrated that overburden folding and dra g 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 duri ng the later stages of salt diapir and salt wall growth when buoyancy dominates. The quantitative significance of these alternatives to buoy ancy 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 conditi ons, these mechanisms produce more salt movement than buoyancy. Differ ential loading dominates when the surface slopes become more than a sm all fraction of the slope of the salt top. Overburden buckling dominat es if the in-plane stress exceeds a critical value. Drag dominates whe n the salt layer is thinner than a few hundred metres. (2) The strengt h of the overburden inhibits formation of salt diapirs, even those due to buoyancy, on wavelengths less than about 12 km.