Initiation of diapirs is one of the least understood aspects of salt tectonics. Differential sedimentary loading or erosion are both effective but not universal. A survey of 18 major salt-diapir provinces shows that salt upwelling is closely linked in time and space to regional extension. Extended salt basins typically develop salt structures, whereas nonextended basins typically do not. Even in salt basins overprinted by inversion or orogenic contraction, the diapirs were initiated during extension on divergent continental margins or in intracontinental rifts.
Regional extension thins brittle overburden by forming grabens and half grabens above flowing salt. These fault structures (1) differentially load the salt by their surface relief and (2) weaken the overburden by fracturing and thinning it. Diapiric walls of pressurized salt rise in reaction to the shitting positions of fault blocks in extending overburdens, regardless of thickness, density, or lithology. If regional extension stops, these reactive diapirs stop rising. Eventually the roof of the reactive diapir can be thinned by extension below a critical thickness. Only then can the diapir break through actively as an independent intrusion.
Diapiric alignments have been ascribed to basement faulting, even where such faults were conjectural or had trivial displacements. Physical modeling shows that extension of the basement has only indirect influence on diapirism by creating space for extension of the overburden, which is the direct cause of diapirism, whether extension is thick-skinned or thin-skinned and whether the salt was deposited before, duing, or after rifting.