REGIONAL EXTENSION AS A GEOLOGIC TRIGGER FOR DIAPIRISM

Initiation of diapirs is one of the least understood aspects of salt t ectonics. Differential sedimentary loading or erosion are both effecti ve but not universal. A survey of 18 major salt-diapir provinces shows that salt upwelling is closely linked in time and space to regional e xtension. Extended salt basins typically develop salt structures, wher eas nonextended basins typically do not. Even in salt basins overprint ed by inversion or orogenic contraction, the diapirs were initiated du ring 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) diffe rentially load the salt by their surface relief and (2) weaken the ove rburden by fracturing and thinning it. Diapiric walls of pressurized s alt rise in reaction to the shifting positions of fault blocks in exte nding overburdens, regardless of thickness, density, or lithology. If regional extension stops, these reactive diapirs stop rising. Eventual ly the roof of the reactive diapir can be thinned by extension below a critical thickness. Only then can the diapir break through actively a s an independent intrusion. Diapiric alignments have been ascribed to basement faulting, even where such faults were conjectural or had triv ial displacements. Physical modeling shows that extension of the basem ent has only indirect influence on diapirism by creating space for ext ension of the overburden, which is the direct cause of diapirism, whet her extension is thick-skinned or thin-skinned and whether the salt wa s deposited before, during, or after rifting.