THE FALL OF DIAPIRS DURING THIN-SKINNED EXTENSION

By dynamically scaled physical modelling, theoretical reasoning and observations from seismic sections, it is shown how regional extension, which initiates and promotes the rise of diapirs, can eventually make diapirs fall. During regional extension, diapirs widen between separating blocks of overburden but begin to subside when the salt supply eventually becomes restricted. The formerly rising diapiric crest rapidly transforms into a site of vigorous subsidence and deposition. This is typically a linear or even circular graben that indents the diapir crest, leaving residual horns of salt, which could be misinterpreted as injections into faults. Potential incompatibilities between deformation in the diapir and in its roof are resolved by local modification of fault geometry or by flow of salt along the diapiric wall from depressions into intervening culminations. Turtle structure anticlines with keystone grabens form between subsiding walls. With extreme extension, diapirs subside until they are segmented into relics by indenting crestal grabens. Such grabens can eventually ground onto the basement and invert to form mock turtle anticlines. Second-cycle diapirs rise from extrusive allochthonous sheets during the fall of the parent diapir. Most of the structures produced by diapir fall during regional extension are conventionally attributed to salt dissolution or forceful intrusion; all three possibilities should be evaluated by the criteria discussed here.