The Rise and Fall of Diapirs During Thin-Skinned Extension

Grabens overlying diapirs have previously been ascribed to intrusion, withdrawal, or dissolution of salt. We propose, however, that many grabens or half grabens above diapirs form by regional thin-skinned extension of a brittle overburden. This regional extension can initiate and promote piercement of diapiric walls through extremely thick overburdens. This Piercement induced by faulting applies regardless of the overburden density. These conclusions cast doubt on some axioms of salt tectonics and are supported by dynamically scaled physical modeling, theoretical reasoning, and observations from seismic sections.A diapir pierces a thick, brittle overburden in three evolutionary stages: reactive, active, and passive. Diapirs initially grow by reactive piercement, a newly proposed mechanism in which the diapir rises in response to faulting during regional extension. The hanging wall of an initial fault sinks into the source layer until resisted by increasing pressure forces in the source layer and bending resistance in the overburden. New faults form repeatedly nearer the axis of the graben. The dwindling central fault block sinks while the diapir rises below it, regardless of overburden density. Progressively smaller fault blocks are supported by the fluid pressure at progressively higher levels flanking a triangular diapir. Sedimentation keeps diapirs in the reactive stage longer by filling the graben. Reactive diapirism is controlled by the rate of regional extension: whenever regional extension ceases, reactive diapirs stop growing. If the diapir becomes tall enough, its roof sufficiently thinned, and the fault trough deep enough, a diapir can pierce actively by lifting and shouldering aside its roof to emerge rapidly at the surface. During subsequent passive piercement, a diapir widens by regional extension and increases in relief by downbuilding during concurrent sedimentation. Diapirs can also be initiated directly into the passive (thereby bypassing the reactive and active) mode of growth by differential loading if the overburden is thin. Faulting, folding, and thickness changes are negligible around passive diapirs. Rounded stocks can evolve passively from walls initiated by grabens.The same regional extension that 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 salt supply eventually becomes restricted. The formerly rising diapiric crest rapidly transforms into a site of vigorous subsidence and deposition. This site 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. During extreme extension, diapirs subside until they are segmented into relics by indenting crestal grabens. Such grabens can eventually ground onto basement and invert to form mock turtle anticlines. Second-cycle diapirs rise from extrusive allochthonous sheets during fall of the parent diapir. Most 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.