THE RISE OF DIAPIRS DURING THIN-SKINNED EXTENSION

Grabens above diapirs are generally attributed to the intrusion, withdrawal, or dissolution of salt. In contrast, this paper proposes that many grabens or half-grabens above diapirs form by regional thin-skinned extension. These conclusions are supported by dynamically scaled physical modelling, theoretical reasoning and observations from seismic sections. A diapir pierces a thick, brittle overburden in three evolutionary stages: reactive, active and passive. Regardless of overburden density, diapirs initially pierce a thick overburden reactively in response to faulting during slow regional extension. The hanging wall of an initial fault sinks into the source layer until resisted by increasing pressure forces and bending resistance. New faults form repeatedly nearer the axis of the graben. The dwindling central fault block sinks while the diapir rises below it. 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 slow rate of regional extension: whenever regional extension ceases, reactive diapirs stop growing. If the diapir becomes tall enough, its roof sufficiently thinned, and the graben 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 bypass the reactive and active modes of growth if the overburden is thin and uneven. Faulting, folding and thickness changes are negligible around passive diapirs. Rounded stocks can evolve passively from walls initiated by grabens.