Deposition, Extension, and the Shape of Downbuilding Salt Diapirs

Abstract

The authors present a new hypothesis for diapir growth in which deposition and extension interact with coeval salt flow. The hypothesis is based on scaled tectonic experiments in which viscous polymers simulated ductile salt and frictional-plastic, dry sand simulated brittle overburden. The model diapirs all grew by downbuilding beneath accumulating overburdens. Without regional extension, sedimentary differential loading causes salt to rise where the overburden is thinnest. The diapir shape depends on the overburden aggradation rate relative to the flow rate of salt up the diapir conduit. With rapid deposition, strata onlap the diapir, which narrows upward and eventually becomes buried and inactive. With moderate deposition, strata uplap against vertical diapir flanks. With slow deposition, salt spreads by extrusion or shallow intrusion and steeply overrides toplapped strata; the diapir widens upward until salt supply is depleted. With very slow deposition, subhorizontal tongues of salt override toplapped strata. Thin-skinned regional extension initially promotes piercement by tectonic differential loading, but it also impedes upward salt flow by horizontally stretching the diapir. With rapid extension, the diapir sags and is indented by growth faults and overlain by depotroughs. With moderate extension rates, the diapir widens with time and thus narrows upward. Only where extensionmore » is very slow can upward-widening or near-vertical diapir walls form. Changes in rates of aggradation, flow, or extension during diapirism can explain complex geometries, such as inverted teardrop shapes, without invoking shrinking stems. Model results are applied to examples from the Gulf of Mexico and other salt regions.« less