Internal Structure of Mushroom-Shaped Salt Diapirs

Mushroom-shaped diapirs have an overhanging bulb fringed by one or more skirts (peripheral pendant lobes), which can curl inward to form vortices capable of entraining cover rocks to various degrees. The highly complex anatomy of mushroom diapirs, some of which have double and eventriple cores, is analyzed in centrifuged and natural diapirs. We conducted 8 centrifuge experiments, which produced more than 100 model diapirs under acceleration equivalent to 1,200 times that of gravity. The experiments were dynamically scaled to U.S. Gulf Coast salt domes, but the qualitative results are also relevant to salt diapirs in other provinces and to granitoid diapirs rising through metamorphic crust. The centrifuged domes grew under overburdens of constant thickness or under aggrading and prograding overburdens, a new experimental approach. Mushroom diapirs are readily recognized in vertical section but less easily in horizontal section. They share three identifying characteristics: the oldest buoyant unit occurs in a peripheral skirt as well as in the diapiric core; the younger evaporites or their immediate cover are folded into the diapir in crescentic patterns in plan view; and most of the internal folds are downward facing.Toroidal circulation in rising diapirs leads to a range of bulb shapes that vary with their maturity and with the contrasts in effective viscosity between the diapir and its surroundings. The resulting shapes comprise external simple mushrooms, external vortex mushrooms, internal simple mushrooms, and internal vortex mushrooms. External simple mushroom diapirs have skirts that infold cover rocks. With increased maturity the skirts can curl inward to form an external vortex entraining cover rocks. In contrast, internal mushroom diapirs have skirts confined entirely within the intrusion. With greater maturity these confined skirts also may curl inward, but they entrain only diapiric material. External mushroom structure results from toroidal circulation of buoyant source and immediate cover having similar effective viscosities. Entrainment of cover by toroidal circulation may be rare in salt diapirs because it appears to require conditions that are realized transiently at certain times and depths in compacting sediments or over long durations in basins where the intruded cover itself contains evaporites, as in Central Iran. Internal mushroom structure results from toroidal circulation confined within the diapir and is probably far more common than external mushroom structure because of isoviscosity within evaporite sequences; we describe natural examples in West Germany and the U.S. Gulf Coast. The internal structure of mushroom salt diapirs elucidates the mechanics of diapiric emplacement and indicates whether an external mushroom shape can be expected and sought by further exploration. Resolving this question is cost effective in exploiting potash ore and vital in the mining engineering of salt caverns. Screens of country rock inclusions entrained into the bulb of an external mushroom diapir could threaten the integrity of a cavern by creating a plumbing system of more permeable rock enclosed in evaporites. In general, engineering within salt structures is likely to be simplest where the evaporites are vertical linear tectonites and more complex where planar fabrics are present.