Abstract

The Bushveld Complex induced the development of a unique type of diapir during its intrusion. These diapirs are preserved as large domal structures exposed on the margin and within the northeastern mafic layered sequence. They were produced by gravitational loading and heating of argillaceous and arenaceous floor rocks. Gravity-induced deformation is restricted to rocks above a detachment level that corresponds approximately with the 550 °C peak-metamorphic isotherm and with lithological contacts of contrasting competency. Strongly foliated and boudinaged contact-metamorphosed sedimentary rocks between domes were deformed by lateral extension along flow lines directed toward dome culminations. At the highest structural levels, domes have bulbous geometries and the enclosing layered igneous rocks are deformed into series of outward-verging folds that define a broad rim syncline. Deformation within the domal cores is represented by constriction that produced radial curtain-type folds that have steeply plunging lineations and concentrically oriented folds in the outer shell. Diapirism is closely linked to magma emplacement mechanisms. Floor folds in the country rocks were initiated in the interfinger areas of a fingered intrusion. With the coalescence of intrusion fingers into a single sheet, interfinger folds matured into large diapiric domes that rose to the upper levels of the magma chamber. Strain rates estimated from strain analyses, dome geometry, and model cooling calculations are on the order of 10−14 s−1, corresponding to diapiric uplift rates of 0.6 cm/yr.

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