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Type: Thesis
Title: Diapirs and diapirism in the Adelaide `Geosyncline’ South Australia
Author: Mount, Trevor Jaunay
Issue Date: 1976
School/Discipline: Dept. of Geology
Abstract: Approximately 180 diapirs define a 500 km belt coincident with the Flinders-Mount Lofty Ranges. New observations on these structures are prefaced by reviews of the concept of diapirism in general and of the history and regional geological setting of the province - the Adelaide 'Geosyncline'. The proposed model for diapirism in the 'Geosyncline' is based on a detailed map of the Arkaba Diapir. Primary control of diapir distribution in the trough can be related to fracture patterns in the pre-source-bed rocks. Outcrop of diapiric material is distinctive over a wide area, and may be accentuated by patterns of vegetation. Weathering is deep and intense but cap-rock or solution megabreccias arc absent. Typical forms are very complex, varying from massifs, domes, dykes, and plugs, shapes that have been controlled by host-rock anisotropy, notably patterns of fracture. Diapir/host-rock contacts are invariably abrupt and coincide with planes of weakness in the host. The contacts, despite sculpting and quarrying by invading diapiric material, can often be matched in 'continental drift' type reconstructions across the cores. Host strata are rarely brecciated or upturned against a diapir, Illustrating the passive nature of the intrusions. Where such deformation occurs, it usually pre-dates diapirism and is due to faulting. Permitted intrusion under local extension in the cover, plausibly induced by regional compression, is implied, Alteration of host-rock adjacent to contacts is absent but for minor dolomitization in certain zones. The intrusive material is an intensely mixed chaotic breccia but one which includes many well rounded and subsphericalxenoclasts, from kilometres across to the finest dust. The size spectrum appears to obey Rosin's Law of Crushing. The breccias, but for rare basement and host-rock xenoclasts, involve a restricted and characteristic range of shallow-marine lithologies including terrigenous clastics, carbonates (especially dolostones), and saline evaporites. This suite may well be assigned to the Callanna Beds of Late Precambrian age. Petrographic studies have revealed a suite of metamorphic minerals, notably carbonate, chlorite, clay, felspar, haematite, magnesioriebeckite, quartz, stilpnomelane, and talc, developed in the core rocks. All mineral components may reasonably have been derived by simple processes entirely from rocks of the type that comprise typical xenoclasts. Many reactions involved dedolomitization and/or saline evaporates. A low pressure, hypersaline, aqueous, oxidative metamorphic environment (zeolite facies) is indicated; replete with C0₂, open-system, and low temperature (150°C-250°C-300°C). Affinities are with natural hydrothermal and geothermal systems. Igneous rocks with a wide range of ages occur in the cores and include both intrusive and extrusive types, mainly basic to intermediate in character. They are essentially xenoclastic but include some in situ post-diapiric intrusions. The occurrence of igneous rock is fortuitous and non-essential to diapirism. Typical breccias have a banded fabric and other features such as the shaping, disruption, mixing, and alignment of xenoclasts that must be attributed to flow. Movement was slow, rather passive, and plug-like, described by non-Newtonian, Andradean law. Mobility of the source material, rather than factors such as density, was paramount to diapirism. The mobility is explained by the former presence of saline evaporites in the interstices of the breccias and by appeal to the concepts of dilatancy, fluidization, and rheidity. The overburden was relatively brittle, its weight the prime driving force to the intrusions. Emplacement was at least partly syn-tectonic, linked to pulses of deformation of the cover, as well as to basement evolution in the 'Geosyncline'. Decollement at the source layer is implied. The host was not explosively breached in the manner of a diatreme; the diapirs are not carbonatites. Although further problems have been outlined, a study of diapirs in the Flinders-Mount Lofty Ranges has clarified many aspects of the global theory. The essence of the new observations is embodied in a proposed classification of intrusions that includes diapirs.
Dissertation Note: Thesis (Ph.D.) -- University of Adelaide, Dept. of Geology, 1976
Keywords: Flinders Ranges diapirs; Arkaba diapir, Thompson Gap diapir; breccia intrusions
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