| Summary: | The Proterozoic Mary Kathleen Fold Belt is one of the most intensely deformed fold belts within the Mt Isa Inlier (northwest Queensland), and contains key evidence for both extension and shortening, as well as a protracted thermal history culminating in a high temperature, low pressure amphibolite facies metamorphism. An early phase of extension (1780-1730 Ma) may have initially been responsible for basin development, and was subsequently associated with intense ductile shearing and multiple magma injection in a lower plate, and minor folding, extensional fracturing and emplacement of discrete intrusions in a more brittle upper plate. The subsequent regional deformation (D) and metamorphism (1600-1500 Ma) involved east-west directed compression, locally producing a variety of fold interference patterns by overprinting of F folds. This deformation resulted in tight folding of the early zone of high D strains into a large anticlinorium (the Wonga Belt). East of this zone, the effects of D dominate. The peak of regional amphibolite facies metamorphism (550-650°C, 300-400 MPa) was synchronous with the F2 folding event. Retrogression (and F deformation) involved initial isobaric cooling, in common with some other Australian and Antarctic Proterozoic terranes. The belt is thus dissimilar in many regards to modern fold belts developed during plate collisional tectonics. The two-phase history of extension and shortening, with both phases involving elevated geothermal gradients, is explained in terms of a mantle-plume model in an intra-cratonic setting. The extension is inferred to be a consequence of mantle-plume ascent, with subsequent shortening due to thermal decay as the plume subsided. Elevated geotherms in the shortening phase may have been generated by magma underplating at the base of the lithosphere, delamination of the lithosphere, upward magma migration, and/or redistribution of radiogenic elements by magmatism. The general similarity of tectono-thermal styles and synchroneity of radiogenic age groupings across many Australian mid-Proterozoic regions suggests that this proposed two-phase process may have been an important factor in Proterozoic crustal evolution.
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