Long-term thermal consequences of tectonic activity at Mount Isa, Australia: Implications for polyphase tectonism in the Proterozoic

Abstract

AbstractMount Isa is a Palaeo-Mesoproterozoic terrane in Northern Australia characterized by >300 Ma of episodic tectonic activity prior to effective cratonization. This tectonic activity has resulted in dramatic changes in the heat production distribution in the crust and must have been accompanied by long-term changes in thermal regimes. Primary differentiation of crust initially enriched in heat producing elements has been achieved by felsic magmatism over much of the 300 Ma history, often associated with extensional deformation. The flux of heat producing elements from lower to mid-upper crustal levels associated with this magmatism was sufficient to cause long-term lower crustal cooling of at least 200°C. The accumulation of the radiogenic intrusives (which comprise c. 230f surface outcrop and have heat production rates averaging 5.2 µWm−3) in the mid-upper crust resulted in a highly stratified heat production distribution. One consequence of this distribution is that small changes in the depth to this heat production, through processes such as deformation, erosion and the deposition of sediments, lead to significant changes in deep crustal temperatures (up to 100°C) and consequently lithospheric strength. These considerations suggest that the long-term evolution of the Mount Isa region partly reflects the progressive concentration of heat-producing elements in the upper crust leading to a long-term increase in lithospheric strength, and eventually to effective cratonization. The long-term cooling and strengthening trend was locally countered by the role of subsidence during basin formation which, through burial of heat producing elements in the existing crust and the accumulation of more heat production in insulating sediments, helped to localize subsequent contractional deformation.