Fission track thermotectonic imaging of the Australian continent

Abstract

Fission track analyses of apatites from a very large data set across Australia provide a first look at the patterns of low-temperature thermochronology over an entire continent. The rock samples are mostly of granitic rocks, or their metamorphic equivalents, of Palaeozoic or older age from the exposed basement regions. Sample chemical analyses indicate that the apatite suite is overwhelmingly of fluorapatite composition. Approximately 2750 fission track analyses have been completed, of which >1700 are of sufficient quality to form a coherent data set that can be interpolated to show the variation of central fission track age and mean confined track length on a continental scale. The resulting images exhibit some features which are well known, such as the trend towards young apatite ages along the eastern and southeastern rifted continental margins, but others that are more surprising, such as the lack of clear differentiation, in terms of the range of ages and lengths, between the older Precambrian cratonic areas in the west, and younger Phanerozoic mobile belts to the east. The Precambrian rocks from the western two-thirds of the continent do, however, show distinctly different cooling histories to those in the eastern Phanerozoic mobile belts when the relationships between track lengths and fission track ages are considered. The western craton everywhere shows patterns of prolonged slow cooling, whereas all major regions of the eastern part of the continent show discrete episodes of rapid cooling, mostly from the Jurassic to the Palaeogene. Significant areas of unusually young apatite ages (<50 Ma) are found in Tasmania and in Precambrian rocks from the northern Gawler Block in South Australia. The most obvious regional cause of the overall fission track patterns across Australia is variation in surface denudation over time scales of hundreds of Ma. However, in some areas, such as the northern Gawler Block, other mechanisms such as the movement of hydrothermal fluids in former cover sequences must be invoked.