Fission track thermotectonic imaging of the Australian continent

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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.

Introduction

From the earliest studies of continental terranes using apatite fission track thermochronology, it has been apparent that fission track data show broad regionally consistent patterns of variation. These are related to the low temperature thermal history of the crust and frequently have little or no relationship to the original formation ages of the rocks involved (e.g. Gleadow and Lovering, 1978a, Gleadow and Lovering, 1978b). There has been a growing awareness that regional fission track age (and track length) patterns are the result of cooling in the near-surface environment due to the interaction of surface processes and underlying tectonics (e.g. Brown et al., 2000, Gallagher et al., 1998, Gleadow and Brown, 2000, Hill and Kohn, 1999). It has also become clear that low-temperature thermochronology is providing a window into upper crustal processes which are often not discernable by other geochronological methods. However, the significance of these regional patterns is not always obvious and there have often been difficulties in interpreting and integrating the results of such studies with other sets of geological observations.

In southeastern Australia, there has been a progressive elaboration of the apatite fission track age patterns Moore et al., 1986, Dumitru et al., 1991, Kohn et al., 1999, O'Sullivan et al., 1999, O'Sullivan et al., 2000a which has revealed an increasingly detailed structure to the variation of fission track parameters both laterally and vertically in this region. The results have been interpreted in terms of a regional late Palaeozoic cooling over the area modified by later cooling events associated with continental rifting and breakup on the eastern and southern margins. Over much of the rest of Australia, however, only very scattered fission track data have been available and large areas have had no coverage at all. This project was initiated within the Australian Geodynamics Cooperative Research Centre (AGCRC)1 to provide the first reconnaissance coverage of the low-temperature thermochronology of the entire continent. In practice, this has focused on the exposed basement crystalline rocks and in some areas limited existing data sets have been upgraded to provide a comparable standard to more recent measurements.

The overall aim of the project was to produce a regional coverage of apatite fission track data across an entire continent in a form which could be combined and compared with other continental-scale data sets, such as heat flow and digital topography. This approach can provide new interpretations of the thermal and tectonic evolution of the Australian continent, at least during that part of the Phanerozoic likely to be recorded through the low-temperature apatite fission track system. The increasingly large data set derived from this study can be used in novel ways to image the evolution of the upper part of the continental crust and reconstruct the denudation history of the land surface. This possibility is explored in another paper (Kohn et al., 2002), which includes the results of thermal history modelling of the data. The purpose of this paper is to present a summary of the entire data set and examine its major qualitative features.

Section snippets

Sampling strategy

Samples were collected from exposed basement terranes over all of mainland Australia, as well as Tasmania and some offshore islands which are part of the continuous continental crust. Samples from Papua New Guinea such as those reported by Hill and Raza (1999), although also part of the Australian continental block, have not been included here. Samples collected were mostly granitic rocks, granitic gneisses and related lithologies. In some terranes samples included various metamorphic and

Regional patterns of fission track age and length

Detailed accounts of various subsets of the analyses included in this study have been published in a number of regional studies by Dumitru et al. (1991), Foster and Gleadow, 1992, Foster and Gleadow, 1993, Foster et al. (1994), Gleadow and Lovering, 1978a, Gleadow and Lovering, 1978b, Gleadow and O'Brien (1994), Hill and Kohn (1999), Kohn et al. (1999), Marshallsea et al. (2000), Mitchell et al. (1998), Moore et al. (1986), O'Sullivan et al., 1995, O'Sullivan et al., 1996, O'Sullivan et al.,

Relationships between fission track ages and lengths

The relationship between track length and fission track age can give insights into the underlying cooling trends responsible for variations within a regional fission track data set. Plots of mean track length against fission track age are often termed ‘boomerang plots’ because of the distinctive concave-upwards trend that is produced in rocks from some cooling environments. This ‘boomerang’ trend was first observed by Moore (1982) and the principles underlying its formation were described by

Conclusions

This study has produced the first broad-scale coverage of the low-temperature thermochronology of the entire Australian continent. In practice, the study has focused on the exposed basement regions and ignored regions of sedimentary basin cover. The samples analysed give a new perspective on the low-temperature thermal history of rocks that are now at the Earth's surface. Regardless of the age of the parent rocks, all of the observed fission track ages post-date the period of formation of the

Acknowledgments

Different parts of this work have been generously supported by the Australian Research Council and the Australian Geodynamics Cooperative Research Centre (Project 2005LO). Neutron irradiations required for the fission track measurements have been supported over many years by grants from the Australian Institute of Nuclear Science and Engineering. Various members of the Melbourne Fission Track Research Group have contributed to development of the Australian fission track data base and

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