LOW-TEMPERATURE THERMOCHRONOLOGY AND DENUDATION MODELLING OF THE AUSTRALIAN CONTINENT

Apatite fission track thermochronology has provided information on the thermal history of rocks in the low-temperature environment of the upper few kilometres of the continental crust (below about 110°C). It is increasingly clear that the results obtained by this method are predominantly related to patterns of denudation at the Earth’s surface. Very large fission track data sets are now enabling reconstructions of thermal and denudation histories to be made on a scale up to that of whole continents, and on timescales up to hundreds of Ma.

In Australia, nearly 3000 fission track analyses of apatites from surface outcrop samples, mostly of granitic composition, have been obtained from exposed basement terranes. Modelled thermal histories enable estimates to be made of palaeotemperatures at various times in the past and the amount of surface denudation, assuming that cooling is dominated by denudation, and that thermal gradients are known. In turn, estimates of the amount of removed section can be ‘backstacked’ onto a present-day digital topographic model, and isostatically adjusted, to provide a first-order view of the evolution of palaeotopography through time.

Some aspects of the resulting interpolated fission track images are well-known, such as the general decrease in apatite age towards rifted margins in the East, while others are rather unexpected, such as the lack of clear differentiation between the older cratonic areas in the west and younger mobile belts to the east. Regardless of the age of the parent terrane, virtually all of the observed low-temperature thermal histories post-date the period of maximum continental assembly in the Late Palaeozoic, often considerably. In a number of regions in southern and eastern Australia, evidence of young thermal influences, apparently not related to surface denudation, can be seen. Tasmania shows a markedly different cooling history to the mainland. Comparable patterns are now emerging from other continents where large data sets are becoming available, with major implications for the long-term denudation history of these regions.