Gleadow, Prof. Andrew1, McMillan, Malcolm1, Boone, Dr Samuel1, Kohn, Prof Barry1
1School of Earth Sciences, University of Melbourne, Melbourne, Australia
Studies of the apatite fission track (AFT) thermochronology of the SE rifted margin of NSW in the early 1980s were amongst the first to show a relationship between low temperature thermochronology (LTT) and the processes of continental rifting. Similar studies have followed in other parts of the world. Several studies on the SE Australian margin have centered on a transect across the Bega Batholith from Nimmitabel via Bega to Tathra-Bermagui on the coast. Like many rifted margins, this area shows an uplifted plateau of low relief separated by a major erosional escarpment near Brown Mountain from a broad coastal plain of moderate relief to the present Tasman Sea coast.
Apatite fission track ages along this transect fall into three zones, the first on the uplifted plateau with AFT ages of ~270-300 Ma, mean track lengths (MTL) of ~13 µm and unimodal length distributions. The second, from the escarpment towards the coast, with AFT ages of 150-250 Ma and broader, often bimodal, length distributions with means of 10-12µm. The third zone, within ~10 km of the cost, has AFT ages of 90±10 Ma with narrow length distributions and long MTLs of ~14µm. These zones define the original ‘boomerang’ trend of MTL with AFT age where the MTL reaches a minimum for the intermediate ages and represent the progressive replacement of an older age on the plateau (~300 Ma) by a younger cooling event (~90) near the coast representing samples that must have been at temperatures >100°C prior to rapid cooing in the mid Cretaceous. Similar patterns, with some significant variations, have been found on rifted margins in other parts of the world, but are by no means universal.
The tripartite zoning in the AFT data approximately coincides with the principal geomorphic elements of this area which has long suggested that the two records are related. Denudation from around the time of Tasman Sea rifting has been invoked to explain exposure of relatively deeper levels along the coast with much older ages preserved on the plateau. Important corollaries of this interpretation are that the boomerang trend and the youngest ages must always lie below the erosional escarpment, and that the youngest ages should approximately date at least the onset of denudation.
Much larger regional scale AFT data sets now show, however, that the particular relationship between the AFT zones and the escarpment observed around Bega is atypical and probably fortuitous. Further north the boomerang trend diverges inland and crosses the escarpment implying that the underlying thermal history, tied to the ~90 Ma cooling event, must pre-date formation of the escarpment and be independent of the subsequent post-breakup landscape evolution. The minimum of the boomerang trend closely parallels the western margin of the Sydney Basin and we propose that the AFT thermal history relates largely to Cretaceous erosion of a formerly more extensive Permo-Triassic sedimentary cover. Evolution of the present-day landscape is poorly constrained by the thermochronology and may be of substantially younger age, probably Cenozoic.
Andy Gleadow has pioneered fission track thermochronology in Australia for understanding the thermal and tectonic evolution of the continental crust. His work has provided tools that are now used routinely by earth science researchers around the world to extend our knowledge of the age and evolution of earth materials.