Thermal evolution of rifted continental margins: new evidence from fission tracks in basement apatites from southeastern Australia

The Palaeozoic granitic basement of southeastern Australia shows a regular pattern of apatite fission track ages which is dominated by a rapid decrease in apparent age towards the rifted continental margin. Apparent ages away from the margin vary between about 230 and 360 Myr but drop sharply to minimum values of around 80 Myr near the coast just south of the Sydney Basin and 150-175 Myr elsewhere along the coast. Fission track length information allows the age pattern to be interpreted as the result of partial to complete resetting of the older ages by a widespread thermal event at about 80-100 Myr. This event accompanied the rift phase of passive margin development and peaked just before the onset of sea-floor spreading in the adjacent Tasman Sea. The zone of reduced apatite ages is deeply embayed around the Gippsland Basin supporting the suggestion that it originated as a Cretaceous "failed" arm of the Tasman rift system. The thermal event has affected apatites up to about 130 km from the margin and has produced a characteristic set of bimodal track length distributions in apatites with partially reset ages. The bimodality gradually disappears towards the oldest apatite ages which have negatively skewed unimodal distributions. The youngest apatites also tend towards unimodal length distributions indicating that thermal annealing of pre-existing tracks in these samples went almost to completion. Maximum temperatures reached in samples presently exposed in the coastal area were thus probably about 110-120°C. Thermal activity was accompanied by considerable uplift across the region followed by gradual erosion to expose the present land surface. Post-breakup erosion was generally greater towards the new coastline, with near-coastal samples being previously buried a probable 1.5-2.5 km.