A large
and increasing data base of apatite fission track results from sedimentary and
granitic rocks exposed within the south-eastern highlands and along the
continental margin ofAustralia provides constraints for
the tectonic development of the south-eastern passive margin. The fission track
results suggest that many regions of the south-eastern margin of
Australia experienced distinct
episodes of relatively rapid cooling during the middle Cretaceous to early
Tertiary that can be attributed to the effects of rifting. One cooling episode
is middle Cretaceous in age and related to denudation of the south eastern
highlands from Tasmania to northern New South
Wales between ~90 - 100 Ma. Rocks exposed along the
present coast line record a later episode of rapid cooling, which is diachronous
and decreases in age northward from ~80 to 90 Ma in northern Tasmania to
southern New South Wales, ~60 - 80 Ma in central and northern New South Wales,
and to ~50 - 60 Ma in southern Queensland.
The results are consistent with models suggesting that
underplating occurred inward of the rift along the eastern margin of
Australia as a result of the
onset of continental extension in the
Tasman
Sea at ~96 Ma. This resulted in a significant change in base level
and km-scale denudation of much of the southeastern highlands. Subsequently, as
breakup commenced during the Late Cretaceous at ~80 Ma and continued into the
Paleocene at ~60 Ma, the second stage of cooling was imprinted on the rocks
along the margin. This cooling is probably due to the relaxation of elevated
geothermal gradients associated with rifting as well as some denudation. Because
rifting was oblique to the present-day coast, different regions record different
ages of cooling associated with the rift. The southern regions of
New South
Wales and
Victoria yield ages associated with earlier rifting while
regions of northern
New South
Wales and
southern
Queensland
give ages associated with later
rifting.