A combination of apatite fission track and (U-Th)/He thermochronometers to constrain the escarpment evolution in south eastern Australia: a case study of high elevation passive margins.
PhD thesis, University of Glasgow.
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In this project apatite fission track and (U-T)/He thermochronometers are used to determine the cooling history of rocks from the coastal (south eastern New South Wales) and the interior (Bathurst area) regions of the eastern Australia high elevation passive margin. Two traverses across the coastal lowlands, escarpment and plateau top are used to determine the tempo and styles of response of the landscape to the continental breakup and sea-floor spreading of the Tasman Sea (85 Ma). The three prevailing models of escarpment evolution namely retreat into a downwarped rift shoulder, escarpment retreat and excavation in place on a high elevation rift shoulder with flexural rebound are described and tested using a previously untested combination of apatite fission track and (U/Th)/He data. The thermochronological data indicate that the coast was affected by a denudational pulse that peaked around 120-100 Ma and that was extinguished by the time of sea-floor spreading. The rapid denudational event caused the removal of 3-4.5 km (depending on the geothermal gradient) of crust at the coast and of approximately 2 km at the present base of the escarpment. The thermochronological data are inconsistent with the downwarped rift shoulder model and the apatite (U-Th)/He data indicate that, while the coast was denuded very rapidly, the coastal lowlands were excavated in place at a much lower pace, and the escarpment reached its present position no later than 60 Ma. This suggests that during continental extension and breakup, rates of denudation at the coast were approximately 80-30m/Myr (depending on the geothermal gradient), whereas at the base of the present escarpment they were about 10-5 m/Myr. The period after sea-floor spreading was characterised by stability and low rates of erosion. The pre-breakup topography, reconstructed using the backstacking technique, is characterised by a considerable relief in the area of the present escarpment. This result confirms the hypothesis that the escarpment evolved pinned to its present position.
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