Late Tertiary tectonic subsidence on the northeast Australian passive margin: response to dynamic topography?

The carbonate platforms and adjacent troughs off northeast Australia record a phase of accelerated tectonic subsidence in the late Miocene/Pliocene. P revious attempts to analyse tectonic subsidence in this area were hampered by poor core recovery and dating uncertainties at some of the ODP (Ocean Dr illing Program) Leg 133 sites on the Queensland Plateau, in particular at s ites 812 and 814. In order to establish a general chronostratigraphy for th ese sites, we have integrated geophysical logs, biostratigraphic, lithologi c, and seismic reflection data to identify and correlate Miocene hardground s on the Queensland Plateau to third-order eustatic sea-level lowstands on the Haq et al. [Haq, B.U., Hardenbol, J., Vail, P.R., 1987. Chronology of f luctuating sea levels since the Triassic. Science, Vol. 235, pp. 1156-1167] curve. This enables us to estimate ages where biostratigraphic information is poor. We have used sediment backstripping to compute tectonic subsidenc e histories for sites on the Queensland Plateau (811/825, 812, and 814), th e Marion Plateau (815) and the Queensland Trough (823) in order to determin e regional subsidence patterns. Results of these calculations show that, al though the Leg 133 sites are located on a passive margin approximately 1000 km south of the Pacific-Australian plate boundary, they record a greater a mount of subsidence than can be predicted from simple elastic models (post 9 Ma subsidence of 1300 +/- 200 m in the Queensland Trough, 650 +/- 200 m o n the western margin of the Queensland Plateau, post 5 Ma subsidence of 500 +/- 30 m on its southern margin, and 660 +/- 50 m on the northern margin o f the Marion Plateau). These models show that it is difficult to account fo r observed subsidence, either by means of thrust loading in Papua New Guine a, or by a combination of the latter and in-plane stresses originating from collision along the Australian-Pacific plate boundary. Alternatively, shea r wave tomography [van der Hilst, R.D., Kennett, B.L.N., Shibutani, T., 199 8. Upper mantle structure beneath Australia from portable array deployments . In: Brown, J., Dooley, J., Goleby, B., van der Hilst, R., Klootwijk, C. ( Eds.), Structure and Evolution of the Australian Continent. Geodynamics Ser ies 26, pp. 39-57.] displays a NNW-SSE trending band of anomalously high ve locities in the upper mantle stretching from the Queensland Plateau to Indo nesia. These anomalies likely reflect subducted slab material originating f rom late Eocene-late Oligocene subduction north of Papua New Guinea. Thus, we suggest that the observed post 9 Ma tectonic subsidence of the Queenslan d and Marion plateaus and Queensland Trough is largely caused by dynamic su rface topography due to Australia's northeastern margin overriding a slab b urial ground, modulated by flexural deformation resulting from collision te ctonics north of Australia. The observed rates of tectonic subsidence are m uch slower than those of third-order sea-level changes, and can thus be dif ferentiated from glacial-eustasy. (C) 2000 Elsevier Science B.V. All rights reserved.