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

Abstract

The carbonate platforms and adjacent troughs off northeast Australia record a phase of accelerated tectonic subsidence in the late Miocene/Pliocene. Previous attempts to analyse tectonic subsidence in this area were hampered by poor core recovery and dating uncertainties at some of the ODP (Ocean Drilling Program) Leg 133 sites on the Queensland Plateau, in particular at sites 812 and 814. In order to establish a general chronostratigraphy for these sites, we have integrated geophysical logs, biostratigraphic, lithologic, and seismic reflection data to identify and correlate Miocene hardgrounds 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 fluctuating 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 subsidence histories for sites on the Queensland Plateau (811/825, 812, and 814), the Marion Plateau (815) and the Queensland Trough (823) in order to determine regional subsidence patterns. Results of these calculations show that, although the Leg 133 sites are located on a passive margin approximately 1000 km south of the Pacific–Australian plate boundary, they record a greater amount 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 on 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 of the Marion Plateau). These models show that it is difficult to account for observed subsidence, either by means of thrust loading in Papua New Guinea, or by a combination of the latter and in-plane stresses originating from collision along the Australian–Pacific plate boundary. Alternatively, shear wave tomography [van der Hilst, R.D., Kennett, B.L.N., Shibutani, T., 1998. 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 Series 26, pp. 39–57.] displays a NNW–SSE trending band of anomalously high velocities in the upper mantle stretching from the Queensland Plateau to Indonesia. These anomalies likely reflect subducted slab material originating from late Eocene–late Oligocene subduction north of Papua New Guinea. Thus, we suggest that the observed post 9 Ma tectonic subsidence of the Queensland and Marion plateaus and Queensland Trough is largely caused by dynamic surface topography due to Australia's northeastern margin overriding a slab burial ground, modulated by flexural deformation resulting from collision tectonics north of Australia. The observed rates of tectonic subsidence are much slower than those of third-order sea-level changes, and can thus be differentiated from glacial-eustasy.