Cretaceous Palaeogeography of Eastern Australia: Connecting the Deep Earth to Surface Processes

Abstract

We have used the geodynamic modelling software CitcomS 3.0 to model the surface evolution of Australia since 140 Ma and constrain the location of the Cretaceous aged subduction zone that paralleled its eastern margin. Australia’s palaeogeography was profoundly affected by mantle convection processes during the Cretaceous. Eastward passage of the Australian plate over subducted slab material induced negative dynamic topography in eastern Australia, causing widespread time-dependent subsidence and formation of a vast epeiric sea during a eustatic sea-level low. Although there exists a considerable amount of geological evidence for active convergence between Australia and the palaeo-Pacific at this time, the exact location of the subduction zone has remained elusive. To constrain the location of subduction we tested two end-member models, one with the subduction zone directly adjacent to the continent, and an alternative model with subduction translated 23° east. Our forward geodynamic models incorporate a rheological model for the mantle and crust, plate motions since 140 Ma and evolving plate boundaries, implemented in the GPlates software. While mantle rheology affects the magnitude of surface vertical motions, the timing of uplift and subsidence depends critically on plate kinematic reconstructions and plate boundary geometries. Tectonic subsidence analysis using the backstrippping method was performed on 42 wells from the Eromanga and Surat basins in eastern Australia. This revealed Cretaceous tectonic subsidence trends with which to compare our modelled dynamic topography. Simulations with subduction proximal to the active continental margin resulted in accelerated basin subsidence delayed by 20 Myr compared with these tectonic subsidence data. However this timing offset was reconciled when subduction was shifted eastward. Comparisons between whole mantle seismic tomography images and equivalent model temperature cross-sections further validate our proposed eastward shift in subduction. Finally an absence of subduction zone volcanism along Australia’s east coast in the Early Cretaceous supports our conclusion that a back-arc basin existed east of Australia during the Cretaceous. Our models further allowed us to test alternative Tertiary plate boundary geometries east of Australia, in particular whether or not the proposed short-lived mid-Tertiary eastward dipping "New Caledonia subduction zone" may have been responsible for a prominent fast shear wave anomaly at ~1100 km depth beneath the Tasman Sea. Our models suggest that post 45 Ma westward dipping subduction along the Tonga-Kermadec Trench may have produced the slab material mapped by mantle tomography models in the lower mantle underneath the Tasman Sea. An additional eastward dipping subduction zone does not appear to be required by the tomographic images, as proposed previously.