Meso-cenozoic intraplate magmatism along the Australian southern margin

Abstract

The eastern and south-eastern Australian passive continental margins host a series of Cenozoic basins preserved in onshore and offshore Victoria, Tasmania and South Australia. Deposition in these basins was concurrent with the Cenozoic Magmatic Province that extends along the Australian eastern and south-eastern Australian passive continental margin. Although classified as a ‘non-volcanic’ passive margin, a large record of igneous rocks is preserved within the Cretaceous to Miocene syn- and post-rift successions of the offshore basins. Previous studies have mainly focussed on onshore magmatic activity and resulting geodynamic models have proposed on mantle plumes or edge-driven convection. Offshore 2D and 3D seismic reflection datasets in the Bass and Gippsland Basins analysed in this thesis have shown that the magmatism in these areas occurred during the Late Cretaceous, Eocene to Oligocene and Miocene to Recent times. The majority of magmatism significantly post-dates continental break-up and basin rifting related to the separation of Australia and Antarctica, which started around 85 Ma. This thesis presents major and trace element and isotope geochemistry of Cenozoic igneous rocks in onshore Tasmania and in the offshore Gippsland and Bass Basins. The data presented suggest that magmas have formed over a thermal upwelling with a long time-integrated high 238U/204Pb or μ (HIMU) signature that traversed a Pacific Mid-Ocean Ridge Basalt (MORB) -like asthenosphere and interacted with the mantle lithosphere. Tasmanian lavas formed at different depths with shallow silica-oversaturated melts undergoing larger degrees of melting than deeper silica-undersaturated melts (> 20 kbar). These shallow melts have then mixed with a remnant source of Ferrar Jurassic magma, related to Gondwana break-up, residing in the lithosphere. Magmas formed in the Gippsland and Bass Basins formed under similar conditions as the shallow silica-oversaturated melts with varying Oceanic Island Basalt (OIB) to Upper Continental Crust (UCC) trace element signatures. Regional 3D seismic mapping of the Gippsland Basin reveals a laterally (>40 km) and vertically extensive magmatic plumbing system comprising more than 186 intrusions. This network of sills shallows from the central part of the basin towards the basin-bounding faults at the northern margin, where magmas were ultimately extruded onto the palaeo-surface during the Late Cretaceous. A second style of magmatic activity occurred during the Middle Eocene, resulting in a volcanic cone complex in the centre of the basin, which has likely been fed through vertical to near-vertical dykes or via faults. Palynology of surrounding sediments intersected by petroleum wells indicates that magmatic activity in the Bass Basin is generally younger than that of the Gippsland Basin with activity being most abundant during the Miocene. Cretaceous to Eocene magmatic activity mainly occurred at major normal faults near the basin margins, while Miocene magmatism is focussed in the centre of the Cape Wickham Sub-basin of the Bass Basin. In contrast to the Gippsland Basin, the main direction of magma transport through the upper crust was more vertically through dykes and/or faults. This phase of Miocene magmatic activity is characterised by a southward younging trend similar to the southward younging trends of the hotspot trails observed on the Australian mainland. Although the Gippsland and Bass Basin are adjoining basins, the magmatic plumbing styles observed differ significantly. These results provide insights into the origin, cause and plumbing into intraplate magmatism occurring along the Australian south-eastern margin and magma transport through sedimentary basins, in general.