The Origin of Meso-Cenozoic Offshore Magmatism Along the Australian Southeastern Continental Margin: New Insights From Seismic and Geochemical Data

Abstract

Offshore magmatism along Australia's south-eastern continental margin forms part of a vast area of intraplate magmatism of which the origin and emplacement has confounded geologists for decades. Often overlooked, the offshore component of this intraplate basaltic province has been proven extensive and represents a largely undescribed record of Meso- and Cenozoic magmatic activity. It remains preserved within sedimentary successions of the Bight, Otway, Bass and Gippsland rift basins. Although sedimentary basins along Australia's south-eastern continental margin have originated as rift basins related to Gondwana break-up, the magmatic activity they contain is significantly younger than continental break-up of the basins. For example, continental break-up initiated at 83 Ma in the Bight Basin, while seismic reflection surveys indicate Middle Eocene magmatic activity in the central Ceduna Sub-basin, correlating with accelerated seafloor spreading and major changes in global tectonics. Progressing towards the east, the Otway Basin has undergone break-up around 67 Ma and near continuous magmatic activity with large volumetric peaks at 57–42 Ma and 5–0 Ma. Similar peaks of activity have also been recognized onshore Victoria as the Older and Newer Volcanic Provinces. In contrast to the Bight, Otway and Gippsland basins, the Bass Basin has not seen continental break-up, but preserves a vast amount of magmatic features nonetheless. These features entered the basin in several phases, which can be correlated to magmatic phases observed onshore Tasmania. A first Mid-Cretaceous rift phase was followed by a latest Maastrichtian-Palaeocene phase and a third and more extensive Oligocene to Miocene phase, which correlates with a volumetric peak of magmatism at 30–20 Ma, also present onshore Tasmania. Further towards the east, the Gippsland Basin originated as a failed rift arm of the Tasman Sea with break-up initiating at 80 Ma. This was followed by extensive Campanian magmatism along the major northern and southern fault systems and mid-Eocene intrusions nearby the Bream Field. We suggest that the distribution of igneous activity cannot be solely explained by the classic plume model proposed for Cenozoic magmatism in eastern Australia. Unravelling the magmatic signal remains a key challenge and calls for an amalgamation of lithospheric processes, such as stretching and edge convection.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015