Post-rift magmatism on the continental shelf of the Otway Basin and implications for the igneous plumbing systems in sedimentary basins

Various types of igneous rocks have been frequently encountered during hydrocarbon exploration of sedimentary basins along rifted passive margins, and their impacts on hydrocarbon maturation and migration processes are diverse. Buried volcanoes generally act as migration pathways, whilst magma intrusions are often associated with source rock maturation. Importantly, the high acoustic velocity and density of igneous intrusions, introduces imaging problems. Therefore, understanding the igneous plumbing styles within the sedimentary basins, remained crucial for hydrocarbon exploration for frontier basins. Combining the seismic reflection surveys with magnetic datasets, in this study, we identified six intrusive sills, seven lava flows and feeder dykes beneath them. Thirty cones to trapezium-shaped and twenty-seven dome to eye-shaped mounds are also identified and based on their magnetic and seismic geomorphic attributes, are interpreted as monogenetic volcanoes and hydrothermal vents, respectively. The igneous complexes were emplaced between mid-Eocene to mid-Miocene, while the hydrothermal vents are mainly of mid-Eocene. Most of the volcanic/hydrothermal vents are linked to the underlying rift related faults via vertical zones of disruption, which are interpreted as feeder dykes or hydrothermal fluid pipes, respectively. Dyke-dominated volcanoes and scattered distribution of sills, contrasts to most of the recent studies that highlight the importance of the lateral magma transportation in sedimentary basins. Our comparison with the neighbouring Bight Basin, suggests that the thin crust but relatively thick sediment layers of the Bight Basin, facilitated the lateral transportation of magma and promoted sill-fed volcanism, while the magma ascending through thick crust but relatively thin sediment layers of the Otway Basin, produced sufficient magma pressure and promoted the predominantly dyke-fed igneous plumbing system. This work highlights the critical role of basin structures, such as continental crust, sedimentary thickness, and faults, in controlling the distribution, morphology and style of post-rift igneous plumbing processes in magma-poor margins.