The shallow depth emplacement of mafic intrusions on a magma-poor rifted margin: An example from the Bight Basin, southern Australia

Three-dimensional (3D) seismic data has provided important insights into how magma is transported and stored in sedimentary basins. Much of our understanding is based on volcanic-rifted margins. In contrast, the magma plumbing systems of magma-poor rifted margins are less well studied. This study uses 3D seismic data to describe the magma plumbing system of the Bight Basin Igneous Complex; a volcanic province located along the magma-poor southern Australian margin. Here, magma emplacement occurred in the Mid Eocene, some 40 million years after the onset of seafloor spreading. Our data images a variety of 5-270 m thick, 2-23 km diameter mafic intrusions that are confined to within 1200 m of the paleoseabed. Intrusions emplaced at ≤200 m depth formed hybrid and compound sills and are typified by elongate, lava flow-like morphologies. Intrusions emplaced at depths of 200-1200 m formed saucer-shaped and compound sills, as well as laccoliths. Approximately 60% of the intrusions have lava flows and volcanogenic vents above their shallowest tips, as shown from previous studies. This suggests that the intrusions played a crucial role in transporting magma to the paleoseabed. Furthermore, many of the intrusions are overlain by forced folds, highlighting the role of magma emplacement in inducing overburden deformation. The sills and laccoliths rarely form interlinked complexes and instead most commonly occur as isolated bodies. This suggests that they were fed by dykes. We infer that high rates of magma ascent in the dykes prevented their transition into sills within sediments at >1.2 km depth. Our study highlights that the magma plumbing system of the Bight Basin Igneous Complex contains a diversity of magmatic intrusions, the morphology of which is linked to their emplacement depth, host sediment rheology and the physical properties of the magma. This plumbing system contrasts markedly with those found along better-studied volcanic rifted margins.