3D seismic reflection constraints on the emplacement of mafic laccoliths and their role in shallow crustal magma transport: A case study from the Ceduna sub-basin, Great Australian Bight

Magma plumbing systems transport and store magma in the subsurface, as well as playing an important role in feeding eruptions and inducing overburden deformation. Whilst the important role of sills and dikes in these systems is well documented, the three-dimensional (3D) morphology of mafic laccoliths and their role in magma transport and eruption is less well understood. This is due to a paucity of well-documented examples from 3D seismic reflection data, and the limited exposure provided by field examples. Here we use 3D seismic reflection data to document for the morphology and emplacement of a multi-lobed laccolith of inferred mafic composition (the Hammerhead Laccolith (HHL)). This middle Eocene-age intrusion was emplaced at <320 m beneath the paleosurface within the intraplate Bight Basin Igneous Complex located offshore southern Australia. The HHL is composed of multiple convex-upwards, flat-based, laccolith-shaped lobes between 5 and 270 m thick and 2–14 km long. Inflation of the lobes resulted in the formation of a ≤150 m high, compound forced fold at the paleosurface, whose morphology mirrors that of the underlying laccolith. Subsequent drainage of magma from the HHL to feed small-volume lava flows resulted in the formation of collapse faults which focus on the margin of a magma reservoir, similar to those associated with calderas. To the best of our knowledge, this is the first study using 3D seismic reflection data to document the morphology of a shallow mafic laccolith and its relationship to its erupted products, whilst the HHL is notable for possessing characteristics of both classic laccoliths and shallowly emplaced, multi-lobate mafic sills. We highlight the important role of laccoliths in magma plumbing systems, and their potential for inducing both brittle and ductile host-rock deformation. Finally, our detailed mapping of the HHL and its associated forced fold provides a valuable exemplar that may assist the identification of mafic laccoliths using 3D seismic reflection data in energy-rich sedimentary basins.