Craig Magee , Christopher Aiden-Lee Jackson 

Dyke swarms are common on Earth and other planetary bodies, comprising arrays of dykes that can extend laterally for 10’s to 1000’s of kilometres. The vast extent of such dyke swarms, and their presumed rapid emplacement, means they can significantly influence a variety of planetary processes, including continental break-up, crustal extension, resource accumulation, and volcanism. Determining the mechanisms driving dyke swarm emplacement is thus critical to a range of Earth Science disciplines. However, unravelling dyke swarm emplacement mechanics relies on constraining their 3D structure, which is difficult given we typically cannot access their subsurface geometry at a sufficiently high enough resolution. Here we use high-quality seismic reflection data to identify and examine the 3D geometry of the newly discovered Exmouth Dyke Swarm, and associated structures (i.e. dyke-induced normal faults and pit craters). Dykes are expressed in our seismic reflection data as ~335–68 m wide, vertical zones of disruption (VZD), in which stratal reflections are dimmed and/or deflected from sub-horizontal. Borehole data reveal one ~130 m wide VZD corresponds to an ~18 m thick, mafic dyke, highlighting that the true geometry of the inferred dykes may not be fully captured by their seismic expression. The Late Jurassic dyke swarm is located on the Gascoyne Margin, offshore NW Australia and contains numerous dykes that extend laterally for >170 km, potentially up to >500 km, with spacings typically <10 km. Although limitations in data quality and resolution restrict mapping of the dykes at depth, our data show they likely have heights of at least ~3.5 km. The mapped dykes are distributed radially across a ~39° wide arc centred on the Cuvier Margin; we infer this focal area marks the source of the dyke swarm. We demonstrate seismic reflection data provides unique opportunities to map and quantify dyke swarms in 3D. Because of this, we can now: (i) recognise dyke swarms across continental margins worldwide and incorporate them into models of basin evolution and fluid flow; (ii) test previous models and hypotheses concerning the 3D structure of dyke swarms; (iii) reveal how dyke-induced normal faults and pit craters relate to dyking; and (iv) unravel how dyking translates into surface deformation.