Abstract. Dyke swarms are common on Earth and other planetary
bodies, comprising arrays of dykes that can extend laterally for tens to
thousands 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 that 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 that this focal area marks the source of the dyke swarm. We demonstrate
that seismic reflection data provide 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.
Seismic reflection data reveal the 3D structure of the newly discovered Exmouth Dyke Swarm, offshore NW Australia
