Seismic reflection data reveal the 3D structure of the newly discovered Exmouth Dyke Swarm, offshore NW Australia

Abstract

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.

Can we relate the surface expression of dike-induced normal faults to subsurface dike geometry?

“…These dike-related zones are >100 m wide ( Fig. 2A), but borehole data suggests dike thicknesses may only be 10's of metres (Magee and Jackson, 2020); i.e. the width of a dike's seismic expression may not capture its true thickness.…”

“…We use seismic reflection images of the Exmouth Dyke Swarm, offshore NW Australia (Magee and Jackson, 2020), to quantify the 3D structure of a natural dike and dikeinduced fault system. We tested: (i) whether graben half-width can be used to predict dike upper tip depths (e.g., Wilson and Head, 2002;Trippanera et al, 2015b;Hjartardóttir et al, 2016); and (ii) if extension across dike-induced fault pairs measured at the syn-faulting free surface can be considered a proxy for dike thickness (e.g., Rubin and Pollard, 1988;Rubin, 1992;Trippanera et al, 2015b).…”

Structural controls on the location, geometry and longevity of an intraplate volcanic system: the Tuatara Volcanic Field, Great South Basin, New Zealand

“…activity at one volcano could have been linked to eruption at another as they probably shared a plumbing system. We also note that some volcanoes are underlain by pronounced vertical seismic disturbances, which we interpret as corresponding to dykes (Figure 6f, 7) Magee and Jackson, 2020;Wall et al, 2010). These dykes appear to feed isolated volcanic cones away from the central edifice (Figure 7a).…”

Salt-magma interactions influence intrusion distribution and salt tectonics in the Santos Basin, offshore Brazil

“…Furthermore, we acknowledge that thin sills, with thicknesses below the limit of visibility of the data, may not be recognised within the data [e.g., Eide et al, 2018;Schofield et al, 2017]. No dykes were identified in the seismic reflection data, although their likely sub-vertical orientation commonly inhibits their imaging so we cannot preclude their presence [see Magee and Jackson, 2020 and references therein].…”

“…The lack of observed feeders to the supra-salt sill-complex above the Merluza Graben (e.g., Fig. 7d) suggests it was likely fed by dykes that are not imaged by our seismic reflection data, perhaps because their sub-vertical contacts reflected little or no seismic energy back to the surface [e.g., Eide et al, 2018;Magee and Jackson, 2020;Malehmir et al, 2018]. If dykes did feed the supra-salt sill-complex, their bypassing of the salt during ascent may indicate the underlying evaporites are dominated halite [e.g., units A1 or A3;Rodriguez et al, 2018], which can fracture at high strain rates to accommodate dyking because it has a relatively high melting temperature and thermal conductivity [cf.…”