Magnetotelluric evidence for layered mafic intrusions beneath the Vّring and Exmouth rifted margins

Marine magnetotelluric (MT) surveys at two volcanic passive margins have revealed an enigmatic layer of extremely high conductivity (⩽ 0.1 Ωm) at ∼10 km depth. At the Vøring Plateau off the northwest shelf of Norway, 2D inversion of data from nine sites along a 54 km line resolves a layer with a conductance of ∼104 S. At the Exmouth Plateau off the northwest shelf of Australia, 2D inversion of 122 sites in 17 lines finds a similar layer at similar depth but an order of magnitude higher conductance. At both plateaus, the depth of the high conductivity layer coincides roughly with what seismic studies have identified as an assemblage of sills. We propose that the extremely high conductance is due to well-connected conductive cumulates (e.g. magnetite) precipitated in layered mafic intrusions. In contrast to sill emplacement, the nature of layered intrusion formation requires connection to a magma source over time. Such a connection would not be likely during rifting when the rift provides a preferential pathway for pressure release. This implies emplacement prior to or during a pause in the early stage of continental breakup.