A magnetotelluric sounding across Vancouver Island detects the subducting Juan de Fuca plate

Abstract

Magnetotelluric1 (MT) and geomagnetic2 depth-sounding measurements have been made at 18 sites (7 with remote reference3) across Vancouver Island (Fig. 1), beneath which the Juan de Fuca plate is underthrusting4,5. Vancouver Island is part of the old accretionary Wrangellia terrane6 and is located in the central portion of a 350-km forearc region that extends from the sediment-filled trench7 to the Garibaldi volcanic belt8. Gravity9 and seismic refraction studies10 suggest that the descending plate has a shallow northeasterly dip of 8°–16° with perhaps higher dips beneath northeastern Vancouver Island. A high-resolution seismic reflection survey11 along lines 1 and 3 (Fig. 1) located a zone of strong acoustic reflections (the E-horizon11) near the top of the subducting Juan de Fuca plate at depths of 23–34 km. Our model of the broad-band MT data defines a sloping, highly conducting zone also near the top of the subducting plate at the same depths as the seismic E-horizon. The high electrical conductivity in this zone (the E-conductor) is the result of saline fluid within the pore spaces of sedimentary and mafic materials of the upper oceanic crust that have been subducted to those depths beneath Vancouver Island. These are the first data which clearly define a dipping conductive layer associated with the boundary region between converging plates; they have significant implications for thrust earthquakes and metamorphic reactions that occur in subduction zones.