The electrical conductivity distribution beneath Vancouver Island: A region of active plate subduction

Abstract

Magnetotelluric (MT) and geomagnetic depth sounding (GDS) data were recorded at 25 locations across Vancouver Island, Canada, over the subducting Juan de Fuca plate. The GDS data respond to a number of local conductive features at frequencies above 10 Hz while at longer periods, the in‐phase and quadrature induction arrows are colinear and are aligned approximately normal to the continental shelf with a maximum amplitude response near the southwest coast. Interpretation of the MT data show the presence of a conducting zone at depths greater than 20 km beneath Vancouver Island and correlating with a strong seismic reflector. Extensive testing with two dimensional models shows the MT and GDS data are consistent with a proposed geoelectrical structure containing a conducting layer extending from the Pacific Ocean, dipping beneath Vancouver Island, and in electrical contact with the mainland conductive region. The layer has a horizontal conductance of 200 S that may decrease to 100 S beneath the northeast side of Vancouver Island. Related geophysical studies suggest the conductive zone is above the actual thrust surface of the Juan de Fuca plate. The conducting layer is most likely the result of cracks and pores filled with saline fluids which are supplied by water subducted with the oceanic crust and by dehydration reactions. The presence of fluids has significant implications for thrust earthquakes and for metamorphic reactions that occur in subduction zones. The conductance of the upper mantle (between 90 and 390 km) is best modelled at approximately 1500 S.