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 axe colineax and are aligned approximately normal to the continental shelf with a maximum amplitude response neax 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 axe 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 tlmast surface of the Juan de Fuca plate. The conducting layer is most likely the result of cracks and pores filled with saline fluids which axe supplied by water subducted with the oceanic crust and by dehydration reactions. The presence of fluids has significant implications for thrust eaxthquakes 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. of the plate provides an excellent target for studying part of the western North American plate which is curthe electrical structure of a subduction zone. Kurtz et rently overriding the Juan de Fuca plate. Much of Vanal. [1986] presented a summary of the MT interpre-couver Island is part of the Wrangellia terrane [Jones et al., 1977], a thick assemblage of Paleozoic, Mesotation results including a highly conducting zone near the top of the subducting plate at the same depth as a zoic and Cenozoic volcanic, plutonic, sedimentary and strong seismic reflective zone [Green et al., 1986]. This metamorphic rocks [Muller, 1977]. Wrangellia was acpaper gives a more detailed interpretation and includes expanded discussions of the MT and GDS data.

H2O and Magmatism in Island Arc Mantle Inferred from Seismic Anelasticity and Heat Flow Data.

“…A lower crustal conductor beneath the aseismic front to the west in northeastern Japan has suggested the presence of H2O (e.g., Utada, 1987). Beneath the Oregon coast and Vancouver Island in North America, a high conductivity layer on the top of the descending Juan de Fuca plate has been observed, and lends itself to the inference of H2O-charged sediments that are partially subducted with the oceanic crust (e.g., Wannamaker et al, 1989;Young and Kitchen, 1989;Kurtz et al, 1990). A similar conductive layer has also been proposed in the Japan arc subduction zone by Utada (1987 …”

Two-Dimensional Electrical Conductivity Structure across the Southern Coastline of Australia.

“…The experiment transect intersects the rifted marginal Otway basin, which is a few kilometres wide and deep (Williams and Corliss, 1982;Finlayson et al, 1994). The effect of brinesaturated sediments on the coast effect at subduction zones has been discussed by Kurtz et al (1986Kurtz et al ( , 1990 and Wannamaker et al (1989), but has not been observed at passive margins. For example, the models of White et al (1990) and Kellett et al (1991) for southeastern Australia gave a good fit of the observed data without the presence of sediment beneath the continental shelf and slope.…”

“…Thermal structure and chemical composition may lead to lateral variations in conductivity across the coastlines. At active continental margins in northwestern USA (Kurtz et al, 1986(Kurtz et al, , 1990Wannamaker et al, 1989) and Japan (Ogawa et al,1986) the subduction of fluids gives rise to a very marked lateral variation in upper mantle conductivity across the margin, but at passive margins there is less obvious reason for such a difference. In Fig.…”

“…This model is compared to that determined by White et al (1990) and Kellett et al (1991) for the passive southeastern Australian margin, and more generally with 2D coast effect studies in different tectonic settings of western North America (e.g. Delaurier et al, 1983;Kurtz et al, 1986Kurtz et al, , 1990Wannamaker et al, 1989) and Japan (Ogawa et al, 1986). Figure 1 shows the deployment location of the four seafloor magnetometers (ROBE 1-4) and one land based magnetometer (ROBE 6) in the experiment.…”

Tomographic image of low P velocity anomalies above slab in northern Cascadia subduction zone

“…Magnetotelluric survey and modeling (Kurtz et al, 1986(Kurtz et al, , 1990) revealed a landward dipping highly conductive zone in the 20-50 km depth range beneath southern Vancouver Island. The high conductivity zone was originally interpreted as the subducted oceanic crust but later recognized to be in the lower crust and mantle wedge above the slab.…”