Electrical Conductivity Structure by Geomagnetic Induction at the Continental Margin of Atlantic Canada

Summary Geomagnetic variations measured at 10 stations in Atlantic Canada show significant laterally inhomogeneous induction. Transfer functions giving the systematic dip angle and direction of the variation field lines have been computed for periods from 20s to 120min and the results numerically modelled. High electrical conductivity exists starting at a depth of 15 km or less under the continental shelf off Nova Scotia and Newfoundland and perhaps under the Bay of Fundy. The apparent coast effect with a maximum at 30‐min period results from the contrast between the highly conducting shelf structure and the more resistive inland rocks. The most likely explanation of the high conductivity is that there is highly saline interstitial water in the lower part of a 10km sedimentary section associated with evaporite, salt layers, or that part of the crust is hydrated in this area. Strong electric currents flow in the various arms of the Gulf of St Lawrence for short period (10 s to 10 min) inducing fields. Numerical models show that they can be explained by local induction in the shallow sea water. The details of the current flow indicate that significant conductive channelling must occur. One station on the north shore of the St Lawrence River has large long period (30 min) anomalous vertical fields. They probably result from a contrast in deep conductivity between the Appalachian and Canadian Shield geological provinces.