Electrical conductivity and temperature in the Canadian Cordilleran crust

Abstract High electrical conductivity and high heat flow coexist in much of the Canadian Cordillera, a region of recent accretion of crustal terranes to North America. The relationship between temperature and electrical conductivity in the crust is investigated by means of results from 251 heat flow determinations and 301 magnetotelluric (MT) soundings. We first compared maps of MT phase at period 10 s and of heat flow. Larger phases correlate with higher heat flows in some areas, but the relationship is complicated by other causes of variation in these two parameters. To give a more focussed comparison, we map the depth d 450 to the 450°C isotherm for two assumed distributions of reduced heat flow, and the depth d c to the top of the lower crustal conductor at MT sites away from major fracture zones. The map of d c visually resembles both of the maps of d 450 , and readings at 76 points of a grid give correlation coefficient 0.80. This supports a relationship between high conductivity in the crust and temperatures above 450°C. Both the lower crustal conductor and the 450°C isotherm occur at depths near 30 km above the subducting oceanic plate under Vancouver Island, at 15–20 km in the Intermontane Belt, and of 10–15 km in the Omineca Belt. If there is an upcurrent in the mantle beneath the continent, behind the subduction, as proposed elsewhere, its axis may now be under the Omineca belt. This location is in harmony with evidence of recent extension and uplift there.