Conductive structures in southwestern Canada: a regional magnetometer array study

Abstract

An array of 33 three-component recording magnetometers was operated in June and July 1980 in Alberta and British Columbia south of the Edmonton – Prince Rupert highway. This very large array, with its stations dispersed through 550 000 km2 and on average 150 km apart, had limited resolution and was designed to confirm known conductive structures, discover new ones, and locate them sufficiently for suitable placement of further arrays with closer spaced stations and, therefore, higher resolution. Magnetograms and three sets of Fourier transform anomaly maps are presented. They show the general attenuation of the vertical component of variation fields west of the Rocky Mountain Belt known from previous work and generally attributed to a conductive layer in the lower crust or upper mantle. Two prominent local anomalies are shown by variation fields of periods 15–30 min. The first indicates induced currents near Tête Jaune Cache, west of Jasper. The highly conductive structure carrying the induced currents may include wet sediments in the Rocky Mountain Trench and possibly partial melt at depth associated with recent volcanics. The second local anomaly appears to be associated with a crustal conductive structure that strikes northeast–southwest across southern Alberta and crosses the southeast corner of British Columbia into eastern Washington State. This may be associated with a Precambrian rift in the lower crust discovered by Kanasewich and his colleagues using deep crustal seismic reflections some 15 years ago. Both of these anomalies are under further investigation by means of arrays operated in 1981 in locations indicated by the results of the array reported here. The regional westward attenuation of the vertical fields has been quantified by means of single-station transfer functions and artificial event analysis, as developed by Bailey and others, to show the Z response to unit southwest–northeast horizontal field at three periods, along a profile from Squamish, near Vancouver, to Edmonton. These response curves will be used in model studies of the regional conductive structure.