Quantitative interpretation of geomagnetic depth-sounding data in western Canada

Abstract

Geomagnetic depth-sounding (GDS) data obtained at two stations in western Canada have been interpreted in terms of electrical conductivity structure. The two stations (Penticton and Pincher) are located about 150–200 km to either side of the main discontinuity between an area of low vertical component variations (low Z) and an area of high vertical component variations (high Z) in the frequency range 10−4 to 10−3 Hz. The interpretation is intended primarily as a check on the validity of the use of magnetotelluric (MT) data as ‘calibration’ for the largescale GDS mapping. The combined MT-GDS models confirm the existence of a finite conductive layer (resistivity of the order of 5 ohm m) in the western regions, starting at a depth of 10–15 km and extending to, or into, the upper mantle. Both regions, at least as far east as 113°–114°W, are underlain by moderately conducting upper mantle material (resistivity of the order of 50 ohm m). Extension of the analysis to very low frequencies (diurnal and its harmonics) indicates that the structures being mapped in southwestern Canada differ from structures being mapped in the U.S.; significant attenuation of diurnal variations has been reported from the southern and central Cordillera in the U.S., whereas no such effect has been found across the low Z/high Z discontinuity in western Canada.