An integrated approach using both geomagnetic depth-sounding (GDS) and magnetotellurics (MT) is applied to the determination of the electrical conductivity structure in western Canada; 42 GDS stations and 6 broad-band MT stations are used. These data are combined with other geophysical information to provide self-consistent petrological models. Excluding geochemically improbable solutions, it can be shown that: a) temperatures at depth 35km must be at least 750C in the entire western region (eastwards extent undefined); b) in a smaller, sharply defined, region (eastern boundary within about +50 km of the Rocky Mountain Trench) the lower crust and possibly uppermost 10-25km of the mantle are hydrated, probably with partial melting.
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.
This report describes recent geomagnetic depth-sounding investigations on the inland (Cordillera) anomaly first reported by Schmucker at latitude 32øN. It includes Hyndman's profile at latitude 49.5øN and two subsequent profiles at latitudes 35øN and 51øN. All three profiles crossed the transition between the 'high-/' and 'low-/' zones. Characteristic parameters have been derived from power spectral analysis of the data. The attenuation in the normalized vertical component at the western stations increases with frequency, from 0.97 q-0.65 db at 0.0'25 c/ks (10-hour period) to 8.10 q-1.25 db at 0.35 c/ks (45-minute period). The attenuation is uniform along all three profiles, confirming that the same conductivity structure exists in the southwest United States and in southern British Columbia. Quantitative analysis indicates a rise of highly conducting material (10 -u emu q-20%) to within 25-35 km of the surface under the western region. x Canadian Contribution 155 to the International Upper Mantle Project. entire spectrum are strongly affected by the conductivity and uniformity of the surface medium in which the electric field probes are located. The advantages of GDS over the MT method are partly offset by the difficulties in quantitative interpretation of the results from single-station recordings. When used in multistation arrays, however, it provides an extremely economical method for mapping the location of discontinuities in the electrical conductivity structure of the upper mantle. It is a particularly useful complement to seismic refraction methods, as it provides excellent horizontal resolution (i.e., lateral discontinuities can be located very accurately). This report is concerned with the GDS results obtained near the Cordillera region of western North America. Schmucker [1964] reported a sharp discontinuity in the character of recorded geomagnetic data (specifically in the amplitude of fluctuations in the vertical component with periods of 15-60 min) along an E--W profile erossing the Cordillera at latitude 32øN. The discontinuity occurred between Las Cruces and Cornudas in New Mexico. To the east of this transition, the ratio of vertical to horizontal amplitudes for bay-type features was roughly three times higher than at the western slations. A similar pattern was found on the recordings of the temporary IGY stations around latitude 6335 6336 CANER, CANNON, 40øN; the Z/H ratios at three eastern stations (Burlington, Colorado; Beloit, Kansas; and Carrolton, Missouri) were about 3-4 times higher than those recorded at Leadville, Colorado, and Espanola, New Mexico [Schmucker, 1964]. Sehmueker interpreted the attenuation in the vertical component as being due to a step in a highly conducting substratum from a depth of 320 km under the eastern region to a depth of 160 km under the western region. The sharpness of the transition between Las Cruces and Cornudas led Sehmueker, however, to postulate an additional, more shallow, conductive region between the two stations. In 1963, the Department of Geophysics...
During 1964 an east–west geomagnetic recording profile was operated in western British Columbia to investigate the "coast effect" in geomagnetic variations. The profile extends earlier work by Hyndman in central and eastern British Columbia. Results of the survey are described and interpreted. The restraints imposed on the choice of models have been based primarily on compatibility with other geophysical information, rather than on agreement with idealized theoretical conductivity structures.The "coast effect" has been interpreted as being due to two different effects: the land–sea interface at periods of less than 10–15 minutes, and an associated upper mantle structure at periods of more than 30 minutes. The latter has been tentatively identified with the discontinuity of the seismic low-velocity layer under continental and oceanic regions.On the basis of additional geophysical information, the interpretation has been extended to include the anomalies in the southwest United States reported by Schmucker in 1964. The inland anomalies in western North America have been interpreted on the basis of a moderately conducting zone at the top of the mantle, tentatively identified with the anomalous seismic low P velocity zone. If confirmed by further observations, these results imply temperature rather than density effects as the cause of the low P velocities.
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