Geomagnetic depth sounding and upper mantle structure in the Cordillera region of western North America

Abstract

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...

Electrical Conductivity Structure in Western Canada and Petrological Interpretation

“…At latitude 32N the change in heat flow (from 1.1kcal/cm2 sec in the Texas Foreland to a mean of 2.21+0.43 in the Basin and Range Province) occurs in close agreement with the location of the GDS discontinuity (Warren et al, 1969). At latitude 35N the relation is not clear; Caner et al (1967) inferred an eastward swing of the GDS discontinuity, which is not in agreement with the heat-flow data. However, Porath (1969) indicated that the eastwards persistence of the low-Z zone at this latitude could be interpreted by a separate (shallow) anomaly rather than a swing in the main "western-type" conductivity structure; Caneret al (1970) have shown that such a reinterpretation of the original data is possible (since no MT "calibration" is available at this latitude, profile -type GDS data are necessarily ambiguous).…”

Geomagnetic Deep Sounding and Upper Mantle Structure in the Western United States

“…Caner, Cannon & Livingstone (1967) carried out geomagnetic deep sounding along a profile across northern New Mexico into Oklahoma and found high 2 only at their eastern-most station in western Oklahoma. They suggest that the anomaly related to the eastern front of the Rockies swings towards the east in northern New Mexico and does not follow the topographic front of the Rocky Mountains.…”

A Geographical Relation Between Geomagnetic Variation Anomalies and Tectonics

“…This geophysical work suggests therefore that the edge of the craton lies west of the Rocky Mountains rather than along the line of King drawn in Fig. 1 To the south in the United States, data from Schmucker (1964), Caner et al (1967), Camfield et al (1971, and Porath and Gough (1971) have been used to derive the I values shown on Fig. 1.…”