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

Summary Magnetic field time variations were observed in September 1967, with a two‐dimensional array of 42 three‐component variometers between latitudes 36° and 43° N and longitudes 101° W and 116° W. Fourier analysis of a polar substorm and of a storm shows that the former has a smooth spectrum and the latter a complex spectrum with many maxima. Upper mantle conductivity structure can be seen qualitatively in the original variograms, but is far more sharply defined in maps of Fourier spectral component amplitudes and phases. A ridge of high conductivity runs at a depth no greater than 200 km under the Southern Rocky Mountains between the Great Plains and the Colorado Plateau, which marks a low‐conductivity region within the Cordillera. A strong conductivity anomaly runs north‐south along the Wasatch Front through central Utah, and indicates the presence of an upwelling of highly conductive material at depth no greater than 120 km along the edge of a step structure which brings the conductive mantle to shallower depth under the Basin and Range Province than under the Colorado Plateau. Long‐period maps from the storm suggest a rise in the conductive mantle between the north‐south structures, from the Colorado Plateau southward to the Basin and Range. The daily variation shows the conductivity structures and indicates their great extent in depth. The geomagnetic deep sounding anomalies are found to be in excellent agreement with existing heat flow data, and this supports correlation of electrical conductivity with temperature. There is also good correlation with the available seismic velocity information for the upper mantle.