Abstract
Time-varying magnetic fields have been observed in the summers of 1967 and 1968 by means of two overlapping arrays of three-component variometers, each containing 42 instruments, between 32° and 43°N and longitudes 100° and 116°W. Maps of Fourier spectral component amplitudes and phases sharply define lateral variations in upper mantle conductivity structure. A ridge of high conductivity runs under the Southern Rocky Mounains between the Great Plains and the Colorado Plateau, which marks a low-conductivity region within the Cordillera. The Basin and Range Province both west and south of the Colorado Plateau shows high mantle conductivity, with an additional up-welling of conductive material at its boundaries with the Colorado Plateau and Great Plains. Close correlation with heat-flow data supports the view that temperature variations are the cause of the conductivity structures. These structures modify the daily variation fields and thus extend to great depths. A deep basin of conductive sediments in southwest Oklahoma causes a pronounced anomaly. Such crustal anomalies are distinguishable from upper mantle anomalies through their frequency dependence and phase lead.
