Magnetic time-variations between Tucson, Arizona and Sweetwater, Texas indicate that a zone of high electrical conductivity underlies the southwestern United States. The interpretation of this zone by Schmucker as a rise of the isotherms in the upper mantle is supported by six heat flow observations along the line of the geomagnetic profile. These and other observations indicate a high but variable heat flow in the Basin and Range Province which contrasts strongly with the uniform values of 1.1 mu cal/cm 2 sec reported for the Texas Foreland. The width of this high heat flow anomaly, which may extend across the entire Basin and Range Province, suggests anomalously high temperatures in the upper mantle. This interpretation is further supported by magnetotelluric data between Phoenix, Arizona and Roswell, New Mexico and by the low seismic P n velocity and negative gravity anomaly. It is suggested that the 'anomalous mantle' may be related to the tectonic evolution of the western United States and the late Cenozoic fault system.