Mantle Conductive Structures in the Western United States from Magnetometer Array Studies

Abstract

Summary Geomagnetic substorm fields were simultaneously recorded during the summer of 1968 at stations of a two-dimensional array of variometers covering western Texas, New Mexico, Arizona and eastern California. This work formed part of a continuing study of upper mantle conductive and thermal structure under the western United States. Magnetograms and maps of Fourier spectral components of two substorms are used to describe conductive structures in the upper mantle and crust. Conductive structures under the Southern Rockies and Wasatch Fault Belt, discovered in a similar array study in 1967, are shown to continue southward. The Basin and Range Province as a whole overlies a highly conducting (and, by inference, hot) region in the mantle. Conditions are discussed which must be satisfied by any assumed regional field used to isolate and normalize the anomalous fields. Anomalous fields satisfying these conditions are derived from the 1967 array data supported by the 1968 array. Numerical calculations are used to show that the anomalies cannot be associated with conductors of limited thickness within the crust, unless these have improbably high conductivity. Two-dimensional ridges on the surface of a semi-infinite conductor, on the other hand, satisfactorily model the observed fields. The conductive structures can therefore be assigned with some confidence to the upper mantle. The anomalies are well represented by induction in a half-space of conductivity 0.2 (ohm m)−1 with a ridge under the Southern Rockies and a step with superposed ridge under the Wasatch Fault Belt. The depths to the surface of the conductor in our model are 190 km under the Basin and Range Province, 120 km under the Wasatch Fault Belt, 150 km under the Southern Rockies and 350 km under the Colorado Plateau and the Great Plains. Such models are naturally not unique.