Magnetometer Array Studies in the North-Western United States and South-Western Canada

Abstract

Time-varying magnetic fields were recorded during the summer of 1969 with a two-dimensional array of 46 variometers in the north-western United States and south-western Canada between latitudes 44° and 51° N and longitudes 100° and 121° W. Magnetograms and maps of Fourier spectral components of three magnetic events are used to describe conductive structures in the upper mantle and crust. The most prominent of the anomalies are found in the North American Central Plains and over the Northern Rockies. The Central Plains anomaly runs from the eastern edge of the Black Hills northward along the boundary between Montana and the Dakotas to the Williston Basin. Its large magnitude, with anomalous fields larger than the normal fields, and its small half-width indicate a crustal conductor which concentrates current induced in a large region. In the western part of the array, attenuation of the vertical variation fields is attributed to a westward rise in the highly-conducting mantle. Two small anomalies in the vertical and horizontal fields, over the eastern front of the Northern Rockies and the Rocky Mountain Trench, may be associated with ridges or steps on the upper-mantle conductor or with crustal features.