Anomalies in Daily Variation Magnetic Fields and Structure Under North-western United States and South-western Canada

Abstract

Summary Thirty-one magnetometers of a large array secured records through a magnetically quiet 5-day period in July 1969, between latitudes 44 and 51 N and longitudes 100 and 121 W. The North American Central Plains variation anomaly persists in the Z component at periods 8 and 12 hr, but not in Y. Even qualitative explanation requires a channelled current of complicated geometry. Induction by Y is dominant at periods less than 2 hr but induction by 2 appears significant at daily-variation periods. The northern end of the conductive ridge in the upper mantle under the Wasatch Front produces prominent induction anomalies in Y and Z at periods 8 and 12 hr. At period 24 hr the amplitude maps show complex patterns of anomalies with features which do not correspond to those at shorter periods. The phase of 2 shows a steep gradient to the south-south-west and approaches that of X. This phase shift of over 5 hr is attributed to induction by X of currents in the highly conductive upper mantle under the Basin and Range Province, south of the array. The anomalies in 2 and Y amplitudes then arise from variable transmission of fields due to these currents through the electrically heterogeneous upper mantle and crust to the stations of the array. The use of a phase-difference test shows that anomalies in Z and Y amplitudes at period 24 hr, near the Northern Rockies, are such variable transmission (vartran) anomalies, whereas those at periods less than 2 hr are induction anomalies. West of the Northern Rockies the conductivity-depth profile which best fits the normal fields differs from the Great Plains profile by the addition of a conductive layer 15 km thick, in the depth range 50-100 km. The top of the ultimate mantle conductor is at depth 350 km. Small local anomalies near the Northern Rockies are modelled by elaborating on the conductive layer.