Electrical conductivity under Western North America in relation to heat flow, seismology, and structure.

Abstract

Western North America between the coastal ranges and the Great Plains, and from the Mexican-United States border to the Trans-Canada Highway has been studied by means of three large two-dimensional arrays of recording magnetometers. The paper discusses the conductive structures found by these arrays in the upper mantle of the region. Highly conductive mantle material at least 100 km thick for conductivity 0.2 S/m underlies the Basin and Range Province and ridges of still higher conductivity (or greater thickness for a given conductivity) underlie the Wasatch Fault Belt and Southern Rockies. Under the Great Plains the upper mantle is more resistive and is of sub-shield type. North of the Basin and Range the Cordillera have a thin layer (10-20 km at 0.2 S/m) of conductive material in the upper mantle and the conductivity indicates a lower level of tectonic activity than in the midlatitudes of the U.S.A. Under the Colorado Plateau the conductivity is of Great Plains type or intermediate between this and that beneath the Basin and Range. Alternative conductive models satisfying variation-field anomalies are discussed with special reference to the poor depth and excellent lateral resolution of magnetometer arrays. The distribution of heat-flow is compared with that of electrical conductivity and it is shown that there is generally close correspondence and full support for association of high conductivity with high temperature. Seismological velocity-depth profiles, station terms in P and S teleseismic times and other parameters are considered and are again consistent with the conductivity and geothermal information.The case for partial melting is considered. It is strongly supported by seismological parameters in the lowvelocity layer and gives the most plausible hypothesis for the contrasts of order 100 in conductivity required by the magnetometer array results. Three tentative suggestions are made concerning tectonics of the region.

The geomagnetic coast effect

“…Gough [1973b] has discussed the situation in southern Peru and Japan. The presence of an inland subduction zone in Peru is undoubtedly associated with the absence of a normal GCE there.…”

“…A number of reviews [Schmucker, 1970b;Gough, 1973b;Hutton, 1976] of various aspects of this subject have appeared. A review of theoretical models for electromagnetic induction in the ocean has been given by Ashour [1973], and a review of techniques and instrumentation for the study of electromagnetic induction itt sea by Filloux [1973].…”

Electrical conductivity anomalies in the earth

“…Mantle anomalies seem to be associated with regional high heat flow and low seismic velocities, as clearly represented by the anomalies in the western United States (Gough, 1974). The anomalies in the Rhine Graben and the Southern Caspian Depression were also interpreted in a similar manner (Berdichevsky et al, 1972;Garland, 1975).…”

“…Gough, 1973Gough, , 1974. In the central part of the western United States, anomalous fields were found in both vertical and horizontal components over the Wasatch Front and the Southern Rockies.…”

“…The obtained structure is characterized by a regional upheaval of a mantle conducting layer under the Basin and Range Province with a local conducting ridge beneath the Rio Grande Rift Belt. However, again the structure is not a unique one and the observed anomaly of geomagnetic variations also seems to be accounted for by another model consisting of a conducting layer corresponding to a low-velocity zone in the upper mantle (Gough, 1974). In the northern part of the western United States and the southern part of western Canada, another anomaly was discovered over the Northern Rockies, and vertical field attenuation (high-cut AZ type) was also found in the western part of the survey area.…”