The North American Central Plains Conductivity Anomaly

An array of 41 three-component magnetometers recorded geomagnetic disturbances during August and September 1972, between latitudes 42° and 54°N and longitudes 98° and 109° W. The objective was a detailed study of the North American Central Plains (NACP) conductive body earlier discovered striking northward from the Black Hills roughly along longitude 104° W in the United States. Source fields were provided by polar magnetic substorms and by other events with incident fields more uniform over the array. Northward mapping of the conductor was carried to within 90 km of the exposed Canadian Shield of north central Saskatchewan. Between latitudes 43°N and 48°N maximum depth estimates place the conductor within the lithosphere; the currents probably flow at smaller depths, i. e. within the crust. At the Shield edge the axis of the conductor is parallel to the strike of fold belts and fault zones in metamorphic rocks, including mylonites, characteristic of intense grinding and crushing. Graphitic conductors are known in many fracture zones. The conductivity anomaly links these structural elements in the Churchill Province Shield to the metamorphic belt mapped by Lidiak in the South Dakota basement, and to the Black Hills. South of the Black Hills the conductor turns southwest to the northern end of the Southern Rockies. There is evidence which suggests, but does not demonstrate, a conductive link there between the mantle conductor under the Southern Rockies and the NACP crustal conductor. It is postulated that the linear crustal structure may be a major continental fracture zone now mapped over a total length of 1800 km, of which 300 km is exposed in the Shield. Two of three earthquakes located in southern Saskatchewan have epicentres close to the axis of the conductive body.