Long-lived Thermal Structure under the Southern Rocky Mountains

Abstract

HOT regions in the upper mantle can be located in two ways. The first is the classical technique of mapping heat flow through the crust from measurements at numerous points. The more recent method of geomagnetic deep sounding (GDS) uses recording magnetometers at a number of stations to locate regions of enhanced electrical conductivity in the Earth through the magnetic fields of transient currents. Some, at least, of the ultra-basic silicates of the mantle are semiconductors which show a sharp rise of conductivity¹ at temperatures above 1,000° C. While there is no one-to-one correspondence, it is probable that many of the known upper-mantle conductivity anomalies result from high temperatures. GDS is therefore an important means of seeking hot regions, hitherto limited by the small numbers of magnetometers available. Ocean floor spreading and plate tectonics have revolutionized earth science in the past few years. GDS affords means of extending study of the dynamic mantle demanded, especially under continents where little is yet known.