Partial Melting and Conductivity Anomalies in the Upper Mantle

Abstract

INVESTIGATIONS with arrays of magnetometers recording geomagnetic variation fields have shown the presence of large anomalies in electrical conductivity under the western United States1-3. Under the Basin and Range Province and under the Southern Rocky Mountains of Colorado, the regions of enhanced conductivity are in the upper mantle. The depth is poorly defined by the magnetometer array data, but is between 50 and 400 km4,5. At about 400 km the mantle conductivity rises steeply with increasing depth in the Earth as a whole6-7. Conductive structures include an elongated ridge on the surface of the conductive mantle (Southern Rockies) and a step-plus-ridge (Basin and Range). The conductivity structure is strongly correlated with heat flow8·9 and with velocities and absorption of seismic waves10. The regional variations of P and S velocities in the upper mantle beneath North America can best be understood11 in terms of a small amount of partial melting in the low velocity zone, at depths between 50 and 150 km, under the western region.