Electromagnetic studies of the lithosphere and asthenosphere

In geodynamic models of the Earth's interior, the lithosphere and asthenosp here are defined in terms of their rheology. Lithosphere has high viscosity , and can be divided into an elastic region at temperatures below 350 degre es C and an anelastic region above 650 degrees C. Beneath the lithosphere l ies the ductile asthenosphere, with one- to two-orders of magnitude lower v iscosity. Asthenosphere represents the location in the mantle where the mel ting point (solidus) is most closely approached, and sometimes intersected. Seismic, gravity and isostatic observations provide constraints on lithosp here-asthenosphere structure in terms of shear-rigidity, density and viscos ity, which are all rheological properties. In particular, seismic shear- an d surface-wave analyses produce estimates of a low-velocity zone (LVZ) asth enosphere at depths comparable to the predicted rheological transitions. He at flow measurements on the ocean floor also provide a measure of the therm al structure of the lithosphere. Electromagnetic (EM) observations provide complementary information on lith osphere-asthenosphere structure in terms of electrical conductivity. Labora tory studies of mantle minerals show that EM observations are very sensitiv e to the presence of melt or volatiles. A high conductivity zone (HCZ) in t he upper mantle therefore represents an electrical asthenosphere (containin g melt and/or volatile) that may be distinct from a rheological asthenosphe re and the LVZ. Additionally, the vector propagation of EM fields in the Ea rth provides information on anisotropic conduction in the lithosphere and a sthenosphere. In the last decade, numerous EM studies have focussed on the delineation of an HCZ in the upper mantle, and the determination of melt/vo latile fractions and the dynamics of the lithosphere-asthenosphere. Such HC Zs have been imaged under a variety of tectonic zones, including mid-ocean ridges and continental rifts, but Archaean shields show little evidence of an HCZ, implying that the geotherm is always below the mantle solidus. Anis otropy in the conductivity of oceanic and continental lithosphere has also been detected, but it is not clear if the HCZ is also anisotropic. Although much progress has been made, these results have raised new and interesting questions of asthenosphere melt/volatiles porosity and permeability, and l ithosphere-upper mantle heterogeneity. It is likely that in the next decade EM will continue to make a significant contribution to our understanding o f plate tectonic processes.