Electromagnetic studies of the lithosphere and asthenosphere

Abstract. In geodynamic models of the Earth’s interior, the lithosphere and asthenosphere are defined in terms of their rheology. Lithosphere has high viscosity, and can be divided into an elastic region at temperatures below 350 C and an anelastic region above 650 C. Beneath the lithosphere lies the ductile asthenosphere, with one- to two-orders of magnitude lower viscosity. Asthenosphere represents the location in the mantle where the melting point (solidus) is most closely approached, and sometimes intersected. Seismic, gravity and isostatic observations provide constraints on lithosphere-asthenosphere structure in terms of shear-rigidity, density and viscosity, which are all rheological properties. In particular, seismic shear- and surface-wave analyses produce estimates of a low-velocity zone (LVZ) asthenosphere at depths comparable to the predicted rheolo-gical transitions. Heat flow measurements on the ocean floor also provide a measure of the thermal structure of the lithosphere. Electromagnetic (EM) observations provide complementary information on lithosphere-asthenosphere structure in terms of electrical conductivity. Laboratory studies of mantle minerals show that EM observations are very sensitive to the presence of melt or volatiles. A high conductivity zone (HCZ) in the upper mantle therefore represents an electrical asthenosphere (containing mel