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.