Application of EM and seismology to the same target area and depth has so far been restricted to a few case studies. However, where EM and seismological measurements have been performed jointly, the two geophysical techniques have proved complementary. At certain depths, (e.g., at the base of the lithosphere or at the 410 km discon- tinuity) compositional or physical changes in mantle materials might be expected to result in abrupt changes of both seismic
9.2 EM and seismology 167
velocities and electrical conductivity. Changes associated with the lithosphere–asthenosphere boundary or the mid-mantletransi- tion zones generally result in increased conductivity with depth, and the problem of finding the depth to the transition is identical to determining the upper boundary of a conductive structure – a problem for which passive EM techniques are well suited (Section 2.5). The asthenosphere has been identified as a zone of low seismic velocity and enhanced electrical conductivity (e.g., Praus et al.1990; Jones, 1999). Early attempts to jointly interpret these asthenospheric signatures rested on the idea that a few percent partial melt, which influences elastic, anelastic and electrical prop- erties of rocks (e.g., Schmeling, 1985; 1986), existed in the astheno- sphere. However, we now believe that it is unlikely that the continental geotherm crosses the dry mantle solidus at the depth in which the asthenosphere is imaged in many regions. The idea that partial melt is primarily responsible for the conductivity increase at the base of the lithosphere is also challenged by quan- titative conductance models, because reasonable amounts of par- tial melt cannot explain the high conductance found in some field studies (e.g., Leibecker et al., 2002; see also Section 9.3). The physical properties of the asthenosphere may rather be explained by the presence of small amounts of water. Hydrolytic weakening can readily explain the low viscosity of the asthenosphere (Karato and Jung, 2003), and hydrogen diffusivity can significantly enhance electrical conductivites (Karato, 1990). In regions where the mantle geotherm crosses the wet mantle solidus, partial melting may actually increase the viscosity and reduce the electrical resistivity of the asthenosphere owing to dewatering of the sub-lithospheric mantle.
In olivine, which is the dominant mineral in the upper mantle, hydrogen diffusivities are anisotropic (Kohlstedt and Mackwell, 1998), and alignment of olivine owing to viscous drag at the base of the lithosphere may lead to electrical anisotropy (Lizzaralde et al., 1995). Electrical anisotropy at the base of the electrical litho- sphere has been detected from MT measurements in central Australia (Simpson, 2001b; see Section 9.3). The electrical anisot- ropy below central Australia occurs at the same depth (150 km) as SV-wave anisotropy, and the direction of high conductivity matches the fast direction of SV-waves (Debayle and Kennett, 2000).
However, the seismic high-velocity lid extends deeper than the depth to the anisotropic layer (Gaherty and Jordan, 1995).
Kurtz et al. (1993) and Mareschal et al. (1995) also detected evidence of electrical anisotropy in Archaean mantle where seismic
168 The special link to other geosciences
anisotropy has been detected (Se´ne´chalet al., 1996). In this case, the direction of maximum conductivity and the polarisation direc- tion of the fast shear wave are oblique to each other, and the electrical anisotropy has been attributed to shape-preferred orien- tation of graphite within ductile shear zones in the lithospheric mantle (Jiet al., 1996).
EM can provide an important tool for resolving and constraining mantle anisotropy for three reasons.
1 Theelectromagnetic strikeestimation can be very accurate and can result inconfidence intervalsas small as 78(Simpson, 2001b).
2 Comparing the maximum and minimum conductances allows the strength of the anisotropy to be quantified.
3 Provided that the static shift problem (Sections 5.1 and 5.8) can be addressed, the depth of the anisotropic structure can be well constrained from the frequency dependence of the MTtransfer functions. Surface- wave studies can provide a similar depth control because of the period dependence of the group velocity (e.g., Debayle & Kennett, 2000), but most studies of continental seismic anisotropy use shear-wave splitting results (e.g., splitting of SKS waves), which provide inherently poor control on the depth in which anisotropy occurs.
The classical seismological tool for detecting mantle heterogeneities is tomography. P-wave tomography (Ritter et al., 2001), S-wave tomography (Keyser et al., 2002) and array electromagnetics (Leibeckeret al., 2002) have been applied in the same target area in the western Rhenish Massif. Ritteret al.(2001) found a P-wave velocity reduction of 2% or less within a 100-km-wide columnar structure within the upper mantle, which they interpreted as a temperature increase of 150–200 K associated with a mantle plume. Leibecker et al. (2002) found an electrically anisotropic structure under the Rhenish Massif with an E–W-trending direction of high conductivity. If partial melt were the dominant electrical conduction mechanism, then enhanced conductivites might be expected to be restricted within the columnar structure inferred by Rittteret al.(2001), which is not the case. A larger array experiment indicated that electrical anisotropy could extend under most of central Germany, and that the conductance maximum of 50 000 S does not occur under the Rhenish Massif but rather 200 km to the east (Gatzemeier, personal communication). Furthermore, the seis- mological results place an upper limit of 1% on the melt fraction that might be present within the low-velocity column (Keyser et al., 2002), which is insufficient to explain the high conductance found by Leibeckeret al.(2002). Other conduction mechanisms such as
9.2 EM and seismology 169
hydrogen diffusivity or electronic conduction must therefore be invoked.