P. Schmoldt, PhD - MTNet - DIAS

5. Earth’s properties observable with magnetotellurics
sLAB eLAB
Depth (km)
Fig. 5.8.: Depth of the lithosphere-asthenosphere boundary (LAB) beneath Europe, defined in terms of seismic anisotropy observed
with teleseismic body waves (sLAB) and in terms of electric conductivity observed with magnetotellurics (eLAB). Figure taken from
Jones [2009] using seismic data from Babuska and Plomerová [2006] and magnetotelluric data from Korja [2007].
the upper mantle at depth usually within the range 50 to 160 km; in cratonic regions the
LAB can reach significantly greater depth, as much as 250 km [Eaton et al., 2009]. Depth
estimates of the LAB for the same region may vary between different geophysical methods.
Different LAB depths have been reported for example for Europe by Babuska and
Plomerová [2006] using teleseismic body waves and by Korja [2007] using magnetotellurics
(Fig. 5.8). This discrepancy might originate from different definitions of the LAB
in terms of the related property, i.e. changes in mechanical properties, electric conductivity,
seismic velocity, temperature gradient, or anisotropy (seismic and electromagnetic)
[e.g. Eaton et al., 2009, and references therein] (Fig. 5.9). The discussion about the LAB
thickness evoke the question of whether the LAB is indeed a sharp boundary or rather a
smooth transition zone with considerable vertical extent [e.g. Cavaliere and Jones, 1984;
Praus et al., 1990; Jones, 1999; Artemieva, 2009; Eaton et al., 2009; Jones, 2009; Meier
et al., 2009]. Discrepancies between the depth estimates of the LAB from different methods
might therefore result from their varying sensitivity to different properties, which are
located at the top, bottom, or within the LAB. In EM induction studies (presuming that
the data possess an adequate period range) the LAB can be identified as a significant reduction
in resistivity, i.e. from values between 10 3 – 10 4 Ωm to values as low as 5 – 25 Ωm
[Eaton et al., 2009]. However, such low resistivity values are not in agreement with predictions
by integrated petrophysical modelling [e.g. Fullea et al., 2011], which propose
a relatively smooth transition from lithospheric mantle values (10 3 − 10 4 Ωm) to values
around 100 Ωm that are related to the asthenosphere (Fig. 5.10). Three groups of explanations
for the discrepancy between EM induction studies and laboratory studies, which
92