Observations of the electrical asthenosphere beneath Scandinavia
Alan G. Jones
The recent recognition that long period (i.e., of the order of hours) electromagnetic induction studies
could play a major role in the detection of the asthenosphere has led to much interest amongst the
geophysical and geological communities of the geomagnetic response functions derived for differing
tectonic environments. Experiments carried out on the ocean bottom have met with considerable success
in delineating the "electrical asthenosphere", i.e., a local maximum in electrical conductivity (minimum in
electrical resistivity) in the upper mantle.
In this paper, observations of the time-varying magnetic field recorded in three regions of Scandinavia,
northern Sweden (Kiruna-KIR), northern Finland/northeastern Norway (Kevo-KEV), and southern
Finland (Sauvamaki-SAU), are analysed in order to obtain estimates of the inductive response function,
C(w), for each region. The estimated response functions are compared with one from the centre of the
East European Platform (EEP), and it is shown that the induced eddy currents, at periods of the order of
10^3 -10^4 s, in the three regions flow much closer to the surface than under the platform centre.
Specifically, at a period of approx. 3000 s, these currents are flowing at depths of the order of: KEV - 120 km;
KIR - 180 km; SAU - 21O km; EEP - 280 km; implying that the transition to a conducting zone, of
sigma=0.2 S/m, occurs at around these depths. One-dimensional inversion of C(KEV) and C(K1R) shows that
there must exist a good conducting zone, of sigma=0.1-1.0 S/m, under each of the two regions, of 40 km
minimum thickness, at depths of: KEV 105-115 km; KIR 160-185 km. This is to be contrasted with
EEP, where the rho-depth profile displays a monotonically decreasing resistivity with depth, reaching sigma approx. 0.1
S/m at >300 km.
Finally, a possible temperature range for the asthenosphere, consistent with the deduced conducvitity,
is discussed. It is shown that, at present, there is insufficient knowledge of the conditions (water content,
melt fraction, etc.) likely to prevail in the asthenosphere to narrow down the probable range of
Tectonophysics, 90, 37-55.
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Alan G Jones / 28 February 2010 /