Fróðskaparrit - 01.01.1982, Page 46

54 Some Magnetotelluric Measurements on the Faeroe Islands
ions at approximately 167°E or 77°E from the geographic
north. The good coincidence of the first direction, with the
trend of the Faeroese fjords and the tuff and agglomeratic
zones, is remarkable and suggests that the deeper resistivity
structures reflect a correlation with these surface structures.
This seems to be in agreement with the interpretation of
NE-SV striking rifts as the suppliers of the lower basaltic
layers and the tuff and agglomeratic deposits (Rasmussen,
Noe-Nygard, 1969).
In the last ten years, seismic evidence supports the idea of a
continental crust rather than an Icelandic type beneath the
Faeroe Islands (Bott, Sunderland, Smith, Casten, Saxov, 1974)
(Bott, Nielsen, Sunderland, 1975), and the thickness of the
crust was estimated at approximately 30 km. Resistivities for
the upper mantle below the Scandinavian Shield are presented
in a paper by A. G. Jones (1980), and the bounds on the
resistivities found from the Monte Carlo inversion for this
area are seen to be consistent with the resistivity level for the
two Faeroese MT stations. The Schmucker inversion (e. g. Wei-
delt 1979) for the Scandinavian area, however, does not show
an increase in the resistivity level with depth found for our
two stations, and shows instead a decrease in resistivities to
90 Í2m at 100 km.
Resistivity/temperature curves for various mantle materials
are shown in fig. 6 (from Haak, 1980), and we note that an
increase in resistivity with depth for a given rock composition
would result in an unlikely negative temperature-depth grad-
ient. Therefore, we have to consider one or a combination of
at least three possibilities to explain this problem:
1) Effect of crustal inhomogeneities on transfer functions
2) Significant variations in mantle composition
3) Influence of partial melting
Our data indicate the existence of deep crustal inhomo-
geneities, which we believe are related to the preferred
directions of the Faeroese fiords. This, in turn, can seriously