Greinar (Vísindafélag Íslendinga) - 01.01.1976, Blaðsíða 10

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of Tertiary flood basalts: layer 1 with P-velocity of 4.1 km/s and
layer 2 with 5.2 km/s. These three layers are underlain every-
where in the island hy a layer with P-wave velocity of 6.5 km/s,
layer 3, which is considered to correspond to the oceanic layer.
This layer is nowhere exposed on the surface hut the depth to its
upper boundary has been mapped in detail by Pálmason (1971).
The temperature gradient in the Icelandic crust has been mea-
sured in boreholes by Pábnason (3,4). He finds values ranging
from 37°C/km on the eastern flood basalt zone up to 165°C/km
in the Quaternary area in SW-Iceland. Within the neovolcanic
zone the gradient is close to zero in the uppermost few hundred
meters. This is caused by high fissure and crack porosity and hence
a flow of cold ground water in the surface layers of the active zone.
Pálmason (3) used the gradient and seismic measurement to
estimate the temperature at the 2-3 layers boundary. He concludes
that the oceanic layer could be formed by a conversion of basaltic
crust to amphibolite. Another possibility to explain the higher velo-
city in layer 3 is that it is largely composed of intrusives (5), which
are denser than the overlying flood basalts. A combination of both
these possibilities i.e. intrusives and some temperature-dependant
effects, such as metamorphism or filling of pore spaces (1) is pos-
sible. Beneath layer 3 there is a layer in the upper mantle with
P-velocity of 7.2 km/s (layer 4). This anomalous low velocity is
explained by small amounts of partial melting. The temperature in
this layer should therefore be around 1000°C, which is in agree-
ment with values obtained from linear extrapolation of the tem-
perature gradinet (4) and from magnetotelluric measurements (6).
It seemed to be promising to measure in situ the electrical resis-
tivity in the Icelandic crust, especially in layer 3, which cannot he
studied at the surface. From comparing the in situ electrical resis-
tivity with laboratory measurements on different types of rocks
under various conditions, conclusions can be drawn about the mois-
ture content, if electrolyte concentration and temperature in the
deep crustal layers are known. The moisture content or porosity
may play a role in explaining some geophysical phenomena like
seismic activity or thermal anomalies, and may also he of an eco-
nomical importance in exploitation of geothermal energy. On the
other hand if porosity and electrolyte concentration are known in