Greinar (Vísindafélag Íslendinga) - 01.01.1976, Side 90

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cities 5.5-6.0 km/sec. The bottom contains a few layers with velo-
cities 6.5-7.0 km/sec and probably a thin 7.5 km/sec bed. The
total thickness of the crust is 16-18 km, i.e. about two times less
than in the adjacent blocks.
Discussion
The travel-time graphs of the seismic waves and the cross-
sections described above give grounds to consider that as in the
Faeroe-Iceland Ridge the mantle in Iceland displays the velocities
approximating the normal, and lies at the depths typical of the
continental regions. The earth’s crust in Iceland has high velo-
cities and previously reported velocities a little more than 7 km/
sec over the short profiles may correspond to the layers in the
crust and not on the mantle surface.
In the middle of Iceland under the neovolcanic and rifting zone
there has been established a depression confined to the M discon-
tinuity and a significant increase of thickness of the sequences
with velocities 6.5-7.0 km/sec. The data on the travel times of
the seismic waves as well as the shape of the Bouguer gravity ano-
maly suggest a bowl-shaped depression approximately correspond-
ing to the outlines of Iceland. Such a structure favours a simple
explanation for the lower velocities in the Iceland mantle as indi-
cated in the seismologic reports (9). This depression under Ice-
land accounts for the increased values of the travel times at the
stations located on the opposite to earthquake foci side of the island
due to the delays of the waves passing through the thick crust as
compared with the values recorded at the stations ljnng on the
opposite shore. The calculations done by us show the data reported
by Francis (9) to yield the normal velocities in the mantle if the
corrections for the crust are introduced.
Schematically tlie constructed cross-section together with the
column-sections of the adjacent areas is shown in Fig. 8. The
block boundaries in Fig. 8.1 are approximate. A similarity of some
parts of the cross-section and the column of the continental crust
in Norway (15) is conspicuous (Fig. 8.2) in the areas of the
Faeroe and Shetland Islands and in part in the Iceland peripheries.