Rit (Vísindafélag Íslendinga) - 01.06.1971, Side 123

CRUSTAL STRUCTURE OF ICELAND
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relict of an ancient isotherm. The oceanic layer is in general at a
temperature well below 400°C, away from the mid-ocean ridges.
Another piece of evidence, which may be of significance for the
interpretation of layer 3, comes from a comparison of the seismic
data in eastem Iceland with the detailed geological work of Walker
and his collaborators in that area. One of their results is the discovery
of a number of Tertiary volcanic centres, central volcanoes, within
the pile of flood basalts in eastern Iceland. Some of these volcanic
centres have been described by Walker (1959, 1963), Gibson
(1963), Carmichael (1964) and Blake (1964).
Fig. 45, which is reproduced from Walker (1966), shows the
location of the Tertiary volcanic centres in the area discussed here.
The existing seismic refraction profiles have been inserted. The
Fljótsdalur (25) and the Breiddalur (27) profiles are outside the
central volcanoes, the Skriddalur reversed profile (26 and 26a) mns
directly across the Thingmúli volcano, and the Lón profile (28)
crosses at least partly the Lón volcanic centre.
On the Skriddalur profiles the delay time of the P3-wave has a
minimum value on the Thingmúli volcano, increasing slightly in
both directions. The minimum depth to layer 3 is inferred to be about
1.5 km. The Fljótsdalur profile, which runs more or less parallel at
a distance of 10—15 km to the west, has a considerably higher delay
time and gives a depth to layer 3 of about 3.0 km. This profile ap-
pears to be wholly outside the Thingmúh volcano. The Breiddalur
profile does not pass directly over any central volcano, but its westem
end approaches closely to the Breiddalur volcano. The profile indi-
cates a relatively large depth of about 3.5 km to layer 3, but this
depth appears to be decreasing strongly towards the volcano. The
Lón profile gives the shallowest depth to layer 3 found in eastern
Iceland, about 1.2 km, and it runs close to the Lón volcanic centre.
The refraction data thus suggest very strongly that in eastern
Iceland the shallowest depths to layer 3 are associated with the
Tertiary volcanic centres. It may he assumed that the isotherms
have risen to shallower depth below these volcanoes during their
period of activity. This suggests that the same mechanism may here
have been responsible for the creation of the 2—3 boundary, as appears
to be working today in southwestern Iceland. The cmst in eastem
Iceland has later cooled down, leaving the 2—3 boundary fossilized.
Recently, Annells (1968) has completed a geological study of the
Vídidalur-Vatnsdalur area in N-Iceland, which is an area of shallow