Rit (Vísindafélag Íslendinga) - 01.06.1967, Page 164

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Hreppar area both contain normally magnetized basalt from
the Gauss normal epoch.
Trausti Einarsson: This paper prompts several remarks.
Firstly what are the reasons for claiming that the 20 km wide
Thingvellir zone represents drift?
K. Saemundsson: In the 20 km broad Thingvellir zone the
most spectacular evidence for drift are the numerous normal
faults, open fissures and volcanic fissure eruptions occurring
there. If we look at the plateau basalts on both sides of the
Thingvellir zone with these unquestionable dilational fea-
tures it is seen to occ.upy a synclinal structure (not an anti-
cline where tension might be expected as a result of uparch-
ing). This is in my opinion a conclusive evidence for drift as
a primary cause in forming these tectonic features.
Trausti Einarsson: The fissures do not provide a conclu-
sive evidence for dilatation, as I have discussed in my paper
on the fracture system in Iceland earlier in this symposium.
They can be fully explained by shearing movement, and in
fact the Réttargjá and Strandagjá, which indicate shear, are
in a prolongation of the Thingvellir zone.
The fact which I stressed earlier that the width of the ac-
tive zone is commensurate with the thickness of the crust
becomes a curious coincidence in the drift hypothesis.
Further the blocks of the plateau basalts are separated by
anticlines, synclines, and flexures, while within each block
the dip is essentially constant. I cannot doubt that this means
tectonic disturbance of originally flatlying banks. To locate
original volcanic centres using the present dips of the lavas
must be done with extreme care.
K. Saemundsson: The dip within each block of plateau ba-
salts is not essentially constant. In eastern Iceland it has
been shown that the dip of an individual lava series in-
creases from the highest outcrops on the mountain tops down
to the valley floors. This is certainly not in agreement with
a general tectonic disturbance of an originally flatlying block.
It is difficult to understand how the lower part of the plateau