Rit (Vísindafélag Íslendinga) - 01.06.1971, Síða 115

CRUSTAL STRUCTURE OF ICELAND
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with zero dip between Eyjafjördur and the volcanic zone, in a quali-
tative agreement with the basalt dips.
In conclusion it may be stated that the seismic 2—3 boundary and
the surface dips of the Tertiary basalt do not conform to each other
in eastern and western Iceland. In northern Iceland, however, there
appear to be considerable similarities in the overall picture, and in
some places there is a good quantitative agreement between the two
sets of data. This could mean that at the time of deposition of the
visible Tertiary lava flows in northern Iceland, the 2-3 boundary
was largely horizontal, and later by tectonic processes deformed into
its present form. In eastem and westem Iceland the seismic 2-3 houn-
dary would then have been created in its present form after the tilting
of the flood basalts.
11.7. Recent volcanism.
The two volcanic zones in southern Iceland appear to have a very
different seismic structure associated with them. The eastern zone,
which is by far the most productive one in postglacial times according
to estimates made by Thórarinsson (1967), appears to coincide with
the depth change to layer 4 in southem Iceland from 8—9 km to 14—15
km. The three most productive volcanoes in Iceland in the past 1100
years, Lakagígar (12.7 km3), Hekla (9 km3) and Katla (3 km3),
appear to be located near the edges of this transition zone, Hekla on
the westem edge and Lakagígar and Katla on the eastern edge. Petro-
graphically there is a large difference between the volcanic products
of Hekla and Katla (Thórarinsson, 1967; Tómasson, 1967). The
Hekla volcanic products are largely acid and andesitic, but the Katla
product is very uniform alkali-olivine basalt. Perhaps this difference
is related to the volcanoes’ position on the opposite edges of this
transition zone.
The seismic stmcture under the western zone does not appear to
be different from the adjacent areas except for the larger accumula-
tion of low-velocity volcanic products at the surface. On the Reykja-
nes peninsula layer 2 (Vp ~ 5 km/sec) appears to be missing, but this
does not apply to the whole western zone since at Thingvellir it ap-
pears to be present. The 2-3 boundary and the 3-4 boundary do not
appear to be appreciably affected by the volcanic zone. The lack of
any characteristic seismic stmcture associated with the western zone
is noteworthy, especially as this is considered to be the continuation