Greinar (Vísindafélag Íslendinga) - 01.01.1976, Side 114

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be concentrated only within the rift zones proper. And here also,
the factors, hmiting the scale of tension, are the outcrops within
these zones of more ancient rocks, serving as the foundation for
younger ones. For example, the outcrops of plateau-basalts and of
the Pliocene deposits on the Tjörnes peninsula near Húsavík reduce
the possible spreading of the cmstal “plates” in the north of the
Eastem rift zone to 20 km. Approximately the same width is
covered by the outcrops of the Holocene lavas in the south of the
same zone, between the outcrops of older rocks of the Hreppar
uplift and the westernmost exposures of the analogues of “ancient
grey basalts” along the southern coast of Iceland on the eastern
margin of the rift zone.
In the north of the West rift zone the Quaternary volcanics
are missing and its bottom is formed entirely by the Pliocene and
the Upper Miocene rocks, which experienced considerable frag-
mentation.
It should be emphasized, that tlie given figure (20 km) of the
possible spreading for the Eastern rift zone is true only in the case,
if it is proved that there are no depositions of more ancient rocks
beneath the young lavas in the band of that width. If future de-
tailed geophysical or drilling works discover the Miocene plateau-
basalts in the base of the succession of the rift zone, then the size
of spreading shall be much less.
What is Iceland, after all?
The structure of the Earfh’s crust under Iceland was studied by
seismic techniques many times during the last 15 years (4, 7, 18,
28. 29, 30). However, only in the process of works of the Soviet
Geodynamic Expedition were quite reliable data obtained about
the deep structure of the interior under Iceland and some of the
adjoining regions of the sea. Seismic studies, carried out under the
leadership of S. M. Zverev and I. P. Kosminskaya (this volume),
have shown that Iceland is underlain by extremely thick crust
reaching 50 km thick. About 30 km thick is the cmst under better
part of the Iceland-Faroes threshold. It becomes much thinner only
under the relatively deep water link on this threshold. It is known,
that such thickness of the cmst is not typical of the oceanic cmst