Greinar (Vísindafélag Íslendinga) - 01.01.1976, Page 113

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swarms and contain negligible number of dikes. This induces one
to assume that even if the dikes indicate crustal tension, the latter
cannot be universal, but local, associated only with the area of
individual swarms.
Finally, there is doubt that the appearance of each dike was
preceded or accompanied by separation in the crust with the ampli-
tude equal to the thickness of the dike. We observed many dikes,
in the vicinity of which the enclosing rocks were much deformed,
thus obviously testifying in favour of an “aggressive” intrusion of
the dike’s material. Perhaps, initial fissures were narrower and the
dike widened them during intrusion; this widening could be partial-
ly compensated by compression in the adjoining areas1.
Faults in the rift zones and also the numerous faults connected
with the block structm-e of the whole Irrner zone, apparently, all
together indicate tension. However, the total size of tension is diffi-
cult to establish, since perhaps not less but possibly even greater
importance in the development of the structure of this major part of
Iceland belongs not to spreading but to vertical movements of blocks.
In the light of all these remarks, the horizontal tension, necessary
for the imderstanding of the observed structures of separate zones
and subzones, would hardly exceed several kilometers, while in the
scale of the whole island it would be several tens of kilometers.
In accordance with the “ocean floor spreading” hypothesis we
should have supposed a much greater tension, measured by hun-
dreds of kilometers. Is there place for it? We have just seen that
the dikes, dissecting the plateau-basalts, cannot be applied to con-
firm the tension on such a scale. On the vast spaces of the Inner
zone, except rift, the exposures reveal superpositions of the Phocene
and Eopleistocene “grey ancient basalts” or of their stratigraphic
analogues on the Miocene plateau-basalts, and also of the rocks of
the Pleistocene formation on the “ancient grey basalts”. This fact
excludes all such areas from the zone of spreading. The latter can
1. Repeating high precision geodetic measureraents of the width of the East rift
zone in its southem part made in 1967 and 1970 (10, 44, 45) showed that in
connection with the eruption of Hekla in 1970 adjoining to it in the north-east
region 15 km wide experienced the tension of 86±24 mm, but it was at least
partially compensated by compression of 20±17 mm in a more westem area
10 km wide, and also hy compression of a more eastem area.