Greinar (Vísindafélag Íslendinga) - 01.01.1977, Page 128

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the mass. In the other case the volcanism is the initiation of
the orogenic stage, i.e. movement and folding within the geo-
syncline. Let us for a moment assume that the arcs are in the
later stage of volcanic activity.
In that case, there was a past time, not necessarily long ago,
during which the geosynclinal content was very weak and
also reached a considerable depth. The geosynclines them-
selves may have been 5—10 km deep, and the early volcanic
phase may have made still deeper layers plastic. Finally, what
is of particular importance, crustal layers may have become
plastic due to deep burial, in a way that is evidenced by
the shallow seismicity on submarine ridges, and we discussed
earlier.
If a strong crustal stress field was rather suddenly put on
at this time (cf. Chapter 3), such as occurred certainly in
the beginning and final stages of Pleistocene glaciations,
(cf. Chapter 3), yield may most easily have taken place in
the geosynclines, and this would have to be on such slightly
dipping planes, as the arcs themselves very strongly suggest.
But why are the island arcs then characteristic of the Pacific?
Gliding in slightly dipping planes is known elsewhere in oro-
genic belts, and it is rather generally suggested by arc-like
structures. But the fact remains that the circumpacific zone
is typical for these clear and regular arcs.
It is difficult to see any obvious explanation of this — at
our time in geological history. But there was something very
striking at an earlier epoch.
In (39) the present author claimed that the submarine ridge
system in the Atlantic area fitted a global torsional stress field,
but he thought it sufficient to add, at that stage of his theory,
that “torsion gives a good representation for large parts of
the system” of submarine ridges. Therefore, he omitted what
he had already presented in university lectures in St. Andrews,
and the Imperial College in 1961, i.e. the two possible positions
of the axis of the torsional stress fiéld. These are a) left screw
torsion with one pole in about Pamir, and the other just west
of southernmost South America. b) right screw torsion with