Greinar (Vísindafélag Íslendinga) - 01.01.1977, Side 127

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high-Q tongue itself, with the sharp bend, is a feature of Her-
cynian orogeny. But the present stresses in the bend might
have been created by a slight and fairly recent movement.
Drillings into the sediments in the trenches will in due time
reveal their ages, if these cannot be inferred in another way,
but we suggest provisorily that, as the trenches are of such a
wide occurrence in the circumpacific area, their origin was a
global event. A stress field as discussed in Chapter 3, is then
a likely cause of the present seismicity, not unlikely of late
Tertiary or even Pleistocene age. After finding the date, as
indicated, quantitative data as to strength in the bend, and
possibly the rate of stress loss by flowage could be found
quantitatively.
These tongues would then, furthermore, provide an insight
into the reason for the below discussed inheritance of Her-
cynian tectonic structure in the present system of submarine
ridges, and clarify the possibility of the reappearance of that
old structure by the action of Pleistocene and Tertiary stress
fields, to become the ridges of present day.
Speaking of earthquakes, let us emphasize very strongly,
that just as the world stresses must be highly variable through
geological history and even within such times as centuries,
in accordance with Chapter 3, and are steadily fluctuating, the
present inferences from focal mechanism are just of temporary
meaning. They are representative for perhaps the last 1—2
thousand years at the outmost, as to intensity and sign, and
in special cases, found by corresponding analysis, they may
be significant just for a couple of years. There can, therefore,
be no sense in an extrapolation of present seismic findings
150 million years back into geologic history.
In Indonesia and along eastern Asia, and also in other cases,
the arcs are Alpine geosynclinal in origin, although quite pos-
sibly of older design, and they are in a state of volcanic ac-
tivity. Such activity is often a certain stage of a geosyncline,
either one at the end of orogeny, when the content of the geo-
syncline has been consolidated, and the plasticity, due to the
new volcanism may possibly aid the tectonic squeezing up of