Greinar (Vísindafélag Íslendinga) - 01.01.1977, Page 70

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Nevada trend in Carboniferous, Permian, and Mesozoic mate-
rial. For this and further reasons, we have since then con-
sidered the magnetization of old sedimentary rocks to be a
tectonic phenomenon, not one caused directly by a geomagnetic
field, unless the latter interpretation is in certain cases proved
to be correct.
But it was an overwhelmingly sedimentary material with
which the two famous “pole”-curves for Europe and America
were constructed, and by so many taken to be a proof of conti-
nental drift. We think, it is on the basis of shear planes in
global stress fields, not on geomagnetic basis, that these cur-
ves should be interpreted.
But as to basalts, we have shared uncritically the general
opinion for 20 years: that their magnetization is geomagneti-
cally significant and trustworthy. Now, we must doubt this
view in light of the above facts, which should have been ob-
vious all the time. These facts about age and deep burial, we
consider to be more fundamental than the criteria used for
significance in paleomagnetic work.
To give examples, the Coal Measures of Scotland must have
been kept at temperatures of 100—150° C under the load of
3—4 km of rocks for times of the order of 100 million years.
The enclosed basalt lavas must have been subjected to the
same conditions.
What do we really know about viscous change of magne-
tization under such pressure-temperature conditions for times
which are at least a factor of million longer than any intervals
possible in a laboratory? And here we mean not least the
change due to non-hydrostatic stress. It seems to us that there
are no laboratory tests of such a process. Can we then deny
to take that process into account, when its effects seem to be
clear where geological time intervals were available?
On the other hand, there may be field circumstances which
show in some cases that basalts have very probably retained
their original direction of magnetization after such a burial
history, as we shall mention later.
Let us consider the Deccan Traps. Their age is Paleocene