Greinar (Vísindafélag Íslendinga) - 01.01.1977, Page 74

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mas, has a central SW-NE trend, or just the tectonic one
which has prevailed in this region for millions of years, and
we have interpreted as maximum shear direction in Part I.
(15).
The anomaly is due to altered basalt which, in a drill core,
shows the surprisingly high magnetization of 1-2 orders of
magnitude higher than is usual for basalts. The present author
suggested at a conference in Reykjavík a few years ago, when
the magnetic results were presented, that this would just be
a case of increased magnetization by long-duration shear action
and aligning of domain vectors by flowage.
It so happens that the seismic Layer 3 in Iceland (50) has
just at Stardalur the exceptionally shallow depth of 400 m,
whereas the usual depth to its surface is 3—4 km. Thus the
Stardalur rock is at any rate a short distance above Layer 3
which seems seismically to correspond to the Oceanic Layer.
Normally the temperature at the surface of Layer 3 in SW-
Iceland is estimated on the basis of geothermal gradient data
to be about 350°C (50), at which temperature a large part of
the original TRM of the basalts in the layer would have been
lost. But the layer has been under non-hydrostatic stress since
it was formed (cf. Chapter 3). The depth is such that flowage
is to be expected, when we keep in mind the state at a shallow
depth inferred from the shallow foci in ridge systems.
A strong magnetization due to flowage and alignment would
then not be unexpected, and this could survive the exhumation.
Taking into account that along ridges the focal mechanism
shows steep dip-slip motion, it would be the vertical component
of magnetization which would be influenced by shear flowage.
There must be a horizontal shear too, parallel to ridge ele-
ments, which follows from an elementary theorem of elasti-
city theory, to compensate the horizontal shear stress along
transcurrent faults, which are just conjugate lines to the
ridge elements ((39) and Part I.). But the horizontal com-
ponent along ridge elements is small in comparison with the
vertical, step-forming motion on faults, and will hardly be