Greinar (Vísindafélag Íslendinga) - 01.01.1977, Side 75

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recognizable in seismic studies. The corresponding shear-
magnetization is then here and on transcurrent faults such
that it does not show up in measurement of the total magnetic
intensity. Hence there are linear magnetic anomalies parallel
to ridge elements but not or mueh less along transcurrent
faults.
In Part I we pointed out that the regular linear magnetic
anomalies along the crestal uplifted zone of the Reykjanes
Ridge near Reykjanes, would be due to shear fractures. We
were then thinking in terms of horizontal shear. But the rule
tells that in addition, there is a much larger vertical shear
motion on ridge faults. This should now be taken to be the
main explanation of the crestal linear anomalies along the
Reykjanes Ridge, although alteration caused by sea-water in
fractures may also play a role as we suggested in Part I.
When Hart and Press (51) chose a magnetic anomaly, which
they called the 50 million years anomaly, to mark the outline
of the Mid-Atlantic Ridge in the North Atlantic, their choice
was actually that of a fault or structure line, which was far
better than to use a certain axial distance in kilometers. Thus,
by this sensible choice, they made their seismic work indepen-
dent of the sea-floor spreading hypothesis. The name of their
demarcation line (also the “10 million years line”) is irre-
levant. If one really wants to put ages on the magnetic ano-
malies, there is some sense in it, being a reference to a certain
system, widely known. And it is probably true that the ano-
malies decrease in age, in a broad sense, the closer they are
to the ridge, because the zone of activity seems to have nar-
rowed with time, cf. the narrowness of the present axial zone
of seismicity. But in any other sense the ages of the anomalies
are meaningless, as they are based on an unacceptable hypo-
thesis. We have proved directly in Part I that in the very
prolongation of the Reykjanes Ridge, the birthplace of the
hypothesis of sea-floor spreading, there has been no spreading
for a time of the order of a million years, and our description
in Chapter 2 here, of the uninterrupted extension of the Upper