Greinar (Vísindafélag Íslendinga) - 01.01.1976, Side 160

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sediments of the shelf SW of Reykjanes — these lack natural ther-
mo-remanent magnetization — and why the elongate anomalies on
the Reykjanes peninsula are quite different from those on the sub-
marine ridge (26).
Two further points of general natin-e should be mentioned in
this connection. First, the magnetic picture of the Reykjanes penin-
sula (26) strongly suggests that the late volcanic activity plays
directly a role, whether or not tectonic activity per se also enters
the picture. Volcanism may, therefore, play a role on submarine
ridges also. How large a role, is a more difficult question. First, it
very much depends on the accuracy of the picture of the submarine
anomalies to what degree the magnetization of primary volcanic
bodies can be separated from the picture due to alteration or fault-
ing or structural boundaries. Secondly, where anomalies are very
broad, the magnetic body may be relatively deep or thick, and the
faulting beneath the younger sediments must also be considered.
The distal anomalies of the ridge (21, Fig. 2a) are much broader
than those of the narrow axial zone. Fractures in the former region
would almost certainly be much older than those of the narrow
axial zone and the lines of alteration might be correspondingly
broader. But the difference has probably also a deeper root: the
two types of anomaly are of different categories. The broad and
very long ones could be intimately connected with such a phase of
areal faulting as is evidenced by the Second Main Tectonic Phase
in Iceland (11, 9), during which most of the country was uplifted
in blocks, while the Tertiary plateau basalts subsided around the
country, especially clearly south of eastem Iceland, i.e. in the
oceanic lane in which the broad magnetic anomalies are so clear.
The subsidence here seems to be indicated by the sinking of the seis-
mic Layer 3 in Iceland from 4 km to 10 km depth (27, Fig. 37).
In other words, the narrow ridge-anomalies are a category belong-
ing to a late phase of the zonal activity which set in after the
Second Main Tectonic Phase, whereas the distal anomalies are a
category belonging to an areal tectonic phase, perhaps that of the
Second Tectonic Phase. In that category the anomalies are most
likely to be due to faults in the basalts, hidden morphologically
under a veneer of marine sediments. And when we look at the
mosaic of anomalies in the intermediate field between the axial