Greinar (Vísindafélag Íslendinga) - 01.01.1976, Side 159

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nes, at least as far as Eldeyjarbodi (63.5°N), and most likely right
along the northeast part of the axial zone of the Reykjanes ridge,
where there is ailso such a narrow crestal, clearly uplifted chain.
It is suggested that the dolerite shieldvolcanoes on both sides of
Thingvellir Lake may correspond in time to similar shieldvol-
canoes found along the sides of Langjökull, in the middle of the
Median zone, for there is correspondence in time between the for-
mer and the volcanic activity at the mouth of Skagafjord, the north-
emmost visible part of the Median zone.
If then, the shieldvolcano phase was characteristic for the Median
Zone as a whole, one may wonder whether the subsequent subsi-
dence within the zone, best seen now at the Thingvellir lake, was
also realized in the Langjökull part. In that case the tuff-breccias
of the LangjökuII socle might have been formed in deep lakes and
uplifted during the uplift phase of the Reykjanes peninsula, whose
evidences we have already traced before to the Skjaldbreidur area.
The socle of the Thórisjökull is very suggestive of such a history
(9, Fig. 28), for its structure is quite similar to that of the moun-
tain chain along the peninsula.
f) Some conclusions on sea-floor magnetic anomalies.
The crestal uplifted zone of the Reykjanes ridge links the ridge
in a natural way with the chain of young uplifted blocks on land
and, hence witli the SW-NE shear in our region of study. This
leads to the question: What are those parallel linear magnetic ano-
malies, so clearly restricted to a 10-12 km broad axial zone of the
ridge (21, cf. also the same width of the active zone on the Reykja-
nes peninsula)? Shear fractures, seems a rather obvious answer,
when we have shown that “spreading” has been non-existent in
the Thingvellir zone, at least for the last one million years or so.
Sea-water is a most active agency of altemation in rocks (22).
Fractures open the way for sea-water under presstu-e to penetrate
into the rock, and it would not seem surprising if this leads to
stripe-wise influence on the thermo-remanent magnetization of vol-
canic, especially basaltic rocks on the sea-floor (23, 24) or in rocks
of marine origin (25). This process would explain at the same
time why the anomalies on the Reykjanes ridge fade out in the