Greinar (Vísindafélag Íslendinga) - 01.01.1976, Page 36

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an anomaly caused by an infinitely deep square prism; magnetiza-
tion contrasts of order (5—10) • 10“3 Gauss have been obtained, and
the bodies appear to extend to within a few hundred meters from
the sea bottom. AU these anomalies are listed in Table I.
By comparison with on-shore geophysical data (see section 1),
Thors and Kristjansson (14) in a preliminary report on the south-
em part of area 7 therefore postulated that a zone having an un-
usually high concentration of central volcanoes continues west-
wards offshore from a similar zone on the Snaefellsnes peninsula.
Kristjansson (15) further postulated, on the same grounds, that
significant gravity anomalies should be occurring at or close to
the magnetic ones in areas 9 and 7.
Free-air gravity data from the 1972-3 surveys off all of West-
em Iceland, digitized at about 4.5 km intervals along the tracks
and contoured, were subsequently made available to the author
by Dr. G. Palmason of the National Energy Authority. These
showed indeed gravity anomalies, mostly positive, at or close to
many of the labelled regions, and also at several others which are
shown by the letters J through V in Figs. 2 and 3. Most of the
latter are also associated with distinct magnetic anomalies, which
had not been considered large enough to label in the first round.
A list is given in Table II.
In Tables I and II there are altogether 25 positive magnetic
anomalies, averaging 2100y, and 10 negative ones, averaging
-1100y. There are 14 positive and 4 negative gravity anomalies.
As is evident from Fig. 6, most of the larger magnetic anomalies
are sharply bounded, indicating that they are due to discrete in-
tmsive bodies rather than diffuse swarms of dykes or sheets. How-
ever, smooth anomalies like those labelled N and V, and back-
ground in some of the others, may originate in dyke swarms.
Anomaly A1 resembles the steep anomaly at Stardalur (6).
The age of the structures causing the localized anomalies is un-
known, but most probably it is Upper Cenozoic. As the localized
magnetic anomalies are dominantly positive, it is assumed that
the causative magnetization is mostly viscous and/or induced,
rather than the respective bodies all being formed during normal-
polarity epochs. The only localized anomalies definitely associated
with major topographic expressions are B2 and a similar anomaly