Rit (Vísindafélag Íslendinga) - 01.06.1967, Side 107

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caused by structures with fairly steep boundaries between
different values of magnetization. A further argument for
vertical boundaries in our case is the change of 4000 y over
a distance of 50 m on the profile with the shallowest depth
to the magnetized rocks (Fig. 3). However, the possibility
of flood basalts, dipping 30° or more, causing the anomalies
cannot be excluded.
Vertical tectonic movements in structures with horizontal
boundaries between different values of magnetization can
create a change in magnetization as the fracture is crossed.
A tectonic scarp need not be accompanied by any vertical
displacement on the surface. Preglacial displacements are
likely to have been obliterated from the surface by subse-
quent erosion or lava flooding. In Ljósavatnsskard this could
explain the disparity between geological observations indi-
cating a large vertical displacement on the boundaries of the
Tertiary area (Hospers 1954), and seismic exploration which
fails to show any sudden change in the depth to seismic hori-
zons in this area (Pálmason 1963). The amplitude and shape
of the anomalies shows that the boundaries between normal
and reversed TRM reach from the surface of the magnetized
rocks down to considerable depths. It is difficult to suggest
a tectonic activity which could produce the large number of
such fractures required to explain the observed anomalies.
As usually in the interpretation of magnetic anomalies
where little other information is available, there is a wide
choice of possible explanations. In this case my preference
is for dykes. Dykes are younger than the surrounding rocks
and have cooled under different conditions. Their magnetiza-
tion can therefore be different. From the prospecting of the
Department of Natural Heat we know that the magnetic
anomalies of dykes can follow straight lines over distances of
several km. The anomalies in Ljósavatnsskard would be the
accumulated effect from many dykes, as individual dykes are
too thin to account for them (Walker 1959).
If the dykes all belong to the same magnetic epoch, they