Rit (Vísindafélag Íslendinga) - 01.06.1971, Page 111

CRUSTAL STRUCTURE OF ICELAND
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made by Einarsson (1967a) to trace some of the anomalies found on
the Reykjanes Ridge into Iceland. It has furthermore heen shown by
a detailed aeromagnetic survey (Sigurgeirsson, 1969) in southwest
Iceland that the central magnetic anomaly on the Reykjanes Ridge
continues into Iceland in the way indicated by Searson’s survey.
Strong linear anomalies with a SW—NE direction are apparent in
southem Iceland. They appear to coincide with the two branches of
the volcanic belt. These linear anomalies appear to break down near
65° latitude, although the easternmost one may perhaps be traced on
with smaller amplitude and a changed direction along the volcanic
belt in northem Iceland (Schönhartling, 1969).
It does not appear to be possible to find in the seismic stmcture
in SW-Iceland an explanation of what may cause the central ano-
maly. Perhaps this is because the profiles have not been located speci-
fically to study this question. The thickness of the young volcanics
layer (layer 0) appears to be relatively great, 500-1000 meters, in
the zone of the positive anomaly, and this may contribute somewhat.
There are no signs of major disturbances in the deeper boundaries
associated with the central anomaly.
The eastemmost anomaly (designated RE by Einarsson) appears
to be directly above the major structural change in the 3-4 boundary
which has been found under the eastern branch of the Neovolcanic
zone. It could thus be related to this structure or to the volcanic zone.
The breakdown of the linear magnetic anomalies near 65°N lati-
tude could be related to the change in depth to the 3-4 boundary
which is inferred from the delay times of the P4-wave along pro-
file WP.
Many of the small wavelength gravity anomalies mentioned ear-
lier also have magnetic anomalies associated with them, as has been
shown by the aeromagnetic surveys of Sigurgeirsson (1969).
It is of interest to note that the width of the magnetic axial zone,
given as 200 km on the Reykjanes Ridge by Heirtzler et al. (1966),
is very close to the width of 230 km of the zone in SW-Iceland, which
is characterized by a relatively shallow depth to layer 4 (8—9 km).
This axial zone on the Reykjanes Ridge has a characteristic magnetic
pattem of large amplitudes and short wavelength compared to the
anomalies in the flank areas on both sides of the axial zone (Heirt-
zler et al., 1966; Godby et al., 1968). It is also characterized by a
very thin sediment cover (Ewing and Ewing, 1967; Talwani et al.,
1968). The depth to the seismic layer 4 in the axial zone on the