Jökull - 01.12.1978, Page 40

cularly in the southeastern area (D 0 to E 8)
deviate markedly from values in the remain-
ing part of the profile. This is interpreted as
being caused by partial remagnetization of
the gabbro due to later minor intrusions
(Schönharting, in prep.).
Most of the investigated dykes show a pat-
tern of directions similar to that of the main
portion of the gabbro. This indicates that they
were intruded before the general tilting
movement. The “brown doleritic dykes” are
an exception. Many of these are normally
magnetized with westerly declinations, sug-
gesting a time of intrusion after tilting of the
lava piie. These dykes do not exhibit the
secondary mineralization assemblage which is
typical for the other minor intrusions in the
gabbro area. Insufficient sampling and
demagnetizing effects might be the cause for
the scatter and the average westerly declin-
ation of the “brown doleritic dykes”, which
has to be clarified by future measurements.
Two magnetic profiles across the major
negative anomaly (Fig. 5) are presented in Fig.
7. The upper profile extends over the gabbro
outcrop with the largest magnetization values,
and the lower profile across the sandur plain
SW NE
Fig. 7. Interpretation of the negative mag-
netic anomaly along profiles e, d and c at
Geitafell gabbro locality, magnetization in-
tensity 35X 10-3emu/cc, decl. 240°, incl.
H-60°.
Mynd 7. Túlkun neikvœös segulfráviks í mœlilín-
um e, d og c í Geitafellsinnskotinu, styrkleiki segul-
sviðs 35 X 10~3emu/cc, misvísun 240°, halli
4-60°.
to the south. An interpretation of the mag-
netic anomaly has been attempted, using the
mean intensity and direction of magnetization
values of the strongly magnetized zone. A 150
m thick, sheet-like body, dipping 45° in a
north-easterly direction would give a mag-
netic anomaly which matches the measured
anomaly reasonably well. A depth to the top
of the body of 100 m has been calculated for
the sandur profile.
The somewhat surprising, relatively shal-
low dip compared to other intrusions in the
area applies only to the strongly magnetized
zone within the gabbro and is perhaps not
representative for the intrusion as a whole.
DISCUSSION AND CONCLUSIONS
The magnetic anomalies above the
Hvannadalur and Geitafell major intrusions
are related to strong remanent magnetization
in certain zones within the gabbros. Such
zones could well represent important sources
for aeromagnetic anomalies over Iceland as
well as for magnetic anomalies over the oce-
ans, if the thermal and oxidation environment
and also the types of intrusions are similar to
those discussed here.
From calculations of the magnetic field it
follows that the Geitafell magnetic model of
Fig. 7 displaced to 2.5 km depth would pro-
duce an anomaly of 50 gamma amplitude
with an anomaly width of over 5 km. A few
bodies of this type could therefore, if properly
arranged, give rise to all or part of a typical
ocean-ridge magnetic anomaly.
The stability values S200 found for the
remanent magnetization of the gabbros are
smaller than those known from average oce-
anic basalts. These stabilities are, however,
considered sufficient to preserve the record of
the earth’s magnetic field for at least some 100
m.y., as required for the Vine and Matthews
theory (1963) on oceanic anomalies
(Schönharting, in prep.).
The sheet-like shape of the magnetization
zones, their relatively shallow dip, and
furthermore, the apparent association of these
38 JÖKULL 28. ÁR