Jökull - 01.12.1989, Page 51

60° 90° 120°
Fig. 9. The power spectrum of the magnetic map.
For a general explanation of the spectrum see Figure
6. The NE-SW lineation apparent in the magnetic
map dominates the spectrum, but tums out to be
composed of two directional components, 45° at low
frequencies (near the center of the spectrum) and
30° at higher frequencies. This reflects the en
echelon pattem exhibited by the magnetic anomalies
in Figure 10. A high frequency peak is also seen in
the spectrum at 135°. This corresponds to the mag-
netic anomalies depicted in Figure 11.
Mynd 9. Aflróf segulkortsins. Skýringar varðandi
aflróf eru gefnar með mynd 6. NA-SV stefna gos-
beltisins er ríkjandi í aflrófinu, en reynist samsett úr
tveimur þáttum. Lága tíðnin (nœr miðju aflrófsins)
stefnir 45°, en hœrri tíðnin stefnir 30°. Þetta endur-
speglar skástíga legu segulfrávikanna á mynd 10.
Auk þessa er í aflrófinu hátíðnitoppur sem stefnir
135° og svarar til segulfrávikanna á mynd 11.
glacier, and the fourth over the glacier Eiríksjökull.
The second anomaly is geologically most interest-
tng. It coincides with an apparent gravity minimum
(inadequately defined over the glacier) and intensive
seismic activity in a volcanically productive area.
East of the rifting zone there is an anomaly over
Hreppafjöll as well as several anomalies on the
eastem edge of the map. Two of them are associated
150. with the volcanoes Tindfjöll and Hekla while the
others appear to be topographically induced.
CROSS-GRAIN STRUCTURES
As mentioned earlier, the gravity and magnetic
iæ° spectra both contain a high frequency energy peak
transverse to the main trend along the rifting zone.
Separating these features out of the respective maps
reveals a positive gravity lineament (Hafnarfjall-
Ferstikla) and a suite of negative magnetic linea-
ments (Figure 11). The most prominent magnetic
anomaly extends from the rift zone near the south-
em part of Þingvallavatn northwest across Hval-
fjörður and Borgarfjörður where it coincides with
the positive gravity anomaly. A similar linear ano-
maly in seismic velocities was noted by Pálmason
(1971) and interpreted as reduced depth down to
layer 3. The lineament cannot be regarded as seismi-
cally active. Figure 11 shows the seismic anomaly
superimposed on the positive gravity and negative
magnetic anomalies which have been passed
through a narrow NW-SE oriented directional filter.
Certain portions of these lineaments seem to have
their origin in recognized volcanic complexes such
as Stardalur, Kollafjörður and Ferstikla, but their
apparent organization into lineaments transverse to
the rifting zone requires an explanation. The NW-
SE direction differs from both the relative plate
motion (N280-285°E) and the plate motion with
respect to a fixed hot-spot reference frame (N277E0
and N282E°) as estimated from the plate motion
models of Minster and Jordan (1978). This suggests
that the lineaments cannot be just trails of volcanic
centers, emanating from active spots in the volcanic
zone or embedded in the mantle below.
One possibility is that the lineaments correspond
to contraction cracks within the cooling plate as it
moves away from the platé boundary. The com-
ponent of contraction perpendicular to the boundary
is simply added to the drift velocity while the com-
ponent parallel to the boundary leads to contraction
cracks that are perpendicular to trajectories of equal
JÖKULL, No. 39, 1989 49