Jökull - 01.12.1989, Page 48

60° 90° 120°
Fig. 6. The power spectrum of the gravity map. The
spectrum is symmetric about the origin. A protractor
is superimposed on the upper half of the spectrum to
help in its inspection. Frequencies are lowest (long-
est wavelengths) at the center and increase radially
outwards to the Nyquist frequency at the edges.
Spectral anomalies are perpendicular to the space
domain lineaments they represent; hence, linea-
ments running north-south in the space domain
result in horizontal anomalies while east-west linea-
ments create vertical anomalies in the spectrum.
At low frequencies the energy in the gravity spec-
trum is located in the upper-left and lower-right qua-
drants, corresponding to a broad gravity anomaly of
a general NE-SW direction, but at higher frequen-
cies this separates into two major peaks. A third
high frequency peak appears in the lower-left and
upper-right quadrants, corresponding to narrower
anomalies running NW-SE in the gravity map.
Mynd 6. Aflróf þyngdarkortsins er samhverft um
miðjuna. Yfir efri helminginn er lagður gráðubogi
svo auðveldara sé að lesa úr aflrófinu. Tíðni er lœgst
í miðjunni og vex síðan í Nyquist tíðni á brúnunum.
Frávik jafngildislína í aflrófinufrá sammiðja hringj-
um standa hornrétt á þœr meginlínur í kortinu sem
valda þeim. Þannig veldur norður-suður stefna á
kortinu láréttu fráviki í aflrófinu og lóðrétt frávik í
aflrófi svarar til austur-vestur stefnu meginlína á
kortinu.
I aflrófinu er lágtíðniorkan einkum í efri fjórðungi
til vinstri og neðri fjórðungi til hœgri, sem svarar til
breiðs þyngdarfráviks með stefnu NA-SV, en við
hœrri tíðni klofnar þetta í tvo toppa. Þriðji há-
tíðnitoppurinn í aflrófinu er í neðri fjórðungi til
vinstri og efri fjórðungi til hœgri, en það svarar til
mjórra þyngdarfráviks með stefnu NV-SA.
As mentioned earlier, Sæmundsson (in press) sug-
gests two rows of volcanic complexes which would
stretch across the gravity map in Figure 5. Most of
the complexes are accompanied by positive gravity
anomalies. The first row starts at Kollafjörður in the
west3, crosses the rifting zone at Hengill and
extends across the gravity high in the southem low-
lands to Hekla (a gravity low). The second row
starts at Hafnarfjall and Ferstikla in the NW comer
of the map, crosses the rifting zone at the relatively
high gravity values between Þingvellir and Skjald-
breiður and reaches east to the positive anomaly at
Þjórsárdalur.
The proposed rows of volcanic complexes' follow
the direction of relative plate motion, N280-285°E,
as calculated from the plate rotation pole of Minster
and Jordan (1978), and might be interpreted as trails
created by foci of high volcanic productivity in the
rifting zone. It must be noted, however, that an indi-
cation of lineation with this direction is conspicu-
ously missing from the power spectrum in Figure 6.
THE MAGNETIC DATA
The aeromagnetic data were mostly acquired in
1968-1980 with several lines added in 1985-1986.
The magnetic field values in the latter have been
corrected for secular variation to make them compa-
tible with the earlier data. The flight altitude was
3. The Kollafjörður and Stardalur central volcanoes, which
belong to this group, are discussed in the following chapter
on interpretation of the magnetic anomalies.
46 JÖKULL, No. 39, 1989