Rit (Vísindafélag Íslendinga) - 01.06.1971, Page 114

114
GUÐMUNDUR PÁLMASON
major fault zone along Skagafjördur with a downthrow of approxi-
mately 1 km to the east is here required and is indicated on the re-
fraction profiles on both sides of the valley (cf. Fig. 24).
Starting in Hrútaf jördur, the basalts dip about 4° to the south. The
seismic structure as deduced from profiles 65a and 65b is shown in
Fig. 21. The dip of the 2-3 boundary is variable, but by taking the
average dip over the length of the profile, a value of about 4-5° to the
south is obtained, in good agreement with the dip of the flood basalts.
The 1—2 interface also dips to the south, but much less, on the average
about 1.5°.
Between Midf jördur and Langidalur the dips shown in Fig. 44
are irregular. A domelike or an anticlinal structure is indicated which
is roughly in agreement with the depth contours to layer 3 in this
area. Recent work by Annells (1968) (cf. sect. 12) shows that a
major Tertiary volcanic centre is located in this area, and that the
dips are therefore highly irregular.
In the area between Skagafjördur and Eyjafjördur the plateau
basalt dips are mainly to the south and southwest, 1-4°. The seismic
2-3 boundary is also shallowest in the northeastern part of the area,
increasing in depth to the south, and in the northern part of the area
also to the west. The agreement in the directions of the two dips is
thus relatively good. Quantitatively, a comparison is first made of
the southward dips between Dalvík and Akureyri, which is about 4°.
Layer 3 is at a depth of 2.7 km near Dalvík and at 4.7 km near Akur-
eyri. This gives an average southward dip of the 2-3 boundary of 3.0°
in fair agreement with the plateau basalt dip. South of Akureyri the
dips in the plateau basalts are smaller, about 2°, according to Ein-
arsson’s map, which is qualitatively in agreement with the seismic
data. Still farther to the south a reversal in the direction of dip of the
plateau basalts would be expected from the seismic data, but condi-
tions are here less clear because in the central part of Iceland the
plateau basalts are covered with a surface layer of younger rocks. In
the northern part of the area between Skagafjördur and Evjafjördur
the seismic dip also has a westward component which is qualitatively
in agreement with the surface basalt dips.
To the east of Eyjaf jördur, south of Akureyri, is a zone with zero
dip of the basalts, while still farther east the dip is mainly towards
the volcanic zone. The seismic data do not allow a quantitative com-
parison to be made here, but they indicate an anticlinal structure