Jökull - 01.12.1992, Blaðsíða 22

(Kaitt, 1963)
Figure 4. Crustal structure at
OBS stations in Axarfjarð-
ardjúp (left) along with the
currently used crustal model
for northem Iceland (NOR)
and standard oceanic crustal
model (right).
— P-bylgjuhraði semfall af
dýpi eftir bylgjubrotssniðinu
(til vinstri) í samanburði við
P-bylgjuhraða á Norður-
landi (NOR) og í úthafs-
skorpu (til hægri).
most crustal structure next to the coast where we see
relatively higher velocities within the upper sedimen-
tary section, which is underlain by lower velocities in
the lower sedimentary section. This velocity change
can be explained by higher velocity sediments which
thicken towards land (Figure 5). The seismic velocity
of a clastic sedimentary layer depends on the nature
of the clastic material, grain size and the depth of
burial. Lithological variations, such as grain-size dif-
ferences within volcaniclastic material, can create con-
siderable variations in density and P-velocity. Coarse
sediments or well-cemented volcaniclastic sediments
could generate the higher velocities observed in the
southem part of the Axarfjarðardjúp trough. Such
sediments with higher velocities have been found on
land in Axarfjörður, where they were drilled. A range
in densities from 2.0 g/cm3 (silt) to 2.7 g/cm3 (con-
glomerates) was observed in the 550 m deep Flatey
borehole (Gunnarsson et al. 1984). The higher veloc-
ity/density sediments may have been created during
erosional episodes which changed the depositional fa-
cies or caused a major depositional break in associ-
ation with sea level changes and variations in glacial
deposition.
Rapid fluctuations in sedimentary facies observed
in the Flatey core reflect altemating subaerial and near-
shore marine sedimentary environments of Pliocene-
Pleistocene age. A total of five glacial/interglacial
cycles were detected in the 550 m borehole. The sed-
iments are mainly of volcaniclastic origin, with some
lignite layers and only a few lava layers were recorded
in the sedimentary sequence (Gunnarsson et al. 1984;
Eiríksson et al. 1987). The rate of sedimentation in the
Flatey core is estimated at 500-1000 m/Ma (Flóvenz
and Gunnarsson 1991). If we assume similar rate
of sedimentation within the Tjömes fracture zone, in
spite of different tectonic environments, we find that
the 2.2 km sedimentary pile in the Axarfjarðardjúp
trough was deposited over a period of 2.2 to 4.4 Ma
and thus that the 4.3 km/s basement may consist of
Tertiary flood basalts. Using the same sedimenta-
tion rate for the 4 km thick sedimentary sequence in
the Eyjafjarðaráll trough we obtain a basement age
of 4.4-8.8 Ma. This suggests that the Eyjafjarðaráll
trough is older than the Axarfjarðardjúp trough which
is in agreement with Sæmundsson’s (1974, 1979) hy-
pothesis that the N-S troughs and volcanic chains of
the TFZ developed successively eastwards into the
20 JÖKULL, No. 42, 1992