Jökull - 01.12.1980, Page 47

dilation in a section near the west end of the
Vogar fissure swarm (partly outside the map
in Fig. 2). His section is 4 km long and inter-
sects 12 fractures with a total dilation of 17.65
m. T. Einarsson (1965) believes that all the
major fractures, in the Holocene lavas on the
peninsula, have formed near the beginning of
that period. Jónsson (1978) has carried out the
most detailed geological investigation in the
area. Some of his results can also be found in
Jakobsson et al. (1978).
Various tectonic studies have been under-
taken in later years. Tryggvason (1968, 1970,
1974a, 1974b) measured vertical movement
across the Vogar fissure swarm. He estimates
the maximum subsidence, during the 10000
years since the lava formed, as 50 m. The
measurements are only relative, i.e. not refer-
red to sea level. Movement is irregular; both
subsidence and uplift occur, but the general
tiiting of the area is 2 microradians per year.
Nakamura (1970) made a study of the fissure
swarms on the western part of the peninsula.
He concludes that the Reykjanes Peninsula is
an oblique spreading ridge. Brander et al.
(1976) measured horizontal movement. In a
profile near the west end of the peninsula they
inferred an average 9 mm dilation per year,
during the interval 1968—1972. New
measurements were carried out in the summer
1979, but the results have not been published
yet. .
Seismology has given important infor-
mation. Pálmason (1971) has examined the
crustal structure of Iceland from explosion
seismology. Some of his profiles are in the
Reykjanes Peninsula. The main results for
that area are as follows: the surface layer
(layer 0), composed of a mixture of young
lavas and hyaloclastites, is 0.35 to 0.54 km
thick. Below are layers 1 and 2. Both are es-
sentially flood basalts, reaching down to depth
of about 3 km. Layer 3, below this depth, is
composed of metabasalts, and is probably to
be equated to the oceanic layer. At a depth of
about 8.5 km layers 4 takes over. It represents
the upper mantle. Fjarthquakes are frequent
on the Reykjanes Peninsula. Most of the rele-
vant data is given in Klein et al. (1977) and P.
Einarsson (1979). The majority of the earth-
quakes occur within a 1 —2 km wide zone that
runs along the peninsula, parallel to the zone
of volcanism. The focal depth usually varies
between 2 and 5 km, the maximum being
about 10 krn.
METHODS
Air photographs in scale 1:35700 were used
to map the Vogar fissure swarm and for most
of the measurements on the fractures.
Detailed field work was also carried out. Its
main objective was a detailed study of in-
dividual fractures and to test the accuracy of
the laboratory methods. The accuracy of the
data given in this paper is 0.5 m for width and
throw of fractures, and about 1 ° for orien-
tation of fractures.
THE VOGAR FISSURE SWARM
- RESULTS
General comments
For various reasons, the Vogar fissure
swarm is the most appropriate for detailed
measurements. In the first place, it lies ahnost
Fig. 3. Location of profiles across the Vogar
fissure swarm. (3) is column number and (3)
is profile number. — Mynd 3. Staðsetning sniða
yfir sþrunguþyrpmguna við Voga. (3) er númer dálks
og Q) er númer sniðs. Önnur tákn eru eins og á mynd
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