Jökull - 01.12.1980, Page 63

Fig. 16. A probable explanation of the Vog-
ar fissure swarm. The intrúsion complex is
composed of sills, fed by dykes. — Mynd 16.
Líkleg skýring á tilurð sprungupyrpingannnar við
Voga. Innskotin eru sillur og gangar.
the moment see serious objectiöns to this
model, it needs to be worked out irt detail, and
tested, before it cán be said to account for the
Vogar fissure swarm.
SUMMARY AND CONCLUSIONS
This paper describes the Vogar fissure
swarm on the Reykjanes Peninsula, SVV-
Iceland, and discusses the origin of the frac-
tures. Detailed measurements on the swarm
were carried out. For various reasons the
Vogar fissure swarm is the most appropriate
one on the peninsula for detailéd
measurements. First: it lies completely within
the same 10000 years old pahoehoe lava, which
is probably one of the oldest Postglacial lavas
on the peninsula. Second: it is the biggest
fissure swarm in this area, not covered by
younger lavas. Third: detailed precision
leveling measurements have been carried out
in this swarm over many years. Fourth: be-
cause of the age öf the lava, and size of the area
covered by it, the total dilation measured in
this swarm is a good estimate of the total
dilation on the Reykjanes Peninsula, as a
whole, during the Flolocene.
The main results of the measurements áre as
follows: Only about 25% of the fractures are
faults, or 34 out of 141 fractures. The mean
orientation of all the fractUres is 54°, referring
to azimuth angle, and the distribution follows
well the normal curve. The standard deviation
is only 17°. There is no significant change in
the orientation of the fractures along the fiss-
ure swarm. The average length of the 120
fractures, that lie completely inside the same
lava, is 611 m. Fracture length has a reason-
able correlation to maximum dilation (r = 0.65)
and to maximum throw (r = 0.64), but its
linear relatión to óther parameters is small.
The average width, in the 1076 points of
measurements inside the Vogar area, is 0.6 m.
The mean of the maximum width of all the
fractures is 1 m, but the maximum width of a
single fracture is 7.5 m. Maximum width has a
reasonable correlation to maximum throw
(r = 0.62). The maximum dilation across the
fissure swarm is 15 m. The dilation is greatest
near the middle of the swarm and diminishes
towards both ends. The mean of the
maximum throw on all the fractures is 1.2 m,
but 4.7 m on the faults. Maximum throw is 20
m. All the faults are vertical or slightly rever-
sely inclined. The fault walls are generally
very irregular. All the fractUres end in very
small fissures.
Various hypotheses have been proposed to
explain the fissure swarms on the Reykjanes
Peninsula in general, and the Vogar fissure
swarm in particular. On exámination they are
all found to be inadequate and a new explan-
ation, mágmatic intrusions, is proposéd. The
types of intrusions discussed are: (i) A vertical
dyke or dykes. Of the various possibilities here,
a dyke swarm is found to be the most prob-
able. Flowever, the inclination of the faults
appears to speák against such a model: but it
does not refute it. (ii) A horizontal sill, or sills.
The single sill inodel is considered in detail. It
is found that a 9.5 m thick sill (maximum
thickness), at depth of about 2 km below the
surface can account for the measured dilation
in the Vogar area. However, the inclination of
the faults is again difficult to explain by this
model. It is concluded thal many sills is a more
probable model and it appears to be able to
explain the Vogar fissure swarm.
JÖKULL 30. ÁR 61