Jökull - 01.12.1980, Síða 56

their en echelon arrangement, the ends usu-
ally become parallel. However, after a certain
amount of growth the ends bend towards each
other, the rock in between breaks and the ends
combine. (4) As a result of continuing “ten-
sion” in the area, the fractures develop into
normal faults, i.e. one block subsides relative
to the other. It should be noted that normal
faults, developed on the basis of tension frac-
tures, are either vertical or very steep (Beloussov
1962), as is the case in the present area.
I have especially chosen fracture number 64
to indicate this general development — as
shown in Fig. 13. From 64 lie a number of
minor fractures: 76, 77, 118, 1 19 and 137. All
these minor fractures occur at joinings on the
main fracture. Fracture 64 is relatively sinu-
soidal, although not as regularly as e.g. frac-
ture number 36. The joining at fracture 77 is a
particularly good example about the accretion
of fracture 64. There it can be clearly seen how
the fracture, lying from fracture number 76 to
fracture number 77, has curved towards the
fracture from 77 to 119 until they joined.
All the fractures end in very small fissures
that die out at the very ends. These small end
fractures are the growing parts of the main
fractures. It should be noted that some of the
end fractures have different orientation from
their main fracture. This is clearly seen on
fractures number 22, 52, 82, 102 and 114.
Obviously, this cannot be explained, like some
en echelon tension fissure systems, by rotation
of the older parts; and there are in fact only
two possible explanations: (1) The end frac-
tures follow some weakness in the beds be-
neath the surface lava, or (2) the end fractures
are curving towards other fractures. The latter
is, though, ruled out in some cases as the dis-
tance between the fractures is too great. Not-
withstanding this, combination of both ex-
planations seems to explain these deviations
satisfactorily. I also emphasize that these
curved fractures are exceptions; the great
majority of end fractures follow the same
direction as the main fractures.
The Vogar fissure swarm is cracking, i.e.
lengthening, towards the east. It is, however,
difficult to say if the cracking has been con-
tinuous or was periodic. Measurements by
Tryggvason (1968, 1970, 1974a, 1974b), and by
Brander et al. (1976) indicate continuous
movement in the area. But these movements
are very slow and irregular, and some may be
explained by movement of warm water
(Tryggvason 1970). The lavas east and west of
the mapped area are without fractures, and
these are 1—2 thousand years old (J. Jónsson
and S. Einarsson, personal communication,
1978). It is known that the fractures from the
Vogar fissure swarm continue beneath these
young lavas, especially the one at the western
end of the swarm. As no fractures are visible in
these lavas, it appears that fracture formation
in this part of the Reykjanes Peninsula has
been small, if any, during the last 1—2
thousand years. Apparently therefore, the
fracture formation has not been continuous,
but whether it was periodic, or all the fractures
formed suddenly, is impossible to assert at
present.
ORIGIN OF THE VOGAR
FISSURE SWARM
Outline and crilicism of oíder hypotheses
Various hypotheses have been proposed to
explain the fissure swarms on the Reykjanes
Peninsula in general, and the Vogar fissure
swarm in particular. In this section I will
briefly state my objections to the most im-
portant of these older hypotheses.
Riedel shears. In this model each of the bigger
fractures in the fissure swarm is supposed to be
composed of smaller en echelon fractures.
These smaller fractures are called secondary
fractures and are believed to be the surface
expressions of “hidden deep fractures”, along
which the “actual movement has taken place”
(77 Einarsson 1967). The surface fractures are
therefore essentially Riedel shears, and are
explained as such by the' above author.
I see some difficulties in this explanation.
First: Riedel shears are, as the name implies,
shear fractures (i.e. faults), and with a hori-
zontal component of movement (Tchalenko
54 JÖKULL 30. ÁR