Jökull - 01.12.1980, Page 57

1970, Tchalenko and Ambraseys 1970). In the
case of the Vogar swarm, the angle between
the underlying fault and the second order
fractures is about 10°, according to T. Ein-
arsson. A large horizontal component of
movement is therefore to be expected. But no
evidence of horizontal movement is found,
although the vertical throw is up to 10 m
(omitting fracture number 105) and the width
up to 7.5 m. Horizontal movement should
easily be demonstrated, if it existed, because of
matching jags and notches on the fracture
walls.
Second: there is no reason to suppose that
the crust beneath the peninsula meets the
requirements for Riedel shears. In Riedel
shears the different behaviour of the upper
and lower part of' the strata is due to difference
in ductility; the upper layers being more duc-
tile than those below. These conditions are
met where sediments overlie crystalline base-
ment. But certainly not where brittle lavas
alternate with hyaloclastites, down to at least
1800 m depth — as is the case on the Reykja-
nes Peninsula. In the much quoted clay ex-
periments, the ductile material is on the top,
but in the present case it is vice versa; due to
an extremely high heat gradient, the crust be-
comes ductile at shallow depths (a íew km).
Third: the depth to the assumed principal
fault is only half the width of the proposed
shear zone. On the surface, these shear zones
are often less than 100 m wide. This would
mean that the principal fault is only about 50
m below the surface, hence the surface frac-
tures should not be deeper than this. Such a
situation is, in my view, extremely unlikely as
all the evidence indicates that the surface
fractures are in fact much deeper than this.
A transform fault. Many authors have pro-
posed a left-handed transform fault running
along the Reykjanes Peninsula (e.g. Ward et al.
1969, Scháfer 1972). Movements on this trans-
form fault are supposed to give rise to the
fissure swarms on the surface. The mechanism
is therefore essentially analogous to the Riedel
shears, discussed above, except that here only
one deep-seated fault (the transform fault) is
supposed to give rise to all the fissure swarms.
Although Ward (1971) has modified the above
picture in that he proposes many transform
faults on the peninsula, most of the objections
below also apply to his model.
Excepting the last one, all the above
arguments against the Riedel shear hypothesis
also apply here. But there are also other ob-
jections that especially apply to the single
transform fault hypothesis. First: the great
variety in mean orientation of the fractures, in
different parts of the peninsula, does not agree
well with a single transform fault as the cause.
This variation amounts to 27° (Guðmundsson
1979). Second: the fractures are clearly
grouped into definite fissure swarms, and es-
sentially no fractures occur between them. If a
single transform fault (or a wrench fault)
caused the fracture formation, one would ex-
pect thenr to be roughly evenly distributed
along the deformation zone. This is, indeed,
observed where sediments overlie an active
wrench fault (Tchalenko and Ambraseys 1970),
but is clearly not the case on the Reykjanes
Peninsula. Third: clay experiments, set up to
test this hypothesis, show it to be very doubt-
ful. All the faults formed in these experiments
“show a major strike-slip offset” (Courtillot et al.
1974). As said before, no evidence of horizon-
tal movement has been found.
An oblique spreading ridge. This hypothesis has
been proposed by e.g. Nakamura (1970). The
term “an oblique spreading ridge” is certainly
not a very clear cut one, but the main points
seem to be the follwing: “A N75E — trending
sinistral strike-slip movement is implied below
the surface”, and there is also “an opening
component”. “The faulting, trending N75E,
has the same amount of strike-slip and
dilation components.”
Regarding the proposed strike-slip move-
ment below the surface, the before mentioned
objections are repeated; the geology of the
peninsula does not give any positive evidence
of such a fault. Nor do I know of any evidence
that the whole peninsula is dilating or
spreading at a constant rate, e.g. 1 cm/year as
is often quoted in the literature. It is of course
JÖKULL 30. ÁR 55