Jökull - 01.12.1984, Blaðsíða 91

Fig. 12. The upper end of
four dykes. All the dykes end
in lava flows. The red inter-
beds and scoria between the
lava flows are hatched, the
dykes are black. The sections
are steep or vertical. The
illustrations are schematic
but are approximately to
scale. (?) means that this part
of the dyke is not well ex-
posed. Dyke (a) is in vertical
cliffs. It is of tholeiite basalt
and contains many vesicles,
which are up to 0.5 cm in
diameter. The dyke does not
occupy a dip-slip fault, and
no fracture is apparent ahead
of its upper end. The host
rock is tholeiite basalt. Dyke
(b) is in vertical cliffs. It is of
tholeiite basalt, and small
vesicles (1 mm in diameter)
are common. It does not
occupy a dip-slip fault. The
dyke is divided into two parts
in this section. C is a joint.
The host rock is porphyritic
basalt. Dyke (c) ends in vert-
ical cliffs. It is of tholeiite ba-
salt, but contains hardly any
vesicles. It has chilled selvage
on its edges. The host rock is
scoria in the lower part of the
section and tholeiite lava in the upper part. Dyke (d) is of tholeiite basalt with inconspicuous vesicles.
The host rock is scoria in the lower part of the section, but tholeiite lava in the upper part. No evidence
was found that dykes (c) and (d) occupied dip-slip faults.
Mynd 12. Myndin sýnir hvernig fjórir gangar enda upp á við í hraunlögum í svo til lóðréttu sniði.
Teikningarnar eru í réttum hlutföllum.
east, and the remainder to the south, west, or are
vertical. Because many of the faults dip in a
direction opposite to the dip direction of the
lavas, then if they formed before the tilting of the
lava pile their initial dip must have been steeper.
Taking the tilting of the lava pile into account
would, however, not much change the average
dip of the faults because the dip of the lava pile is,
on the average, only a few degrees. Also, it is
likely that the tilting of the lava pile was a con-
tinuous process, accompained by faulting and
dyke intrusion, so many of the faults probably
formed after a part or all the tilting of the lava
pile had occurred. The initial dip of most of the
faults, therefore, was presumably similar to that
observed today.
The throw of the faults is from 0.5 to 25.0 m,
with an average of 5.3 m (Fig. 16). In the cliffs, I
followed some of the faults upwards to see if any
noticeable change in the throw occurred. For the
few faults which could be followed upwards, the
throw was essentially the same 80-100 m higher
JÖKULL 34. ÁR 89