Jökull - 01.12.1984, Side 90

Fig. 11. Lateral offset (lo) and vertical offset (vo)
of a dyke (D). The parts of the dyke are con-
nected by thin veins. L = lava flow, S = scoria
(modified from Guðmundsson 1983a).
Myndll. Lárétt og lóðrétt hliðrun gangs. Ganga-
hlutarnir eru tengdir með þunnum œðum. Hliðrun
ganga er skýrð í Guðmundsson (1983a).
Lithology, vesicles and amygdales
All the dykes are basaltic and the great major-
ity (73%) belongs to the tholeiite type, according
to the criteria given by Walker (1959). The
remainder are mostly of the olivine-tholeiite
type, but about 2% of the dykes are of the
porphyritic basalt type (with more than 5% of
phenocrysts).
Vesicles are common in the dykes. Most of
them are circular and only a few millimeters in
diameter. Occasionally, however, one finds much
larger vesicles, up to a couple of centimeters in
diameter. In some dykes the vesicles form bands,
parallel to the dyke walls; in other dykes the
vesicles are confined to the outermost parts, but
in most dykes the vesicles are evenly distributed.
Amygdales are less common in the dykes than
empty vesicles. Amygdales occur mainly in those
dykes that are observed in sections that are
deeply eroded below the original surface of the
lava pile. Secondary fillings are not restricted to
the vesicles but occur commonly as thin veins that
often follow joints in the dykes. It is also common
to see such veins in the contact between the dyke
and the host rock.
Host rock lithology is similar to that of the
dykes. All the lavas are basaltic and the majority
is tholeiite. The remainder is either oli-
vine-tholeiite or porphyritic basalt. Vesicles and
amygdales are more common in the lavas than in
the dykes, and they are also much larger in the
lavas.
Some of the dykes show evidence of rapid
chilling, the chilled selvage being variable in
thickness but generally less than 1 cm. The
majority of the dykes, however, do not have
chilled selvage, although the grain of the dyke-
rock is often finer near the edges.
Age
No radiometric age determinations on the
dykes have been made. Nevertheless, it is
reasonable to assume that most dykes are of
age similar to the lavas. Thus, the dykes should
generally be 12—14 m.y. old.
An attempt was made to find the relative age
of dykes from field relationships. Where two
dykes intersect, the one that is offset is assumed
to be older. However, because individual dykes
are commonly offset without being crosscut by
other dykes, this assumption may not always be
justified; but generally it should hold true. Only
in eleven cases were dykes seen to intersect. Only
in Vatnsfjördur and Geirthjófsfjördur are the
results clear-cut, but in other areas there is some
doubt as to the relative age of the dykes. The
results are indicated in Fig. 2.
FAULTS AND JOINTS
Faults
Faults are common in the area and the only
type seen to occur are normal faults. Excluding
the “faults" occupied by the dykes (see the sec-
tion on Dykes and Normal Faults below), 68
faults were observed or inferred in the profiles.
The data on some faults is, however, incomplete
as regards strike, dip, or size of throw, and such
faults are omitted in the discussion below.
Fig. 14 shows the strike distribution of the
faults, and Fig. 15 shows the poles to the faults.
The majority of the faults have a strike that is
similar to the dominating strike of the dykes.
About 62.5% of the faults have an azimuth angle
between 40° and 90°, that is they strike NE to E.
The average dip of the faults is 69°. About 47%
of the faults dip to the north, about 27% to the
88 JÖKULL 34. ÁR