Náttúrufræðingurinn - 1980, Blaðsíða 36

the K-Ar method 13.2+2.0 m. y. old. The
lavas become progressively younger away
from the anticline axis. It is suggested that
the youngest rocks in the Snaefellsnes sync-
line belong to Epoch 6. This is supported by
K/Ar. dates of 6.7 + 0.4 m. y. The lavas
below the Hredavatn unconformity are
12— 13 m. y. old in the Borgarfjördur region
but become younger farther west and are
about 8.0—8.3 m. y. old in Hitardalur. The
flows above the unconformity are about
5.5— 6.0 m. y. old in the southern part of the
Borgarfjördur region but increase in age
farther north and west and are about 8.0 m.
y. old in Hitardalur. Thus the time gap re-
presented by the unconformity is greatest in
Borgarfjördur but is relatively small in
Hitardalur where the unconformity might
represent a time of low volcanic activity
rather than a pause in the activity.
A great variety of faults and joints occur
in western Iceland (Fig. 5). The faults are of
two different origins. Firstly, fault swarms
which correspond to the fissure swarms of
the active rift zones. These faults are formed
in a pronounced tensional stress field inside
a rift zone. A central volcano is always
situated in the centre of each swarm and
may be regarded as an integral part of it.
The fault swarms are usually also ac-
companied by dyke swarms. Secondly there
are faults which are formed in a stress field
characterized by lateral shear forces. The
Snaefellsnes Fracture Zone stretches from
the Snaefellsnes peninsula to the Borgar-
fjördur region (Fig. 5). It is about 90 km long
and 30 km across. The faults of the Fracture
Zone can be divided into three groups based
on their trend and age: NW — SE-, N—S-
and NE—SW-trending faults. The N — S-
and NE—SW-trending faults are only pre-
sent in rocks older than 8.0—8.3 m. y. and
they disappear underneath the Hredavatn
sedimentary horizon. NW—SE-trending
faults are all younger than 8.0—8.3 m. y.
where they cross the Hredavatn sedimentary
horizon in Hitardalur. Nearly half of them
disappear underneath the sedimentary
horizon in Borgarfjördur and thus are older
than 6.5—7.0 m. y.; the rest disappears
underneath a minor unconformity farther
east which has been dated 4.5 m. y. old.
Minor movements have been observed up to
present on the old NW — SE-trending faults.
The suggested evolution of the rift zones
and crustal accretion is presented in Fig. 9.
The Snaefellsnes rift zone was active at least
from 16 m. y. up to 6.7 m. y. ago. The lava
flows in western and northwestern Iceland
were produced in this zone (Fig. 9a). The
bend in the rift zone caused lateral shear
stresses to develop and consequently
inducing a conjugated system of N—S- and
NE—SW-trending faults.
The southern part of the Snaefellsnes rift
zone became extinct about 5—6 m. y. ago and
a new spreading axis was formed farther
east, the predecessor to the Reykja-
nes-Langjökull rift zone. This new segment
was not in line with the northern part of the
Snaefellsnes rift zone but some tens of kilo-
metres east of it. The shear stresses of this
new arrangement of the rift zones produced
the NW—SE-trending faults (Fig. 9b).
The northern part of the Snaefellsnes rift
zone became extinct about 3—4 m. y. ago
and a new rift started farther east, a
predecessor to the northern part of the
Eastern rift zone (Fig. 9c). By this time the
lateral shear stresses had ceased to operate in
western Iceland and the area has been more
or less tectonically inactive since. At this
stage the rift zones displayed a similar patt-
ern as did the Snaefellsnes rift zone ori-
ginally. Under these circumstances a conju-
gated system of N — S- and NE—SW-trend-
ing faults developed in central southern Ice-
land (Fig. 9c) similar to the earlier ones in
western Iceland.
About 2 m. y. ago the rifting and volcanic
activity spread southwards from the newly
formed northern segment and lead to the
present pattern of rift zones in Iceland (Fig.
9d). The next step may be the extinction of
the Reykjanes-Langjökull rift zone and the
establishment of a fracture zone joining the
Reykjanes ridge and the Eastern rift zone.
The eastward shift of the rift zones in Ice-
land may be caused by westward drifting of
the plate boundary in the North Atlantic
region across the assumed mantle plume
beneath Iceland. The mantle plume, being a
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