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SUMMARY
Evolution of rift zones in western
Iceland
by
Dr. Haukur Jóhannesson
Department of Geology
Museurn of Natural History
P. 0. Box 5320. Reykjavík, Iceland
The active rift zones in Iceland form syn-
cline structures due to loading and sub-
sequent sagging. By mapping dip and strike,
the whereabouts of extinct rift zones may be
found in the form of syncline structures. A
major syncline has been identified in west-
ern Iceland, the Snaefellsnes syncline (i. e.
Snaefellsnes rift zone), which runs obliquely
across the Snaefellsnes peninsula and east-
ward along the fjord Hvammsfjördur where
it joins a syncline in Vatnsnes in northern
Iceland (Fig. 2). The Snaefellsnes syncline is
situated about 70 km west of the presently
active Reykjanes-Langjökull rift zone. Mid-
way between the two is the Borgarnes
anticline.
The Hrcdavatn unconformity (Fig. 2)
plays a major role in the geology of western
Iceland. The lava flows below the uncon-
formity have been tilted and eroded before
bcing covered by the Hredavatn sedi-
mentary horizon and the overlying rock
series (Fig. 3 and 4). The Hredavatn uncon-
formity is on the eastern and southern flanks
of the Snaefellsnes syncline but its counter-
part on the northern and northwestern
flanks is the Tindar unconformity and
Tindar sedimentary horizon (Fig. 2 and 4).
To study the age relations a continuous 5.5
knt thick section of the lava pile was mapped
and by studying the palaeomagnetic
polarity of the lava flows a polarity time
scale for the region was produced and fitted
to the revised polarity time scale of La-
Brecque et al. 1977 (Fig. 6).
By using all available data a distribution
map of the palaeomagnetic epochs could be
drawn (Fig. 7 and 8). The oldest rocks are
found in the axis of the Borgarnes anticline
as expected; they were previously dated by
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