Rit (Vísindafélag Íslendinga) - 01.06.1967, Page 159

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A similar situation is observed north of lake Thingvallavatn.
There single faults of more than 100 m throw are seen in
outcrops of Pleistocene rocks, whereas only several m (up to
30 m in Almannagjá) are displayed by postglacial lavas. This
demonstrates well the repeated movements along the same
fault planes within this graben structure.
The faults within the westem branch of the central Ice-
landic graben are all normal faults (dip-slip faults). Strike-
slip faults have nowhere been demonstrated there so far.
Open fissures show that a considerable lateral tension has
occurred. Walker (1965) has estimated this lateral tension
to be of the order of 0,5 cm/year during the last 5000
—10000 years in the Thingvellir lava plain.
Several central volcanoes occur within the westem branch
of the central Icelandic graben. The occurrence of a high
temperature hydrothermal area and acid volcanic rocks in
the Hengill-mountains suggests that this mountain complex
could be considered as a central volcano. Other such volca-
noes certainly exist in Kerlingarfjöll, and west of Hvera-
vellir. A fourth is found near Geysir on the eastern border
of the volcanic belt.
The Reykjanes-Langjökull volcanic belt must be a very
old structure. This is evident for the following reasons:
(1) the strike of the plateau basalts and dykes dissecting
them is parallel to the trend of this structure on either side
of it; (2) there are no major breaks in the stratigraphic suc-
cession — a thick series of basalts with intercalated cold cli-
mate vestiges is intermediate between the Tertiary rocks and
the young volcanic rocks of the volcanic belt; (3) central
volcanoes are equally present in the Tertiary and Quaternary
volcanic pile as in the present volcanic belt. All this favours
the idea of a continuous evolution of the present volcanic belt
since Tertiary times.
In westem Iceland a very conspicuous fault zone, 10—15
km broad, stretches from Helgafellssveit on the northem
shore of Snaefellsnes to the valleys of the upper Borgar-