Náttúrufræðingurinn - 1980, Síða 35

briiche. Eiszeitalte und Gegenwart. 17:
85—86.
— .1967. An outline of the structure of
SW-Iceland. í: Sveinbjörn Björnsson
(ritstjóri); Soc. Sci. Islandica. Rit 38:
151 — 161.
— 1974. Evolution of the axial rifting zone
in northern Iceland and the Tjörnes
Fracture Zone. Geol. Soc. Am. Bull. 85:
495 — 504.
— 1978. Fissure swarms and central vol-
canoes of the neovolcanic zones in Ice-
land. Geol. J. Spec. Iss. 10: 415-432.
— & H. Noll. 1975. K-Ar ages of rocks from
Húsafell, western Iceland, and the deve-
lopment of the Húsafell central volcano.
Jökull. 24: 40—57.
Talwani, M. & O. Eldholm. 1977. Evolution of
the Norwegian-Greenland Sea. Geol.
Soc. Am. Bull. 88: 969—999.
Talwani, M., C. C. Windisch & M. G. Langseth.
1971. Reykjanes Ridge crest. A detailed
geophysical study. J. Geophys. Res. 76:
473-517.
Vogt, P. R. 1974. The Iceland phenomenon:
Implications of a hot spot on the ocean
crust, and implications for flow below
the plates. 1: Leó Kristjánsson (ritstjóri):
Geodynamics of Iceland and the North
Atlantic. 102—126. D. Reidel, Dor-
drecht.
Walker, G. P. L. 1959. Geology of the
Reydarfjördur area, eastern Iceland.
Quat. J. Geol. Soc. London. 114:
367—393.
— 1963. The Breiddalur central volcano,
eastern Iccland. Quat. J. Geol. Soc.
London. 119: 29—63.
— 1964. Geological investigations in east-
ern Iceland. Bull. Volc. 27: 1 —15.
Ward, P. L. 1971. New interpretation of the
geology of Iceland. Geol. Soc. Am. Bull.
82: 2991—3012.
Watkins, N. D., & G. P. L. Walker. 1977.
Magneto-stratigraphy of eastern Ice-
land. Am. J. Sci. 277: 513—594.
Wilson,J. T. 1973. Mantle plumes and plate
motions. Tectonophysics 19:149—164.
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
29