Jökull - 01.12.1979, Side 5

1 The shelf area around Iceland
LEO KRISTJANSSON
Science Institute, University of Iceland, Reykjavík
The Mid-ocean ridge and volcanic zones in Iceland
From the results of various surveys of magnetic
anomaly lineations it has been inferred that the
ocean floor to the north and to the southwest of
Iceland has been spreading at a constant overall
half-rate of 1 cm/year in both areas since at least 12
m. y. ago, but possibly at a slower rate before this
time. The direction of the Kolbeinsey (or Ice-
land—Jan Mayen) Ridge, however, is about due
north near Iceland, while that of the Reykjanes
Ridge is 35° east of north. A close link between this
circumstance and the tectonics of Iceland is bound
to exist, but evidence of its specific nature has been
obscured due to the great volcanic productivity of
Iceland and due to shifts in its active zones with
time. As on some other slow-spreading ridges, the
direction of spreading may be stable for millions of
years although not at right angles to the overall
ridge trend. On the Reykjanes Ridge, the spreading
direction appears to be about 100° east of north.
From 60° N the Reykjanes Ridge becomes gra-
dually shallower towards Iceland, and its central
region more subdued, until the ridge merges with
the Iceland margin at 63° N. These changes are
mirrored by changes in the chemistry of the
tholeiite basalts produced at the ridge crest. North
of 63° N most magnetic anomalies are also subdued
or have lost their linearity, and the crestal region is
displaced farther and farther east, while its seg-
ments trend N to NE, until reaching the Reykjanes
Peninsula. Although there is reason to believe that
spreading of the North Atlantic mid-ocean ridge,
may occur by localized episodes of rifting, some
major magnetic anomaly lineations are easily fol-
lowed (Fig. 1) along the entire length of the
Reykjanes Ridge, notably the Anomaly 5
lineations inferred to be of 9—10 m.y. age. Of these
two symmetrical anomalies, the western one can be
followed to the tip of the active Snaefellsnes Pen-
insula and from there to the NE across the bay
north of Snaefellsnes. Datings on outcropping
basalts from the Northwest Peninsula, though still
fragmentary, are not inconsistent with this corre-
lation.
The eastern Anomaly 5 lineation on the Reykja-
nes Ridge can be followed as far as 63° N, but only
tentatively farther. Continued along its strike, it
would come ashore near the Ölfusá and Thjórsá
rivers, between the active volcanic zones. Similarly,
according to the ridge anomaly lineations, the
eastern volcanic zone of South Iceland which has
no continuation south or southwest of the Westman
Islands, should be of the order of 15 m.y. old.
To explain the large age discrepancy just men-
tioned, a fracture zone has been postulated to run
ESE across the south Iceland shelf, but there is no
positive evidence for it from local topography,
seismic activity or other observations. On the other
hand, geological evidence in Iceland (see chapter 2)
suggests that large eastward shifts in the SW-Ice-
land active spreading zone, to its present Reykja-
nes-Langjökull location, took place subsequent to
Anomaly 5 time. This easterly migration appears to
be continuing. It follows that much of eastern Ice-
land and the shelf may overlie a considerably (by
10—20 m.y.) older ocean bottom, although again
positive evidence for this, say in the form of datable
xenoliths, is lacking.
Turning now to the area north of Iceland, the
Kolbeinsey Ridge has been active for at least 12
m.y. (probably much longer) in itscurrent location,
and magnetic anomaly lineations along it are very
well developed to 69° N (Spar Fracture Zone). The
ridge, on approaching Iceland, splits into two dif-
ferent features just north of 67° N. The western
feature is a fault trough, which together with a
positive magnetic anomaly continues due south
towards the Tröllaskagi Peninsula at 19° W. The
lava sequences on this peninsula, however, are of
approximately Anomaly 5 age. The eastern feature
is a complex series of faults and recent volcanic
JÖKULL 29. ÁR 3