Jökull - 01.12.1979, Page 6

Fig. 1. Iceland and the surrounding ocean area.
The central zones of the Reykjanes and the Kol-
beinsey (Iceland-Jan Mayen) ridges are shown, as
well as anomaly lineations on these ridges.
Anomaly ages: 5 is 9—10 m.y., 6 is 19—20 m.y., 13
is about 36 m.y. and 20 is about 46 m.y. Depths in
fathoms. (After Talwani and Eldholm, 1977)
basement highs extending SE towards the northern
end of the active volcanic zone in Axarfjördur (17°
W). Considerable seismic activity of a transform
fault character is associated with both extensions of
the Kolbeinsey Ridge on the Iceland shelf, reaching
even as far west as to the recently extinct N-S Skagi
volcanic zone at 20° W. As a landscape feature, this
transform fault is trivially small in comparison
with many of the North Atlantic fracture zones, but
gravity results indicate the presence of a sizable
WNW-trending sediment-filled trough in its
southern part, close to shore.
The linear magnetic Anomaly 5 west of the
Kolbeinsey Ridge (Fig. 1) is known to extend at
least to 66° N and can thus be regarded as essen-
tially continuous through the Iceland area. East of
the ridge, on the other hand, we encounter similar
cornplexities as in the case of Reykjanes Ridge: the
Anomaly 5 lineation becomes indistinct south of
67° N, but its continuation may reach the coast at
16—17° N, i.e. at the active volcanic zone. Basalts
of about Anomaly 5 age are known to outcrop in
Tjörnes nearby, as well as in the fjords of eastern
Iceland, but detailed research is needed to follow
these continuously through all the eastern Iceland
area by aeromagnetic lineations and/or surface
mapping.
Differences between Reykjanes and the Kol-
beinsey ridges include the fact that the transition
from the latter to the Iceland shelf area is more
abrupt than off Reykjanes, both in topography and
in basalt chemistry. Certain large scale V-shaped
topographic lineations on both ridges, interpreted
by some as evidence of pulsing flow of material
from a mantle plume under Iceland, are better
developed and at a narrower angle on the Reykja-
nes Ridge, which also has much less sedimentary
cover within 100 km of the crest.
Inactive ridges
The broad inactive ridges or swells extending
between the Lower Tertiary volcanic areas of East
Greenland and Northwest Britain were for a long
time considered to represent a foundered land
bridge. Since the 1960’s, however, many have
viewed the swells as being the continuous trace of
the same mantle plume activity as has created Ice-
land. Similarities between Iceland and these trans-
verse ridges, setting them apart from typical North
Atlantic ocean floor areas, include their large
crustal thickness, geochemical features, poor deve-
lopment of magnetic lineations, and the occurrence
of a number of central volcanoes, inferred from
localized gravity and magnetic anomalies of several
km extent.
The land bridge hypothesis has recently been
revived in modified form by the discovery of
lateritic soil within basalt basement in a Glomar
Challenger core at 1300 m below sea level on the
Iceland-Faeroe Ridge.
The original mantle plume trace hypothesis has
also been modified, as researchers working on the
Iceland-Faeroe-Scotland ridges and associated
banks now tend to divide them into blocks, some of
which may be of continental character. The block
arrangement is largely due to a major westward
shift in the location of the rift axis, but the timing of
this shift on the Iceland-Faeroe Ridge is not quite
certain.
It is most likely that the mantle plume moved to
its present location about 27 m.y. ago, to begin
building up the E-W aligned Iceland block. This is
also the time when rifting ceased at the Aegir Ridge
in the Norwegian Sea and moved to 10—12 m.y.
older crust at the Kolbeinsey Ridge, splitting off in
the process a slice of the Greenland continental
margin of about 100 km in width. This slice, now
4 JÖKULL 29. ÁR