Jökull - 01.01.2016, Blaðsíða 85

The Kleifakot geomagnetic instability event in NW-Iceland
Research on Icelandic lavas has continued to
provide much robust information on the paleo-
geomagnetic field. The large number of high-quality
directional data which may be used directly without
need for much prior processing, allows one to appre-
ciate various possibilities and limitations of the pa-
leomagnetic method. Paleomagnetic directions from
several thousand Icelandic lava flows have been pub-
lished (Kristjánsson and Jónsson, 2007). These are
mostly obtained from lava flows of > 1 Ma age, as
part of stratigraphic mapping projects outside the cen-
tral active volcanic regions. With polarity zones being
on average composed of 15–20 lava units, they have
often been very useful as aids in correlating lava se-
quences up to tens of kilometers apart.
The overall proportion of VGPs situated below
40◦N or S is around 10%, increasing with age (Krist-
jánsson, 2013, p. 558). These VGPs sometimes ap-
pear to have been recorded during polarity transitions,
while in other cases they seem to be major excursions
of the VGP to mid- or low latitudes.
Clustered and irregularly varying paleomagnetic
directions in Iceland
The relatively few cases where more than four inter-
mediate poles are observed in successive lavas in Ice-
land, mostly include one or two groups each, forming
conspicuous clusters within areas of say 20–30◦ size
on the globe. See Kristjánsson (2015, p. 310) for ex-
amples of these. They tend to be found in thin series
of flows (flow units, or compound flows) which are
likely to have been emplaced in rapid succession com-
pared to thicker flows (McDougall et al., 1984). This
may be concluded both from geological evidence and
from the rate of directional changes in the field. In
the ordinary secular variation, such changes as well as
the resulting VGP movements may be of the order of
5◦ per century, according to observatory records and
archeomagnetic studies.
A steady progression of the VGP in latitude is
only rarely observed in Iceland, the clearest case still
being the early Quaternary „R3-N3“ transition discov-
ered by Sigurgeirsson (1957) and later studied in more
detail by others. This transition was used for strati-
graphic correlation across a distance of some 25 kilo-
meters. Other intermediate paleomagnetic directions
have also occasionally aided in stratigraphic work.
Kristjánsson (1995) cited observations on a single-
lava excursion extending over more than 10 kilome-
ters in East Iceland. Kristjánsson and Guðmundsson
(2001) present an example where a series of lavas with
irregularly varying remanence directions at a polarity-
zone boundary correlate well individually across dis-
tances of 2–7 kilometers in West Iceland.
Figure 1. An unusually wide cluster of virtual ge-
omagnetic pole positions in middle and low lati-
tudes, from lava flows NT 33–46 at a polarity bound-
ary of about 3 Ma age in West Iceland. – Nokk-
uð dreifðar staðsetningar sýndar-segulskauta á skil-
um segulskeiða í hraunlagastafla vestanlands, um
3 milljón ára.
A rather wide cluster of low- and mid-latitude
VGPs in Iceland occurs within a series of thin lava
flows at a reverse-to-normal polarity boundary of
about 3 Ma age. The cluster is found in the thin lavas
numbered NT 33–46 in the large 1973 paleomagnetic
survey by Watkins et al. (1977) in West Iceland. In
that survey, most of these units were omitted. During
subsequent sampling by the author (L. Kristjánsson,
unpubl. data, 1975), faults were found to cause some
repetition in this lava group, reducing it to nine suc-
cessive units. Their VGP positions (Figure 1) were in
northern low and mid- latitudes, based on stepwise AF
JÖKULL No. 66, 2016 85