tto
Geomagnetic Polarity Mapping of lcelandic lavas: C.omparison with
Ocean-Floor Magnetic Uneations
L. Kristja nsson
Science Institute. University of lceland, Dunhaga 3, Reykjavik, lceland
Key Words
Paleomagnetism
Polarity time scale
Stratigraphy
Iceland
Abstract
Recent work on paleomagnetic measurements in dated sequences of lcelandic lava flows is reviewed The polariry
ime s.ale derived from these has been found to fit reasooably well with analogous scales obtained by others from
linear magnetic anomalies over fast-spreading ocean ridges,
taking ioto account the many factors that are likely to comolicate detailed correlations between such time scales derivid by diff"..nt methods. Results from aeromagnetic survcys over Iceland can also be correlated with paleomagnetic
mapping on the ground, and correlation of these with anomaly sequences on the ocean ridges north and southwest of
tceland is likely to aid in the reconstruction of the tectonic
and volcanic history of the lceland area in Late Celozoic
ume.
accessible formations, or by interpreting geomegnetrc anomalies caused by well-defined bodies of rock.
To derive maximum information on the paleomagnetic field
from a geological formation, it is essential both that the
formation can be dated and that the magnetic results from
it cao be ordered in a time sequelce. Thus, studies on intrusions whose time relations are unknown, are much less
useful than scudies on stratigraPhically maPped units, where
e.g. magnetic polarity zones can be conveniently emPloyed
!o test or extend mapping that was based on other stratigraphic markers.
As always in earth science research, there is a number of
complicating factors, mostly unforeseen in the early years
of these studies. One serious complication is that most
rocks carry more than one component of remanent magne_
tization, having different age, intensiry and stability. Therefore, the most reliable paleomagnetic results have been ob-
tained from relatively fresh and undisturbed formations
whose remanence may be assumed
with their emplacement.
Investigatrons of anclenl remanent magnetization in rocks
have had a major impact on research in various neighboring
disciplines, including the magnetic ProPerties of minerals,
the theory of geomagoetic field generation, and global tectonics. It has now been shown beyond reasonable doubt
that this remaqence is generally of the same age as some slgnificanr event in the history of the rock - its emPlacement,
recrystallization, a major heating episode etc. - and that it
reflects the direction of the geomagnetic field at that time
The most characteristic and geologically useful property of
the geomagnetic field are cornplete reversals that take place
relatively quickly compared to the time elapsed bet'.veen
these. The science of paleornagnetism deals with these reversals, as well as with the irregular short term directional
and intensity changes of the field between reversals (the
secular variation) and long-term variations in the mean field
configuration (apparenr polar wandering). Mostly, this is
done by collecting and measuring oriented samples frorn
outcrops, but information on some asPects of the ancient
field can also be obtained from semioriented coring of inearth evolution sciences 2/1982
be contemporaneous
Polarity Time Scales
research on the time scale of geomagnetic polarity reversals mainly employs three different tyPes
of material,
a) Series of lava flows, each lava giving a sPot reading of the
ancient field direction.
b) Sedimentary sequences, particularly marine sediments'
where each sample represents the field direction averaged over tens to thousands of Years.
c) lnversion of linear magnetic anomalies in the oceans.
All these melhods, however, depend very critically on
various assumptions and simplifications which cannot be
tested directly. Observed breaches of many of these assumptions are listed in Table I, and from the table the reader
may consider it remarkable that any useful results at all
have been obtained. Progress in studies of the time scale has
to be made by a careful study of the available data, using
thorough checks and rigid criteria on the consistency with
geological evidence and other data. Sophisticated methods
have now been developed to cilcumvent some of the problems listed in Table I, but the resul$ from this modern research are being fitted into an existing framework that was
pardy built up without the apProPriate tests of physical,
geological and statistical reliability being applied to paleo-
At the present time,
lntroduction
to
127
earth evolution sciences 2/1982
Table I: Problems intheconstructionof atrue geomagnetic polarity
flme scale
a)
Series
of
lava
flows
Few long sequences
of
lavas exist, and even in these volc€nism is
likely to have been 1oo intermittent {or complete recording of
reversals and excursions.
Large lateral and vertical variations occur in the inlensity and
type of volcanic production in these series.
Exposures are often only partial. Faults and unconformities may
complicate correlation between nearby sections.
Dating is technically difficult and results are affected by alteration which is generally present. The most cornmon lava types are
low in radioaclive isotopes. Resolution in dates seldom allows
unambiguous identif ication of chron.
Remanence in some l6vas is unstable and/or overprinted, Self_
reversals may occur in certain minerals,
The reversal process is poorly understood: partial reversals, rebounds. and localized reversals have been suggested,
b)
Ocean and lake sediments
No direct dating possible except in uppermost Ouaternary. Depositional remanence may be acquired below sediment-water
interface and hence not be synchronous with fossils in the sediment.
Time-variable sedlment types and deposition rates (possibly even
erosion), partly due to changes in climate. Compaction of sediment with depth. Biological activity, slumping and liqueJaction
may upset original remanence- Chemical remagnetization duling
diagenesis is common.
Lateral variations in sedirnentation, fossil content, etc.
Very low rernanence intensity. Disturbance oJ grains during
sampling and preparation. Chemical changes on laboratory
heating.
c)
Interpretation of linear magnetic anomalies ln oceanic areas,
Non-uniqueness of data-inversion solutions.
Begional field reference level is onlv estimatedi it may be affected by long-wavelength lateral variations in rock magnetic properti€s, by deep magnetic sources, and by ionospheric currents.
Rapid attonuation of signals from short evenls with increasing
vertical distance to below instrumental noise levels. N4agnetometer probe may be up to 3-4 km above the formations being
studied.
Finite width of active zone and off€xis volcanism will complicate the time pattern of ocean flooi generation.
Thickness and overall remanence intensity of the magnetized
crustal layet are unkown.
Normal and transcurrent Jaulting will upset linearity in anomalies. Centralzone may move laterally.
N/R boundaries are likely to be inclined and irregular. Intrusive/
extrusive volume ratios are unknown. Unexpected layering of N
and R formations has been found by deep sea clrilling.
Topographic effects at faults 3nd volcanic centers.
Time'variable, asymmetric and oblique spreading has been
demonstraied at some ocean rifts.
Viscous decay of primary remanence; new viscous and chemicaL
remanence, becatis€ of alteration at sea floor incl. serpentiniza-
tton.
Ocean floor basalts are generally unsuitable Jor radiometric dat_
ing due to various factors.
As an example we may mention that for a number of years
the lengths of Upper Tertiary geomagnetic polarity epochs
were commonly believed to be of the order of one million
years or more, but now a likely figure for rhe mean length
of these (LaBrecque et 41. 1977, Kristjansson and McDougall 1982 is closer to 0.1 m.y. Another indication of the
difficulties in the construction of a polarity time scale is
ahat several short geomagnetic revelsal events have been
suggested to occur in the last half million years, but none
have been confirmed and dated
to the satisfaction of
the majority of paleomagnetic workers.
However, the general principle of dating by the aPplication
of geomagnetic polarity time scales, has been demonstrated
to be sound for instance by the results of biostratigraphic
of Glomar Challenger ocean-floor cores, whose
maximum ages correspond well with basement ages inferred
from magnetic lineations. The time scale most extensively
used at present is that of LaBrecque et al. (1977) which is
based on the inversion of cenain magnetic anomaly sequences in the Pacific Ocean. It emPloys one arbitrary fixed
point, taken to be the Gilbert-Gauss boundary dated on
shore at approx. 3.+ rf'.y. :]ge (see reviews by McDougall
(1979) and Mankinen and Dalrymple (1979)). With steadily
increasing and improved data, especially from sedimentary
sequences, our estimates of rhe Upper Cenozoic time scale
of polarity reversals appear to be converging and may
hence become a reasonably reliable global stratigraPhic rool
within a few years.
dating
Geomagnetic Polarity Mapping in lceland
Regiooal geological mapping in the Late Cenozoic lava pile
was initiated in the nineteen fifties by Tr. Einarsson and by G. P. L. Walker. Their work has been continued
by other scientists, particularly K. Saemundsson and col-
of lceland
leagues
at the National Energy Authority of lceland,
and
augmented by a number of radiometric age determinations,
detailed maps of major volcanic centers, geochemical and
tectonic studies.
A pattern of age zogation in the slowly spreading and subsiding lave pile of Iceland is emerging from these mapping
efforts. lt has been ascertained that volcanism was essentialIy continuous in both Eastern and Western lceland during
the past 14-16 m.y., and probably since as much as 25
m.y. ago if formations on the insular shelf are included.
It was also found in the nineteen fifties that the basalt
flows of lceland are good material for paleomagnetic work,
being well exposed, stably magnetized, relatively fresh, and
undisturbed by any major tectonic upheavals. Several long
composite key profiles through the lava pile have accordingly been sampled in detail for paleomagnetic laboratory
measurements and K-Ar dating, but in other profiles simpIer magnetic mapping by field testing of hand samples has
been applied.
Individual mapping studies of this kind have been described
magnetic data. This may have resulted in the promotion of
local noise factors to global phenomena by spurious correlation or by circular arguments, causing early misconceptions to become rooted in the literatr:re for a lone while.
by Watkins and Walker (1977), McDougall et ^1. (1977),
Kristjansson et al (1980) and Saemundsson et al (1980),
and their paleomagnetic aspects have been reviewed by
Krisrjansson and McDougall (1 982).
128
earth evofution sciences 211982
The pal€omagnetism of surface outcrops can then be correlated with local low-level aeromagneric anomalies from
the detailed total-field surveys of Sigurgeirsson (1970, 1979,
1o80. 1981). It is found thar rhe mosr recrnr geomagneLic
epochs (0-3.4 m.y.) are among those that are mosr easily
correlatable between ground outcrops and airborne anomalies. This is understandable by referelce ro the fact that
the youngest rocks are less disturbed by faulting, erosion,
viscous decay of primary remanence and olher such processes rhan older forrnations are. However, stratigraphic work
in these young rock units is nor as far advanced as in the
Tertiary formations, because of facies aspects imposed by
recurrent glaciations during their emplacement. Of the
older magnetic zones, the most noticeable one both in
ground mapping and in aeromagnetic anomalies is the socalled Epoch 9 or Anomaly 5 zone. This zone, of about
9-10 m.y. age, has now been dated in Eastern lceland
(see McDougall et al. 1976), in central Northern Iceland
(Saemundsson et al. 1980) and in NWlceland (I. McDougall,
pers. comm. 1980). In all rhese locations it is represented
by of the order of a hundred normally magnetized lava
flows in sequence, including however a few short revelse
interva-ls whose existence had been anticipated from marine
magnetic aromaly studies (see LaBrecque et al. 7977).P.esearch aimed at further tracing occurtences of lavas belonging to this magnetic zone is in progress in NE-Iceland (K.
Saemundsson, pers. comm. 1980), in SElceland (Academy
of Sciences of the U.S.S.R., unpublished report, 1981) and
in central Western Iceland.
It is now thought (Saemundsson 1979, Johannesson 1980)
that a predecessor to the present westen active volcanic
zone of lceland once passed in SW-NE direction through
central western Iceland. This zone become extinct 7 or
8 m.y. ago, and its axis is now marked by a syncline abour
120 km NW of the present volcanic zone. Fig. 1 shows how
vadous magnetic zones have been traced through parts of
Western Iceland by ground mapping; some linearions of
Fig.
1
IVlap ofWestern lceland, showing outcrops of magnetic zones belongang to various geomagnetic polarity epochs (chrons). Odd-numbered
epochs are of normal polarity and will give rise to positive aeromagnstic anomalies. Some known positive anomalies offshore are
indicated. lvlodif ied from Johannesson ('l 980).
this diagram have also been extended with the aid of magnetic anomalies. Note that Epoch 9 age lavas crop out symmetrically on both sides of rhe exrincr z<ine.
I
l
I
Linear Oceanic Magnetic Anomalies in the Vicinity
of lceland
Provided that magnetic lineations within lceland and over
the nearby oecan floor may be assumed to be due to similar
processes of crustal generation, they will be very valuable
in establishing age correlations between the rocks of rhese
areas. This will then provide an independenr check on tectonic conclusions derived from other geological work.
As an example, excellent magnetic profiies over the ocean
north of Iceland have been obtained by the U.S. Naval
Oceanographic Office, and presented by Vogr et al. (1980).
However, the interpretation of results from these profiles
is subject to the many uncenainties listed in Table Ic; some
of these are evident from the sketch in Fig.2, which traces
the more plominent positive linear anomalies in an ocean
area centered on the Kolbeinsey Ridge.
Thus, a fracture zone at 69-N is seen to offset the youngest
magnetic anomalies; another fracture zone then offsets
Fig.2
Trace ot main positive linear magnetic anomalies north of lceland,
as measured by U.S. Naval Oceanographic Olfice (from Fig. 2 of
Vogt et al. 1980). Anomalies number 1 (Brunhes epoch age) and 5
{Epoch I age) heavily drdwn- Inset shows locarions of Figs. 1 and 2,
and also the poshion of the central volcanic zones o{ lceland and
the active ocean ridge crens (R). T.F.Z.: Tjornes Fracture Zone.
S.F.Z.: Soar Fracture Zone.
129
earth evolution sciences 2/1982
these anomalies in the opposite sense just north of the [celand coast. Some anomalies are seen to fade out, rnerge, or
change directioo in response to volcanic or tectonic changes
that have taken place locally on the ridge. This is particularly common for narrow anomalies, because a time interval
of say 50,000 years will on average translate into a pair o{
stripes of ocean floor only 500m wide, which is probably
much less than the effective width of rhe zone of volcanic
activity at the ridge crest.
In correlating with anomalies over Iceland it should also be
kept in mind that magnetic anomalies are most likely to
occur over the eroded edges of magnetic polarity zones in
the lava pile, which may be several km distant from the
dyke swarm that gave rise to these lava flows. Other complications include the effects of the relatively thick crust
in Iceland, the large width of the
active volcanic zones,
and the presence of highly rnagnetic formations at localized
volcanic centers. To resolve these complications, and improve our knowledge of the details of geological processes
taking place at active spreading ridges, it is essential to continue thorough interdisciplinary studies of the Icelandic
lava pile, including stratigraphic rnapping, dating, paleomagnetic measurements and g€ochemical research. The
nature of the traDsilion between typical ocean floor and
Iceland-type cmst at the edges of the insular shelf of Iceland also needs further study.
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13
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Iceland
in