@
J. Geodynamics Yol.25, No. 2, pp. 99-108, 1998
O 1997 Elsevier Science Ltd
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AEROMAGNETIC RESULTS AND THE PRESENCE OF AN
EXTINCT RIFT ZONE IN WESTERN ICELAND
LE6 KRISTJANSSON'
ANd
GEIRFINNUR J6NSSON'
rscience Institute, University oflceland, Dunhaga 3, 107 Reykjavik, Iceland
(Received 15 July 1996: accepted 20 December 1996)
Abstract-Aeromagnetic surveys over western Iceland including Breidafjiirdur and Faxafl6i
bays reveal linear features which mostly have a northeasterly trend. However, this trend changes
to a northerly direction as the lineations cross the Northwestern peninsula. Over the southern
part of the area, the anomaly pattern may be fined to a simple propagating-rift model. This
model is consistent with results from geological mapping in westem Iceland in the 1960s and
1970s which yielded evidence for an extinct (>7 Ma old) rift zone west of the currently active
rift zones. The magnetic data provide constraints on the process of rift zone movement and on
the age of the bedrock offshore. @ 1997 Elsevier Science Ltd
INTRODUCTION
Iceland forms an anomalous region on the mid-Atlantic ridge, whose origin is often attributed
to a mantle 'plume'. The uppermost crust on the island and probably on much of the insular shelf
,and transvefse ridges, is largely composed of a regular series of basalt lava flo.ws. Zones of
rifting, volcanism and subsidence cross the island from south and southwest to northeast (Fig.
1). The age of the lavas increases in general away from the rift zones; in northwestern Iceland
the oldest rocks above sea level are about 15 Ma old, and those in eastein Iceland are a little over
l3 Ma old.
Crustal structure and processes in Iceland differ considerably from those of mid-ocean ridges
in general. Among these differences are thicker crust; the corrunon occurrence of silicic volcanic
centers; the presence of multiple volcanic zones and their relatively great width; eruption of
differentiated and alkalic magmas; absence of topographically distinct transform zones.
Progress in detailed stratigraphic/tectonic mapping and dating of the lava pile of Iceland has
been sporadic. Ages and relationships between formations as well as ages of tectonic processes
in various parts of the island are uncertain. Little is also known of formations on the insular shelf
or of age connections with the surrounding ocean floor.
However, it is clear that bedrock ages do not always increase uniformly with distance from
the volcanic zones. Various authors have pointed out the presence of synclinal and anticlinal
forms in the lava pile. Unconformities occur, sometimes accompanied by clastic sedimentary
beds of tens or hundreds of meters in thickness. Proposed explanations of these forms in past
decades have included alternation of volcanic and tectonic episodes, uplift and subsidence of
large blocks of land, and compression, but most researchers favor an explanation based on
99
L. Kristidnsson and G. J6nsson
100
discontinuous migration ofthe spreading axes, cf. the next section.
The lateral jumps of the spreading axes are attributed to the westerly drift of the plate
boundary in Iceland relative to the mantle plume. The amount of the relative drift is estimated
by J6hannesson (1980) to be of the order of 0.75 cm/year; we are not aware of any more recent
quantitative assessment of this speed based on the local geological evidence. When the boundary
has moved too far west, a new conduit through the uppermost mantle and crust is formed,
generating a new plate boundary. Suggestions have been made that the activity of the hot spot
or plume is variable (e.g. Schilling et al., 1982), and its location in the mantle may also have
varied.
Piper (1973) suggested that the most northerly part of the current western spreading zone of
Iceland has been inactive since late Pleistocene time. Many other investigators have developed
this idea. Accordingly, spreading activity in the eastem volcanic zone in southem Iceland is
propagating to the southwest and that in the western zone is waning. This circumstance gives
15'
10"
__,tt5
I
.".''iI I
ri
!j
.i1...
rL
t.)
ii
i'l l
li.i'
66'
t_,.i
l./
i.:
q
a.-i
.Sr
oi...,l
.)f...., r,,:,,'ru,.";:.:,:-:--.r'\ i ,.
,.;)\:!' ,,,::-'- , ,t-,,.':titr:--'):--' \, i
,.iitl . ..
'Y:
,t
7r'
25"
--1;i"X*-',"'-----t*'(*
2i,'-'.. ;rt
\'"-'
i-'
l. Outline map of Iceland, including several isobaths (in m) in the surrounding seas. Dark gray
indicates the youngest bedrock, of Brunhes chron (<0.78 Ma) age which corresponds approximately to the
Fig.
regions of active spreading. In the light gray regions the bedrock is 0.78-3.2 Ma old judging by K-Ar
dating and magnetostratigraphy. The position of the core of a synclinal structure in a lava pile of estimated
7-l0Maageinwesremlceland(J6hannesson, 1980, 1986)isindicated,andalsothecoreof ananticline
in older lavas. The central mid-ocean ridge anomalies (R: Reykjanes Ridge, K: Kolbeinsey Ridge) and
those correlated with other distinct anomaly lineations on the Mid-Atlantic ridge are shown. Hatching
delineates the seismically active southem Iceland and lidrnes transform zones.
An extinct rift zone in westem Iceland
101
rise to an east-west trending 'fracture front' (Einarsson and Eiriksson, 1982) joining the zones.
The southwards-moving front or deformation region is near 64oN at present (Fig. 1).
OLD PROPAGATING RIFT IN WESTERN ICELAND
An ENE-SSW syncline in the Snaefellsnes peninsula of western Iceland (Fig. l; see Fig. 2
for place names) was first described by Th. Einarsson in a symposium volume (Bjiirnsson,
1967). In the same volume, K. Saemundsson suggested that this syncline might mark the
position of an extinct spreading axis. Following extensive stratigraphic field work in western
Fig. 2. Aeromagnetic survey lines over westem Iceland and parts of the shelf. Thin lines north of Hfnafl6i
bay: "Project Magnet" 1973-:74 survey. Dots: surveys by Th. Sigurgeirsson in 1968-80. Broken lines:
1985-86 surveys (J6nsson et al.,1991). Solid lines: previously unpublished l99l-92 surveys (Kristj6nsson
and J6nsson. 1996). Place names mentioned in the text are shown.
lO2
L. Kristj6nsson and G. J6nsson
as shown in
Iceland, J6hannesson (1980, 1986) plotted the location of the Snaefellsnes syncline
near
Stykkish6lmur
syncline,
of
the
core
from
the
Fig. 1 The only available radiometric date
toin of Fig. 2, is 6.7 Ma (Moorbath et al., 1968; 6.9 Ma if newer decay constants are used)'
Radiometric ages from lavas generated in the currently active zone in western Iceland reach
7.1 Ma (McDougall et al., 1977).
In Borgarfjiirdur, midway between the axis of the extinct Snaefellsnes volcanic zone and that
of the current zone, there is a NE-trending anticline (Fig. l; J6hannesson, 1980). At the core of
the anticline, lavas generated in the Snaefellsnes zone are exposed but not far from this core
the
there is an unconformity above which lavas from the culrent zone are found' The age of
(quoted
K-Ar
dates
available
only
two
as
the
known,
well
is
not
rocks at the core of the anticline
with
by J6hannesson) are 9 and 13 Ma. The higher of these two numbers is in better agreement
of
2.5
km
profile
in
a
composite
magnetostratigraphy
the
of
an age obtained by extrapolation
(H'
and
J6hannesson
Borgarfjdrdur
way
to
the
most
of
Stykkish6lmur
from
totaiava thickness
L. Kristjdnsson, unpublished data).
J6hannesson (1980) states that all lava sequences in the Borgarfj<irdur area which are older
with the
than 4.5 Ma exhibit thinning towards the south. This geological evidence is consistent
and
(Hey
1989),
al.'
et
Pacific
Eastern
in
the
research
from
known
rifts'
'propagating
concept of
of tectonic lineaments (J6hannesson, 1980) and geochemical patterns
so are observations
(Schilling et al., 1982) in western Iceland.
AEROMAGNETIC SURVEYS OVER WESTERN ICELAND AND THE SHELF
The most comprehensive survey of the magnetic field intensity over Iceland was made by
sigurgeirsson (1t70-85) in 1968-80. The flight altitude was generally 900 m' and the line
.pi.ii'g was 3 km. Additional measurements, mostly in offshore areas, were made in 1985-86
(iOnssJn et al., l99l) and in 1991-92 (Kristj6nsson and J6nsson, 1996) at 600 m altitude. The
(after subtraction of
aeromagnetic coverage is shown in Fig. 2 and the results in Fig. 3. All data
request'
on
available
field)
are
regional
and
effects
aircraft
variation,
diumal
The main coherent features of the residual field, to be discussed in the next section, are
lineations and central-volcano anomalies. Additionally, there are topographic effects and other
field deviations of short wavelengths which are difficult to correlate between lines. Amplitudes
are commonly of the order of 1000 nT. The lineations are definitely caused by alternating
polarity of primary remanence rather than lateral variations in induced (and viscous)
magnetization, as typical average Kbnigsberger ratios in Miocene.to Lower Pleistocene lava
flows in Iceland are of the order of 3-4 (Kristjdnsson, 1984)'
, The linear anomaly features are much less pronounced than those over the mid-ocean ridges.
It is probably in part due to the boundaries between magnetic polarity zones in the uppermost
(
m)
Iceland being subhorizontal rather than vertical. The anomaly signal from thin < 100
flight
altitude
"ruriin
at
our
the
anomalies
and
lost,
polarity zones in the lava pile will essentially be
."fl""tih" dominant polarity ofapprox. l-km thick zones in the pile, representing time intervals
over
of the order of 1 Ma (Kristj6nsson and Helgason, 1988). The lineations are best developed
than
older
alteration
and
by
tectonics
less
disturbed
are
formations less than 3 Ma in age which
rocks.
INTERPRETATION OF MAGNETIC ANOMALY LINEATIONS NORTH OF THE SNAEFELLSNES
PENINSULA
types'
Elongated magnetic anomalies north of the Snaefellsnes peninsula (Fig. 3) are of two
X
in
size.
50
km
to
15
of
up
residuals,
negative
or
positive
generally
First, there are rigions of
An extinct rift zone in westem lceland
103
To some extent, they appear to have an en echelon pattem. This pattern is reminiscent of
anomalies seen over stepped spreading-ridge segments, for example on the northernmost part of
the Reykjanes Ridge and in southwestern Iceland (J6nsson et al., l99l). Second, there are small
Fig. 3. Total-field magnetic residuals over western Iceland and the shelf. Positive values are plotted with
double the density ofnegative residuals. Sign convention is shown in the lower left-hand comer, maximum
amplitude 1000 nT.
L. Kristiiinsson and G. J6nsson
104
anomalies which can be tentatively traced between adjacent flight lines. Sometimes this type
lineation may be due to topography, especially over some
of
of the small fjords south of
Isafjardardjrip.
The two types of lineations are indicated in Fig. 4 which also shows some examples of a third
type of magnetic anomaly. These are roughly circular in shape and about 10 km across,
generally associated with volcanic centers such as Setberg and Snaefellsjdkull. The positive
(red) and negative (blue) areas should not be expected to reflect a dike swarm or other relicts
of a rift axis. It is more likely that they represent the upper edge of a tilted lava series where one
polarity domiriates. The tilt is not known in the Breidafjdrdur bay north of Snaefellsnes due to
sediment cover. The thickness of these sediments, judging from limited sparker measurements
in 1972, is probably of the order of tens of meters. However, in the Northwestem peninsula,
published measurements show that the tilt is generally in the range 1-8 degrees, increasing to
the southeast. Where known, the strike of the lava pile (Kristjdnsson and J6hannesson, 1996)
and of dikes is very similar to that of the anomaly trends in Fig. 4.
The most notable feature of the figure is the apparent rotation of trends, from an ENE or NE
24.
ffi
I
Z
7t
offiotm
Fig. 4. Tentative interpretation of the magnetic anomaly pattern over the Breidafjdrdur bay, the
Northwestern peninsula and Hrinafl6i bay. Red: regions with predominantly positive residuals. Blue:
regions with predominantly negative residuals. Lines: minor magnetic anomalies traceable between
adjacent flight lines. Solid lines indicate positive anomalies, broken lines indicate negative anomalies.
Stars: volcanic centers, from Krisddnsson and Helgason (1988), J6hannesson (1986) and H. J6hannesson,
pers. commun. (1996). Righrhand side: geomagnetic polarity time scale 7.5-15 .2Ma ago, after Cande and
Kent (1995) with red for normal polaritv.
An extinct rift zone in western lceland
105
direction in Breidafj<irdur and southwest of this area (Nunns et al.,1983) to a N direction in the
northemmost part and north of the area. A similar rotation of trends is also seen in the currently
active volcanic zones of Iceland.
The areas characterized by positive residuals in central Breidafjdrdur bay and H0nafl6i bay
are presumably underlain by extrusives from the period C5n of the polarity time scale, of
approx. 9.7-10.9 Ma age. The areas of negative residuals west of these can in a similar way be
correlated with the period of predominantly reversed polarity from 10.9 to ll.9 or l2.2Ma age.
The ages correspond well with radiometric ages found for the lava pile of the northwestern
peninsula (McDougall et al., 1984). A l-km thick normally polarized series of lavas correlated
with the C5n.2n chron runs northeast from Gilsfj<irdur, crossing Steingrimsfj<irdur east of
H6lmavik village.
INTERPRETATION OF MAGNETIC ANOMALIES IN FAXAFLOI BAY
In the westem part of Faxafl6i bay (Fig. 3), magnetic anomalies are quite subdued. This is
probably related to the presence of a sediment pile of up to several hundred meters in thickness
(K. Thors, pers. commun., 1988). West of the Snaefellsnes peninsula a clear NE- trending
lineation represents the continuation of 'Anomaly 5' on the Reykjanes Ridge (Fig. 1;Nunns et
al., 1983). We have adopted a different approach to the interpretation of the magnetic anomalies
in this area than in Breidafj<irdur, largely inspired by the northerly trend of a major positive
anomaly lineation in the bay.
Our model assumes the existence of a past rift axis trending 40o east of north which gradually
waned from northeast towards southwest, to be replaced by another rift axis located 36 km to
the southeast. For simplicity these axes are assumed to be parallel and the spreading motion is
assumed to be at right angles to the rifts.
Fig. 5 shows an age pattern for the shelf bedrock of 0-12 Ma age from our model, adjusted
for a reasonable fit with the central and 'Anomaly 5' lineations in the area. The half-spreading
rate used is 0.9 cm/year and the velocity of propagation is 2.5 cm/year. The ridge is assumed to
have started moving 8 Ma ago, the new ridge forming in crust which was 4 Ma old at that time.
Each stripe in Fig. 5 corresponds to a time interval T= 1.0 Ma. The model is consistent with the
following features:
.
The general polarity and trend of anomalies in Breida{iirdur bay (Fig. 3) and west of the
.
The pattern and trend of the anomaly lineations corresponding to the Brunhes, Matuyama,
Gauss and Gilbert chrons in southwestern lceland; these in tum coincide well with the age and
polarity of lava outcrops in the area. Onshore east of the rift zone, however, the anomalies may
be affected by crustal deformation in the fracture front mentioned in Section l.
The trend of the presumed Anomaly 5 lineation (yellow) in Faxafl6i bay, which is different
from that ofyounger anomaly lineations over the present western volcanic zone (Fig. 3). The
anomaly position onshore fits approximately with the known location of a thick series of
normally magnetized lavas in Mt. Fagrask6garfjall and nearby mountains (H. J6hannesson
and L. Kristj6nsson, unpublished data) but farther north, the anomaly pattem becomes less
coherent due to effects of Quaternary volcanism and irregular topography.
The trend of the Anomaly 5 lineation south of Iceland towards much younger formations
onshore, and its disappearance over the shelf (Fig. 1; Fig. 4 of J6nsson et al., l99l).
.
.
Snaefellsnes peninsula.
iJliji:,'ll:l-:i:ll.:t{;:i1i..,r':l::jll;li .i!ll..i:l:li.'.1:i-:.., r=-
a)
age:z T
24"
5
22"
20"
s)
Fig. 5. A simple model pattern to imitate magnetic anomalies over the Faxafl6i bay and surroundings.
Stripes are of altemating polarity, each covering I Ma. They are generated in two parallel rift zones, as
indicated for different times in the upper part of the figure (a-0. Red colors (except yellow for 'Anomaly
5') corespond to positive field residuals, and blue to negative residuals. Dark lines mark unconformities.
See text.
An extinct rift zone in western Iceland
1W
CONCLUSIONS AND DISCUSSION
Aeromagnetic anomaly lineations show a change in trend from east-northeast to north as they
cross the Northwestem peninsula of Iceland and two flanking bays. A similar change is observed
in the strike of tilted lava series and of dike swarms. The most prominent negative lineations
(Fig. a) are most likely due to lava series emplaced ll-l2iN4a ago in a presumed extinct rift zone
crossing Snaefellsnes (J6hannesson, 1980). Younger positive lineations in the area would
accordingly be due to lavas from chron c5n.2n, of lG-ll Ma age. After l0 or 9Ma ago,
spreading may have slowed down as there are few recognizable anomalies of less than l0 Ma
rift zone in central western Iceland.
Positive anomaly lineations crossing Breidafj<irdur bay are probably a continuation of
'Anomaly 5' on the Reykjanes Ridge. The pronounced easterly trend of these anomalies in
southem Breidafjiirdur may indicate that while the Snaefellsnes rift zone was active its tectonic
setting was similar to that of the present Reykjanes peninsula. The position of the extinct
spreading zone in the general region east of Hvammsfjdrdur is not well known. J6hannesson
(1980) indicates that it has a northerly direction onshore south of the eastern part of Hrinafl6i
bay (Fig. 1), coincident with a syncline shown in Fig. 1 of Kristjiinsson et al. (1993). This,
however, corresponds to the position of an anomaly lineation in the bay (Figs 3 and 4) which
in tum seems to be in direct continuation of 'Anomaly 5' north of the island. Lava sequences
covering the period about 7.3-8.4 Ma ago have been mapped just east of the syncline
(Kristji{nsson et at., 1993), so it is possible that the positive lineation in Hrinafl6i bay (Fig.4)
age in the vicinity of the extinct
is due to formations from Chron C4A or C4n.2n rather than C5n.2n.
Sequences dated from 8 or 9 to ll Ma age are found in central northern Iceland
(Saemundsson et al., 1980) but additional dating in this region and to the east is needed to
reconstruct the history of the plate boundary in northern Iceland. Connections with the spreading
history north of Iceland must take into account the complex Tjcimes fracture zone (Fig. l).
In the region of Faxafl6i bay and south of the Reykjanes peninsula, a simple model of rift
propagation can be made to fit several observed features of the magnetic anomaly field. One of
these features is the disappearance of the 'Anomaly 5' lineation on the shelf south of lceland.
The chosen mean rate of rift propagation, 2.5 cm/year (50 km in 2 Ma) is in broad agreement
with J6hannesson's (J6hannesson, 1980) geological observations in the Borgarfjdrdur region.
However, it must be noted that radiometric dating in the region is fragmentary and age estimates
are based only on somewhat uncertain magnetostratigraphic correlations.
The model of Fig. 5 is a very simple one. It doe3 not take into account the possibility of
different strike directions for the extinct rift zone and the current rift zone (J6nsson et al., l99I)
or the presence of oblique spreading in the area (Appelgate and Shor, I 994). There may also be
minor lateral movements cif the spreading axis taking place (Helgason, 1984) in between the
major jumps.
In summary, the aeromagnetic results are consistent with an extinct subsided rift zone in
westem Iceland and the shelf. This rift zone, which had many similarities with the current
western rift zone, seems to have been active from at least 14 Ma ago but slowed down by 9 Ma
ago and in the Snaefellsnes area it had died out by 7 Ma ago.
Acknowledgements--:The authors thank Mr Ii. Henningsson for his valuable cooperation during tbe l99l-92
aeromagnetic surveys, and Dr H. J6hannesson for advice on his geological studies in western Iceland. This work was
supported by the Icelandic Science Fund and by the Research Fund of the University of lceland.
REFERENCES
Appelgate
B. and Shor A. N. (1994) The northem Mid-Atlantic and
Reykjanes Ridges:
108
L. Kristj6nsson and G.
J6nsson
spreading center morphology between 55"50'N and 63.00'N.
17935-17956.
J.
Geophys. Res. 99,
Bjcirnsson S., Ed. (1967) Iceland and Mid-Ocean Ridges. Socieras Scientiarum Islandica,
Rit 3g,
209 pp.
Cande S. C. and Kent D. V. (1995) Revised calibration of the geomagnetic polarity rime scale
for the Late Cretaceous and Cenozoic. J. Geophys. Res. 100,?093_-OOSS. ^
Einarsson P. and Eiriksson J. (1982) Earthquake fractures in the districts Land and Rang6rvellir
in the South Iceland seismic zone. Jbkull 32, ll3-120.
Helgason_J.,(1984)Frequentshiftsofthevolcaniczoneinlceland. Geology12,212-216.
Hey R. N., Sinton J. M. and Duennebier F. K. (1989) Propagating rifts a"nd spreading centers.
ln Geology of North America, vol. N, The Eastern pacifii oceai and Hawaii (winte-rer E. L.,
Hussong D. M. and Decker R. w., eds.), pp. 16l-176. Geological society of America,
Boulder.
J6hannesson
13-3 1.
J6hannesson
3-19.
J6nsson
H. (1980) Evolution of rift zones in western Iceland. Ndttfirufraedingurinn 50,
H. (1986) Chapters in the geology of the Breidafjcirdur area. Breidfirdingur 44,
G-
Krisdr{nsson L. and Sverrisson M. (1991) Magnetic surveys of Iceland.
t89, 229-24 l.
Kristjiinsson L. (1984) Nores on paleomagnetic sampling in lceland. Ji;kull 34,67-76.
Kristjiinsson L. and Helgason J. (1988) Some propertieJ of basalt lava sequences and volcanic
centres in a plate-boundary environment. ln Early Tertiary Volcanism oid tht Opening
of the
NE Atlantic (Morton A. c. and parson L. M., eds.), No. 39, pp. 147-155. ceological"siciety
Te c t o no p
hy s ic s
of lnndon Special Publication. The Geological Society, Londbn.
Krisdi4nsson L. and J6hanne::o_n
!I. (1996) stratigraphy and paleomagnetism of the lava pile
south of isafjardardjrip, NW-Iceland. Jijkull U,i-tO.
Kristjiinsson L. and J5nsson G. (1996) Aeromagnetic Surveys off South and West lceland
in
1990-1992. Science Institu_te, University of lceland, Report nH_OO-96, 31 pp.
Kristj6nsson L., J6hannesson t-I. qrd McDougall I. (1993) Stratigraphy, age and paleomagnetism
of Langidalur, northem Iceland. Jdkull 42,3144.
McDougall I., saemundsson K., J6hannesson H., watkins N. D. and Krisddnsson L. (1977)
Extension of the geomagnetic polarity time scale to 6.5 m.y.: K-Ar dating, geological
ani
paleomagnetic study of a 3,500 m lava succession in westem icel and. Bull. d"il.
Soi. l*. gg,
l-15.
r., Kristj6nsson L. and Saemundsson K. (19g4) Magnetosrratigraphy and
geochronology ofNorthwest lceland. J. Geophys. Res. g9, 70ZVJO6O:
Moorbath s., Sigurdsson H. a_nd Goodwin R. (196g) K-Ar ages of ihe oldest exposed rocks
in
Iceland. Earth Planet. Sci. Lett. 4, l9i-205.
' Nunns A. G., Talwani M., Lorentzen G. R., Vogt p. R., sigurgeirsson T., Kristjdnsson L., Larsen
H' C. and Voppel D. (1983) Magnetic anomalies over Iceland and surrounding seas. In
structure_ and Development of the Greenland-scotland Ridge (Bott M. H. p., saxov
s.,
Talwani M. and Thiede J., eds.) pp. 661-678 and map. plenum, New york.
Piper J. D. A. (1973) Volcanic history and tectonics of ttre North Langitikull region, cenrral
Iceland. Can. J. Earth Sci. 10. l&-179.
McDougall
K., Kristjansson L., McDougall I. and Watkins N. D. (1980) K-Ar dating,
'geological and paleomagnetic
study of a 5-km lava succession in northern Iceland. ./.
Saemundsson
Geophys. Res. 85, 3628-3646.
Schilling J.-G., Meyer P. S. and Kingsley R. H. (1982) Evolution of the Iceland hotspot. Nature
296,3t3-320.
sigurgeirsson Th. (1970-85) Aeromagnetic profile Maps of Iceland in scak I:250,000. sheets
1 to 9. Science Institute, Universiw oflceland.