Download Tectonic and volcanic events at the Jan Mayen Ridge

Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on May 16, 2016
Tectonic and volcanic events at the Jan Mayen Ridge microcontinent
S. T. Gudlaugsson, K. Gunnarsson, M. Sand & J. Skogseid
SUMMARY: Two main tectonic phases were responsible for the formation of the Jan
Mayen Ridge microcontinent: (1) the opening of the Norway Basin in late Palaeocene/early
Eocene times, and (2) subsequent rifting within the Greenland margin by which complete
separation was achieved in early Miocene times. During the first phase the eastern ridge
flank developed as a volcanic passive margin. The initial break-up was associated with
flexuring and the formation of sequences of eastward-dipping basalt flows, which are
considered equivalent to similar features beneath the Voting and Faeroe-Shetland marginal
highs off Norway. Rifting along the Greenland margin during the second phase was
accompanied by uplift, listric normal faulting and the formation of large extensional fault
blocks. To the W and S of the ridge a flat volcanic marker of probable earliest Miocene age
covers the subsided rift and masks the ocean-continent transition. It was formed by a
volcanic event of large magnitude, either as submarine lava flows or as a sill complex.
In 1985 a detailed marine geophysical survey of
the Jan Mayen Ridge area was carried out jointly
by the Norwegian Petroleum Directorate and the
National Energy Authority of Iceland. A total of
4000 km of multichannel seismic reflection data
was obtained (Fig. 1) in addition to gravity,
magnetic and sonobuoy measurements. The
seismic data in particular have provided important information in terms of the tectonic and
volcanic history of the ridge.
The main objective of this paper is to describe
the nature of the igneous provinces at the Jan
Mayen Ridge microcontinent and show how their
formation relates to tectonic events at the ridge
and the plate tectonic development of the
Norwegian Sea.
Plate tectonics
The Jan Mayen Ridge (see Fig. 1) is a bathymetric
ridge complex extending S from the volcanic
island of Jan Mayen to about 67~ The main
northern ridge block is flat-topped with water
depths increasing to about 1000 m in the S. It is
separated from the still deeper southern ridge
complex by the Jan Mayen Trough.
The Jan Mayen Ridge is a crustal fragment
which split from the Greenland continental
margin by a westward shift in the plate boundary
at Oligocene/Miocene time. The evolutionary
models for the Jan Mayen Ridge are based on a
westward shift of the plate boundary and the
observations of Talwani & Eldholm (1977) that
the fan-shaped spreading pattern along the Aegir
Ridge in the Norway Basin required that complementary spreading must have taken place further
W to account for the motion of Greenland relative
to Eurasia. Talwani & Eldholm (1977) proposed
that the Aegir Ridge was active until about
anomaly 7 time and suspected that the complementary crust formed between the southern part
of the Jan Mayen Ridge and the Norway Basin.
At anomaly 7 time a westward jump to an
'intermediate' axis on the Iceland Plateau occurred. This axis supposedly became extinct
when spreading started from the Kolbeinsey
Ridge further W just before anomaly 5 time.
Alternatively, Vogt et al. (1980) have rejected the
existence of an intermediate axis and postulated
spreading from the Kolbeinsey Ridge since
anomaly 6C time.
The three-plate model has been further developed by several investigators (Unternehr 1982;
Nunns 1983a, b; Bott 1985). According to Nunns
(1983a, b) fan-shaped spreading formed two
conjugate wedges of seafloor on either side of the
Jan Mayen Ridge during the interval between
the formation of anomalies 20 and 7. Similarly,
Larsen (1988) suggests northward propagation
of the Reykjanes-Kolbeinsey Ridge combined
with gradual termination of spreading northwards along the Aegir Ridge during the 20-7
time interval.
Structure
The first order geological framework was reviewed by Myhre et al. (1984). The ridge is
bordered by provinces of basaltic rocks on both
sides. A fiat volcanic marker, reflector F, characterizes the western province whereas irregular
lava flows and seaward-dipping reflectors are
From MORTON,A. C. & PARSON,L. M. (eds), 1988, Early Tertiary Volcanism and the Opening of
the NE Atlantic, Geological Society Special Publication No. 39, pp. 85-93.
85
Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on May 16, 2016
Ft6.1. The grid of multichannel seismic reflection lines used in this study superimposed on bathymetry. The new
survey lines are shown as dotted lines. Contour interval 100 m with contours labelled every 500 m. JM = island of
Jan Mayen, JMB = Jan Mayen Basin, J M T = Jan Mayen Trough, SRC = southern ridge complex.
Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on May 16, 2016
Events at Jan Mayen Ridge mierocontinent
found beneath the eastern ridge flank. The ridge
is covered by a thick sequence of eastwarddipping sedimentary rocks, above a strong reflecting interface, which in early single-channel
seismic records defined the acoustic basement
(see Fig. 3, reflector JO). The sedimentary
sequence comprises two main units separated by
a prominent unconformity (reflector JA; J is
prefixed to the commonly-used symbols O and A
to indicate the Jan Mayen Ridge).
The structural map in Figure 2 is based on the
interpretation of existing multichannel seismic
reflection profiles including the new data. The
seismic database is shown in Figure 1 and line
drawings of three seismic profiles crossing the
main geological provinces are shown in
Figure 3.
The Jan Mayen Ridge is strongly affected by
normal faulting. The number of fault blocks and
the general structural complexity increase southward, as does the depth to the individual fault
blocks (see Figs 2 & 3). The main ridge block in
the N has a distinct asymmetric structure. The
eastern flank dips steeply towards the Norway
Basin and is almost undisturbed by faulting
whereas the western flank of the ridge is downfaulted towards the Jan Mayen Basin. These
faults form a listric fault complex in which
individual faults can be seen to sole-out at depth.
The ridges comprising the southern ridge complex
(Pelton 1985) are also tilted fault blocks. The
majority of the blocks, and almost all the large
ones, have fault scarps facing W. Where the
volcanic marker (reflector F) W of the ridge is
absent a number of deep half-grabens are observed between the fault blocks.
Two main fault trends are observed at the Jan
Mayen Ridge. The trends are also reflected in the
bathymetry and intersect at the prominent bend
in the ridge at 69~
N of the ridge bend, the
faults trend N-S. S of the bend, both on the main
ridge block and in the southern ridge complex,
the trend is nearly NNE-SSW. Complex structures are observed in the region where these
trends intersect.
The system of eastward-rotated fault blocks at
the Jan Mayen Ridge is interpreted in terms of
crustal extension. There is also evidence of a later
phase of compression. Reverse faults, similar to
those reported by Skogseid & Eldholm (1987),
have been identified in the new data, but their
detailed correlation has yet to be worked out.
Between the eastern and western volcanic
provinces there is a seismic window into the
deeper crust. The new data show a number of
reflecting interfaces beneath reflector JO, some
of which are found at depths of 6-7 s two-way
time (see Fig. 3).
87
Geophysical investigations and deep-sea drilling have not yet conclusively answered the
question of whether the ridge is continental or
oceanic in nature. In our opinion the balance of
evidence favours a continental crust, probably
thinned and modified by rifting processes.
The seismic velocity structure (Johansen et al.
in press) and the seismic reflector pattern below
reflector JO are not compatible with normal
oceanic crust.
The continent~ocean boundary is difficult to
locate for two reasons: (1)the boundary is
probably masked in many places by lava flows;
and (2) the detailed location of the boundary is to
a certain extent a matter of definition. We can,
however, place limits on the zone where the
crustal transition must occur. The outer limit is
marked by the oldest seafloor-spreading anomalies on both sides of the ridge. At the eastern side
the inner limit lies at the apex of the wedge of
seaward-dipping reflectors. On the western side
it follows the scarps of the westernmost fault
blocks.
Volcanic provinces
The distribution of volcanic rocks on either side
of the ridge is shown in Figure 2. On the eastern
flank we differentiate between a wedge of
eastward-dipping reflectors below reflector JO
and a younger volcanic overprint. Reflector JO
forms the top of the eastward-dipping reflectors.
The wedge is most prominent S of the Central
Jan Mayen fracture zone, where it is underlain
by a sequence of parallel reflectors. On line C (see
Fig. 3) this sequence may be traced up-dip
towards the boundary fault without change in
character, which shows that it predates the
faulting and originally continued further W. On
line B the same sequence may be continued a
short distance beyond the apex of the wedge. The
internal reflectors of the overlying wedge converge only to a certain point and then also become
parallel. On the basis of these observations we
suggest that an equivalent of the E Greenland
plateau basalts may underlie the seaward-dipping
reflectors and cover the southern part of the ridge.
N of the Central Jan Mayen fracture zone the
wedge has a different character. Both the dip and
the divergence of the reflectors is less prominent
and the reflector pattern is more irregular. The
mapping of the extent of the dipping reflectors
presented here is primarily based on the new
survey, and their presence in Figure 2 is only
shown where a well-developed dipping and
divergent sequence is observed.
Reflector JO is overstepped from the E by a
Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on May 16, 2016
FIG. 2. Structural map of the Jan Mayen Ridge and the surrounding areas. Horizontal ruling = western front of
volcanic overprint; diagonal ruling -- area covered by reflector F; stippling = areas of seaward-dipping
reflectors; thin continuous lines = bathymetric contours; thick continuous lines = fracture zones; dotted lines =
seafloor-spreading anomalies (Skogseid & Eldholm 1987; Vogt et al. 1980; Gr~nlie et al. 1978). JMB = Jan
Mayen Basin, JMT = Jan Mayen Trough, C J M F Z = Central Jan Mayen Fracture Zone. Fracture zones from
Skogseid & Eldholm (1987). Dashed lines A, B and C show the location of the seismic profiles in Figure 3.
Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on May 16, 2016
89
Events at Jan Mayen Ridge microcontinent
e.e5
~z
< II
~z
..~
.=
o
~
~E
e~
~a
M~
e~
u~
~g
~.~
; . 7 .~
Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on May 16, 2016
9~
S. T. Gudlaugsson et al.
strongly reflective horizon characterized by diffractions and both irregular and smooth reflector
segments (see Figs 2 & 3). The surface can be
traced from opaque oceanic basement westward,
where it gradually becomes semi-opaque with
irregular structure below before it terminates
within the sediments above reflector JO. We
interpret this layer as being caused by submarine
lava flows, intrusions and pyroclastics interfingering with marine sediments. The western front
of this younger volcanic overprint may be traced
from the southern ridge complex, with decreasing
stratigraphic gap with respect to reflector JO, N
to the Central Jan Mayen fracture zone. At this
point the front turns westwards and merges with
reflector JO (see Fig. 2).
W and S of the ridge we have mapped a strong,
fiat-lying acoustic basement reflector beneath a
thin sedimentary cover. This is the 'opaque
horizon' of Eldholm & Windisch (1974) that
covers a large part of the Iceland Plateau. The
new data allow us to differentiate between an
ultra-fiat opaque volcanic marker (denoted F) in
the area between the ridge and the oldest seafloorspreading anomaly on the Iceland Plateau and a
different more irregular type of basement further
W.
Reflector F covers the area W of the southern
ridge complex locally extending into the Jan
Mayen Basin and the Jan Mayen Trough. On the
eastern side it terminates abruptly at the fault
scarps. The western boundary lies at the foot of
an E-facing escarpment associated with the oldest
seafloor-spreading anomaly. The height difference between the top of the escarpment and
reflector F decreases towards the S. S of about
69~ the relationship between reflector F and
the oceanic basement becomes unclear. Here the
reflector seems to overstep the basement towards
the W, but we cannot verify that it is continuous
with the basaltic basement reflector which was
drilled at Deep Sea Drilling Project (DSDP) Site
348 (Talwani et al. 1976). We note that W of the
escarpment a number of sub-basement reflectors
are observed on lines A and B. On line A the
reflection pattern is westward-dipping and diverging similar to the pattern of a typical wedge
of seaward-dipping reflectors.
Reflector F is very strong and normally no
reflectors are observed below. The acoustic energy
becomes trapped in the water layer and in the
thin sedimentary layer. The reflector is not
perfectly smooth but exhibits small-scale roughness. As Figure 3 shows, reflector F is composed
of a number of offset segments. Because of the
poorly developed stratification in the overlying
sediments it is difficult to determine the nature of
these offsets, but there are indications that some
of them are associated with high-angle normal
faults. The most striking aspect of the reflector
is, however, the extreme flatness of individual
segments.
In our opinion, the possible interpretations of
reflector F are: (1) high impedance sedimentary
layer; (2) volcanic ash layer; (3) intrasedimentary
sills; and (4)lava flows. In the sedimentary
environment of the Norwegian Sea it is unlikely
that such a high impedance contrast could occur
within a sedimentary succession without the
compaction effect of burial and later erosion.
Reflector F does not show any evidence of
submarine erosion. Volcanic ash may generate
highly reflective flat surfaces. An ash layer must
either have been pelagically draped over existing
topography or redeposited as mass flows. The
first mechanism is not viable as there is no
draping effect over the blocks and the reflector
terminates abruptly against fault blocks and
escarpments. It is possible, however, that volcanic
ash from an elevated source area close to Iceland
may have been transported by turbidity flow
northward onto the Iceland Plateau. A regional
intrasedimentary sill is also a possible explanation, but seems unlikely from a mechanical
viewpoint considering the areal extent of the
reflector. However, if the reflector offsets are
interpreted as the limits of individual sills and
not as normal faults this argument does not apply
and reflector F may represent the top of a sill
complex. Nonetheless, we favour an interpretation of extensive submarine lava flows as suggested for the ultra-flat opaque reflectors on the
Reykjanes Ridge (Vogt & Johnson 1973).
On line A (see Fig. 3), which crosses the Jan
Mayen Basin just S of the northern termination
of reflector F, the layer below the reflector is
locally acoustically transparent. A number of
reflections are observed beneath it and the
reflection pattern suggests a continuity of sedimentary layers beneath the volcanic marker from
the main ridge block to a fault block further W
which rises above the floor of the Jan Mayen
Basin. On seismic lines further S reflections from
sedimentary layers and detachment planes in the
fault complex at the western margin of the main
ridge block are observed to dip under reflector F
but are lost a short distance from its edge. In our
interpretation, the listric fault complex continues
underneath the reflector. A similar relationship
is observed in the southern ridge complex where
some of the fault blocks plunge beneath it.
Since the reflector is fiat-lying and covers the
basin fill in the half-grabens between some of the
blocks, its formation postdates the block faulting
by a considerable interval. It follows that most of
the present elevation difference between the Jan
Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on May 16, 2016
Events at Jan Mayen Ridge microeontinent
Mayen Ridge, and the lower areas to the W
covered by reflector F, was already established at
the time of its formation. The flat unconformity
on top of the main ridge block represents the
deepest level that wave erosion ever reached
since the faulting. Therefore, the reflector was
formed under submarine conditions.
Whether reflector F is interpreted as a layer of
volcanic ash, submarine lava flows or sills, it was
formed by a volcanic event of a large magnitude.
Timing of volcanic events and
evolution of the microcontinent
No detailed chronology of events has been
available for the Jan Mayen Ridge. The DSDP
holes drilled during Leg 38 (Talwani et al. 1976)
only sampled part of the sedimentary sequence
and the stratigraphy of the ridge has not been
well understood. We now propose a chronology
for the main tectonic and volcanic events based
on seismic stratigraphy and correlation with the
geology of the conjugate Greenland and Norwegian margins.
We start by examining the age of the oldest
well-defined rock sequence at the Jan Mayen
Ridge, i.e. the sequence of seaward-dipping
reflectors. Comparable sequences at the Norwegian margin are well known (Skogseid & Eldholm,
1987). In the light of their origin as extensive
subaerial basaltic lava flows formed during the
earliest spreading phase, their position relative
to the Jan Mayen Ridge in plate tectonic
reconstructions and the symmetry of the wedges
on both sides, there can be no doubt that the
sequence of seaward-dipping reflectors at the Jan
Mayen Ridge has a similar origin and was formed
at approximately the same time at the conjugate
margin as proposed by Skogseid & Eldholm
(1987). They interpret reflector JO as an equivalent to reflector EE of earliest Eocene age at the
V~ring margin.
Gairaud et al. (1978) divided the cover of
Tertiary sediments at the Jan Mayen Ridge into
two sequences separated by a prominent unconformity, reflector JA. The unconformity was
drilled at DSDP Sites 346, 347 and 349 and
proved to represent a hiatus in the early Oligocene
(Yalwani et al. 1976).
Here, we prefer to divide the sedimentary
series covering the Ridge into three sequences
numbered 1, 2 and 3 from below (sequence no. 1
being the oldest, see Fig. 3).
The base of sequence 3 corresponds to reflector
JA. Sequence 1 is mostly parallel-bedded and
represents a widespread slope or shelf sequence.
91
It predates the block-faulting at the western
margin of the ridge as does the parallel sequence
below reflector JO and the wedge of seawarddipping reflectors. Sequence 2 shows a more
disturbed sedimentary pattern. On line A the
sequence exhibits outbuilding and downlap followed by marine onlap. Over the entire ridge the
apparent instability of the sedimentary environment increases upward and the uppermost part
of the sequence consists of slumps and other
mass-flow deposits. We interpret this as gradual
uplift of the ridge flank associated with doming
and rifting at the western margin. The uplift of
the ridge culminated in subaerial exposure of the
main ridge block, a strong erosional phase and
the formation of a prominent submarine unconformity on the slope (reflector JA). Sequence 3
formed first by outbuilding of submarine fans
and later by passive draping with much reduced
sedimentation rates.
We now turn to the formation of the fault
complex at the western margin of the ridge. Its
conjugate part is the Liverpool Land margin,
where Larsen (1984) describes a buried rift with
an Eocene to early Oligocene graben-fill. He
suggests the main tectonic episode of blockfaulting to be early to mid-Eocene and that the
rifting accompanied the initial formation of the
Norway Basin, inferring that the block-faulting
at the Jan Mayen Ridge dates from this time.
However, the Jan Mayen Ridge seismic data do
not support this model. In fact, the western
margin of the Norway Basin is found at the
eastern flank of the Jan Mayen Ridge, where it is
associated with flexuring and formation of a
wedge of seaward-dipping reflectors. No block
faulting is observed. The only candidate at the
Jan Mayen Ridge for a symmetric counterpart to
the landwards-rotated system of fault blocks at
the Greenland margin is the system of eastwardrotated listric normal faults at the western margin
of the ridge. The two margin types reflect different
tectonic events with different thermal and mechanical characteristics and must be separated in
time. We associate the block-faulting at the Jan
Mayen Ridge with the separation of the ridge
from Greenland, thus suggesting a younger age
for block-faulting at the Greenland margin. In
this connection it is interesting to note that Larsen
(1984) finds no evidence for a later tectonic
episode at the Liverpool Land margin which
might correlate with the separation of the ridge
from Greenland. At the Jan Mayen Ridge the
minimum time gap between the two events is the
interval represented by sedimentary sequence 1.
The upper boundary of the sequence represents
the earliest possible date for the onset of rifting.
Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on May 16, 2016
92
S . T . Gudlaugsson et al.
Thus, we propose that:
(1) Sequence 2 is time equivalent with the
Liverpool Land rift basin-fill and is middle
Eocene to early Oligocene in age. The
deposition of sequence 2 was contemporaneous with the development of the rift
beneath the Liverpool Land shelf. On the Jan
Mayen side the sequence was deposited
outside the rift, on its eastern flank. The
sedimentary fill observed on the Greenland
side was deposited within the rift during the
same period.
(2) Sequence 1 at the Jan Mayen Ridge is early
Eocene in age, possibly extending into midEocene. It was deposited on the continental
slope or shelf of Greenland on top of the
seaward-dipping reflectors concurrently with
extrusion of submarine lava flows in the
Norway Basin. This sequence is probably
found within the fault blocks in the Greenland
Rift as well as at the Jan Mayen Ridge.
Reflector F, the volcanic marker W of the Jan
Mayen Ridge, clearly postdates the block-faulting
at the western margin. Since it lies fiat on top of
the half-graben-fiU in the southern ridge complex
and shows no evidence of being affected by
movements on the boundary faults, neither there
nor W of the main ridge block, the formation of
the reflector postdates rifting between Greenland
and the Jan Mayen Ridge by a significant time
interval. W of the main ridge block it appears
that normal seafloor spreading was not established until anomaly 6C time (24 Ma). Line A
indicates that the final break-up was associated
with the formation of a submarine sequence of
westward-dipping reflectors W of the escarpment
at anomaly 6C. Examining the western termination of reflector F at the escarpment we find it
most likely that the escarpment was formed prior
to the emplacement of the reflector. We propose
an earliest Miocene age for reflector F.
ACKNOWLEDGEMENTS:We wish to thank the Norwegian Petroleum Directorate and the National Energy
Authority of Iceland for their permission to publish this
paper. S. T. Gudlaugsson and J. Skogseid were
supported by a research grant from the Norwegian
Petroleum Directorate, and K. Gunnarsson by a grant
from the Nordic Council of Ministers.
References
BOTT, M. H. P. 1985. Plate tectonic evolution of the
Icelandic transverse ridge and adjacent regions.
Journal of Geophysical Research, 90, 9953-9960.
ELDHOLM, O. & WINDISCH, C. C. 1974. Sediment
distribution in the Norwegian-Greenland Sea.
Bulletin of the Geological Society of America, 8 5 ,
1661-1676.
GAIRAUD,H., JACQUART,G., AUBERTIN,F. & BEUZART,
P. 1978. The Jan Mayen Ridge. Synthesis of
geological knowledge and new data. Oceanologica
Acta, 1, 335-358.
GRONLIE, G., CHAPMAN,M. & TALWANI,M. 1978. jan
Mayen Ridge and Iceland Plateau: origin and
evolution. Norsk Polarinstitutt Skrifter, 170, 25-48.
JOHANSEN,B., ELDHOLM,O., TALWANI,M., STOFFA,P.
L, & BUHL,P. (in press). Expanding spread profile
at the northern Jan Mayen Ridge. Polar Research.
LARSEN, H. C. 1984. Geology of the East Greenland
Shelf. In: SPENCER, A. M., et al. (eds) Petroleum
Geology of the North European Margin. Graham &
Trotman, London, pp. 329-339.
- 1988. A multiple and propagating rift model for
the NE Atlantic. In: MORTON,A. C. & PARSON,L.
M. (eds) Early Tertiary Volcanism and the Opening
of the ArE Atlantic. Geological Society of London,
Special Publication, 39, pp. 157-158.
MYHRE,A. M., ELDHOLM,O. & SONDVOR,E. 1984. The
Jan Mayen Ridge: present status. Polar Research,
2, 47-59.
NUNNS, A. G. 1983a. Plate tectonic evolution of the
Greenland-Scotland Ridge. In: BOTT, i . H. P.,
SAXOV, S., TALWANI,i . & THIEDE, J. (eds) The
Greenland-Scotland Ridge: New Methods and Concepts. Plenum, New York, pp. 11-30.
-1983b. The structure and evolution of the Jan
Mayen Ridge and surrounding regions. In: WATKINS,J. S. & DRAKE,C. L. (eds) Continental margin
geology. Memoir of the American Association of
Petroleum Geologists, 34, pp. 193-208.
PELTON, C. D. 1985. Geophysical interpretation of the
structure and evolution of the Jan Mayen Ridge.
Institute of Oceanographic Sciences Report, No. 205,
38 pp.
SKOGSEID, J. & ELDHOLM, O. 1987. Early Cenozoic
crust at the Norwegian Continental Margin and
the conjugate Jan Mayen Ridge. Journal of Geophysical Research, 92, 11471-11491.
TALWANI, M, & ELDHOLM,O. 1977. Evolution of the
Norwegian-Greenland Sea. Bulletin of the Geological Society of America, 88, 969-999.
- - , UDINTSEV, G, et al. 1976. Initial reports of the
Deep Sea DrillingProject, US Government Printing
Office, Washington, 38, 1256 pp.
Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on May 16, 2016
Events at Jan Mayen Ridge microcontinent
UNTERNEHR, P. 1982. Etude structurale et cinOmatique
de ta met de NorvOge et du Groenland. Evolution du
microcontinent de Jan Mayen. Thesis. University of
Bretagne, 227 pp.
VOGT, P. R. & JOHNSON, G. L. 1973. A longitudinal
seismic reflection profile of the Reykjanes Ridge:
93
Part II--Implications for the mantle hot spot
hypothesis. Earth and Planetary Science Letters, 18,
49-58.
& KRISTJANSSON,L. 1980. Morphology and
magnetic anomalies north of Iceland. Journal of
Geophysics, 47, 67-80.
S. T. GUDLAUOSSON& J. SKOGSEID,Department of Geology, University of Oslo, Norway.
K. GUNNARSSON,Department of Geology, University of Oslo, Norway and National Energy
Authority, Reykjavik, Iceland.
M. SAND, Norwegian Petroleum Directorate, Stavanger, Norway.