Download Lithospheric structure of the Mid-Norwegian Margin: comparison

Journal of the Geological Society, London, Vol. 162, 2005, pp. 1005–1012. Printed in Great Britain.
Lithospheric structure of the Mid-Norwegian Margin: comparison between the
Møre and Vøring margins
1
M . F E R NÀ N D E Z 1 , C . AYA L A 1 , M . TO R N E 1 , J. V E R G É S 1 , M . G Ó M E Z 1 & R . K A R P U Z 2
Group of Dynamics of the Lithosphere, Institute of Earth Sciences ‘Jaume Almera’, CSIC, Lluı́s Solé i Sabarı́s s/n,
08028 Barcelona, Spain (e-mail: [email protected])
2
Norsk Hydro Oil and Gas, Bokharest Bld., Bokharest Street, 15137 Tehran, Iran
Abstract: The lithospheric structure across the Møre Margin has been studied by integrating regional
elevation, gravity, geoid and heat-flow data together with available seismic and geological data. Our results
show that the Moho depth varies from 42 km beneath the Caledonides to 20–15 km beneath the basin and to
13–16 km in the oceanic domain. The lithosphere thins in a step-like manner from 160 km inland to 60–
80 km in the oceanic domain. A comparison with the Vøring Margin reveals that both margins have similar
crustal and lithospheric structures. Major dissimilarities are: (1) the volume of underplating beneath the
ocean–continent boundary; (2) the lithospheric thickness beneath the Caledonian Front; (3) the lithospheric
thickness beneath the distal parts of the margins. The heat transfer across the Vøring Transform Margin, the
secular variation of lithospheric thickness with age, and the jump of the Aegir spreading ridge are responsible
for these differences.
Keywords: Møre margin, Vøring margin, Caledonides, lithosphere.
show that both margins are characterized by significantly thinned
continental crust, anomalously thick oceanic crust, and the
presence of large volumes of magmatic underplated material
emplaced at the later stages of the continental break-up (at c.
55 Ma). None the less, the lithospheric structure of the MidNorwegian Margin is still poorly understood: at the Vøring
Margin it has been studied only recently (Fernàndez et al.
2004a) whereas at the Møre Margin it remains largely unknown.
According to Fernàndez et al. (2004a), the Vøring lithosphere
thins dramatically from c. 190 km beneath the Norwegian
Caledonian Belt to less than 60 km in the oceanic domain.
In this context, the main objectives of this work are: (1) to
investigate the lithospheric structure across the Møre Margin
from the Caledonian Orogenic Belt onshore to the oceanic
domain offshore; (2) to compare the resulting lithospheric
structure with that proposed for the adjacent Vøring Margin by
Fernàndez et al. (2004a); (3) to discuss the structural similarities
and differences between the two margins. For these purposes we
have performed an integrated geophysical study along a profile
that crosses the main structural units of the Møre Margin and
runs subparallel to the Vøring Margin transect (Fig. 1).
The Mid-Norway Margin, comprising two segments, the socalled Møre and Vøring margins (Fig. 1), is a key region to study
the present-day crustal and lithospheric structure of an area
affected by continental extension and sea-floor spreading under
the influence of a mantle plume (e.g. Brekke 2000; Skogseid et
al. 2000). After the Caledonian Orogeny, the Mid-Norway region
underwent several episodes of lithospheric extension during the
late Permian–Triassic, late Jurassic–Early Cretaceous and Late
Cretaceous–Palaeocene, forming the large and deep Møre and
Vøring sedimentary basins. The latest rifting episode led to
continental break-up and ocean-floor generation at the Palaeocene–Eocene transition.
The Møre and Vøring margins are separated by the Jan Mayen
Fracture Zone, a major offset in the Mid-Atlantic Ridge that ends
to the SE in the Vøring Transform Margin (Berndt et al. 2001).
Both margins have similar structural characteristics, which are
from NW to SE: (1) a marginal high, topped by Eocene lavas,
which marks the ocean–continent boundary (OCB); (2) an
escarpment, partly related to landward-dipping sequences of
lavas; (3) wide and thick Cretaceous basins (the Møre and
Vøring basins) with NE–SW-trending axes; (4) the Møre–
Trøndelag Fault Complex, which can be related to the Caledonian deformation, reactivated during subsequent rifting episodes
(e.g. Gabrielsen et al. 1999). The main dissimilarities between
the two margins are: (1) the late Palaeozoic–Triassic Trøndelag
Platform, next to the SE of the Vøring Basin, a tectonic unit that
is not present in the Møre Margin; (2) the Late Cretaceous–
Palaeocene normal fault system affecting the NW Vøring Basin,
which is not evident in the Møre Basin (e.g. Brekke 2000; Ren
et al. 2003).
The crustal structure of the Møre and Vøring margins has been
primarily imaged by seismic methods, although the Vøring
Margin has been the subject of a more extensive investigation
(e.g.Mutter & Zehnder 1988; Zehnder et al. 1990; Planke et al.
1991; Olafsson et al. 1992; Mjelde et al. 1997, 1998, 2001;
Berndt et al. 2001). Different interpretations of seismic data
Fundamentals and data analysis
Our modelling approach is based on the integration of regional elevation,
gravity, geoid and heat-flow data together with available seismic data.
The combination of these observables gives information on the density
and temperature distribution at different depth ranges. Short-wavelength
gravity anomalies and heat-flow variations are more sensitive to shallow
structures, whereas elevation, geoid height and long-wavelength heat-flow
variations provide information on deep-seated features. Near-vertical and
wide-angle seismic data provide very valuable information on both the
geometry of the crustal layers and the depth distribution of P-wave
velocities.
Geopotential, lithostatic, and heat transport equations are solved
simultaneously by using a finite element code based on Zeyen &
Fernàndez (1994). The lithospheric section is divided into a number of
1005
1006
M . F E R NÀ N D E Z E T A L .
Fig. 1. Main structural elements of the
Norwegian Margin. Continuous straight
lines indicate the location of the modelled
lithospheric profiles. Grey strips indicate
the location of MCS profiles used in the
Møre Margin transect. Dashed lines
indicate the location of ESPs from Elf
Aquitaine Norge (E) on MCS profile
MVM1, and from Lamont–Doherty
Geological Observatory (L) on WAP profile
NGT-8 (Olafsson et al. 1992). Location of
well 6205/3-1 is also indicated.
bodies with different material properties: thermal conductivity, heat
production (varying with depth), and density and its variation with
pressure and temperature (Table 1). The top and bottom of the model
correspond to the Earth’s surface and the lithosphere–asthenosphere
boundary (LAB), respectively. The density of the lithospheric mantle is
assumed to be temperature dependent according to rm ¼ ra
[1 þ Æ(T a T (z)], where ra is the density of the asthenosphere, Æ is the
thermal expansion coefficient, and Ta is the temperature of the asthenosphere (Parsons & Sclater 1977). Temperature distribution and surface
heat flow are calculated by solving the steady-state heat transport
equation assuming no heat flow across the lateral boundaries and a fixed
temperature at the top and bottom of the model of 58 C and 13508 C,
respectively. The Bouguer gravity anomaly is calculated using Talwani’s
2D algorithm (Talwani et al. 1959) for each triangular element of the
finite element mesh allowing for density variations depending on
pressure, temperature, and lithology. The geoid anomaly is calculated
using an algorithm based on the resolution of the gravity potential for a
rectangular prism, which is formed by adjacent triangular elements and
extends to infinity in a direction perpendicular to the strike of the
lithospheric section (Zeyen et al. 2005). Elevation is calculated on every
column of the mesh by comparing its buoyancy with that corresponding
to the average mid-oceanic ridge column under the assumption of local
Table 1. Rock parameters used in the 2D lithospheric model
Lithology
Cenozoic sediments
Cretaceous sediments
Pre-Cretaceous rocks
Upper crust
Lower crust
Oceanic crust
Underplating
Lithospheric mantle
Thermal conductivity
(W m1 K 1 )
2.2
2.2
2.5
3.2
2.1
2.1
3.0
3.2
Heat production
(106 W m3 )
1.2
1.2
1.6 e z =15000
1.6 e z =15000
0.2
0.2
0.02
0.02
Density
(kg m3 )
2200
2375 3 5.2 3 1010 P
2670
2650 3 1.4 3 1010 P
2950
2850
3000
3200*
*The density of the lithospheric mantle is temperature dependent: rm ¼ ra [1 þ 3:5 3 10 5 (T a T z )] with ra ¼ 3200kgm 3 . P is
lithostatic pressure (Pa) and z is depth (m).
D E E P S T RU C T U R E O F T H E M I D - N O RW E G I A N M A R G I N
isostasy and following Lachenbruch & Morgan (1990). The depth of
isostatic compensation to calculate elevation, and gravity and geoid
anomalies, is taken at the maximum depth reached by the lithospheric
mantle along the transect. The space between this depth and the base of
the model is filled with asthenospheric material with a constant density.
The resulting elevation, Bouguer anomaly, geoid height variation, and
surface heat flow are compared with the measured values. The geometries
of the LAB and crust are then modified until the best fit is obtained.
Clearly, the degree of freedom in modifying the geometry of the crustal
layers depends on the availability and quality of seismic data. This
modelling technique has been applied in different geodynamic settings to
determine the crustal and lithospheric structure, including the Pyrenees
(Zeyen & Fernàndez 1994), the West Carpathians and the High–Middle
Atlas (Zeyen et al. 2002, 2005), and the Vøring and the SW Iberian
margins (Fernàndez et al. 2004a, b).
Regional elevation, gravity, geoid and heat-flow data have been
compiled from different sources. Elevation and free-air gravity data have
been obtained from TOPEX global datasets (Sandwell & Smith 1997).
Offshore, the two datasets are combined to calculate the simple Bouguer
anomaly with a density reduction of 2670 kg m3 , whereas onshore, we
have used the NGU (Geological Survey of Norway) Bouguer anomaly
map. Geoid heights have been calculated from the EGM96 geopotential
model (Lemoine et al. 1998) developed up to degree and order 360. Seafloor heat-flow data have been compiled from Sundvor & Eldholm (1992)
and Sundvor et al. (2000). Onland heat-flow values come from Cermak et
al. (1993). Figure 2 shows the maps of these observables and the location
of the modelled profiles.
Topography shows constant average values of 800 m along the
Norwegian Caledonian Belt decreasing rapidly to sea level. Offshore,
however, bathymetry shows conspicuous differences between the Møre
and the Vøring margins. The Vøring Margin is characterized by a wide
shelf, with depths between 0 and 400 m across the Trøndelag Platform.
Towards the NW sector of the margin, the Vøring Marginal High is
located at water depths of about 1400 m. The deepest part of the margin
remains at depths of 3000–3500 m. The Møre Margin shows a shelf of
about 80 km width and bathymetry increases steadily from the shelf
break to 3000 m in the Møre Basin some 300 km from the coast. The
Bouguer anomaly map displays values of 50 mGal in the Caledonian
Belt increasing to more than þ250 mGal in the deepest parts of the
margins. The Vøring Margin and, to a lesser extent, the Møre Margin
show short-wavelength gravity highs and lows reflecting the irregular
basement structure. The geoid height map displays an east–west increasing trend from values of 36–45 m in the Caledonides to about 50–52 m
in the deep oceanic basins. This east–west trend contrasts with the SE–
NW trend shown by elevation, gravity maps, and tectonic structure.
Surface heat-flow values of 50–60 mW m2 are measured onshore,
whereas offshore the measurement shows a very high scatter, ranging
from 30–40 mW m2 to more than 100 mW m2 . This scatter is
particularly noticeable in the Møre Margin, where high and low heat-flow
values alternate along the continental slope.
Lithospheric structure of the Møre Margin
The profile along which the present-day lithospheric structure of
the Møre Margin has been modelled starts in the oceanic domain
and crosses the Møre Marginal High, the Faroe–Shetland
Escarpment, the Møre Basin, and the Slørebotn sub-basin, ending
in the Caledonian mountains, with a total length of 640 km (Fig.
1). The profile has been oriented to coincide with deep (10 s
two-way travel time (TWTT)) multichannel seismic (MCS) lines
MB-08-92 and GOM-95-408 provided by courtesy of WesternGeco and NPD and used by Gómez et al. (2004) to construct a
regional geological cross-section (Fig. 3).
The crustal structure along the modelled profile has been
constrained from different seismic datasets, which include multichannel profiles, wide-angle ocean bottom seismometer (OBS)
data, and expanded spread profile (ESP) data. The Tertiary–
Quaternary sedimentary thickness in the oceanic domain has
1007
been obtained from profile C of the ‘1:3 000 000 Bedrock Map of
Norway and Adjacent Ocean Areas’ (Sigmond 1992). For the
structure of the oceanic crust and the thickness of the magmatic
underplating we have used ESP seismic data from Olafsson et al.
(1992) and the seismic compilation data of Planke et al. (1991).
Profile M–M9 of Blystad et al. (1995) has been used to constrain
the geometry of the Pre-Cretaceous rocks in those places where
no deep seismic data were available. The geometry of Cretaceous, Tertiary, and Quaternary sedimentary layers in the Møre
Basin has been taken from MCS data interpreted by Gómez et
al. (2004). Data of Korja et al. (1993) have been used to
constrain the depths of the Moho and the top of the lower
continental crust onshore. The coverage of seismic data along the
modelled profile is uneven and makes use of additional gravity,
elevation and geoid data necessary to interpolate or extrapolate
the crustal structure to those portions of the profile that are not
covered by seismic data.
Because of the similarities in lithologies and velocity distribution between the Vøring and Møre margins, we have used the
same densities and thermal parameters as Fernàndez et al.
(2004a) used for the Vøring Margin (Table 1). The only
differences are the densities of the Cretaceous sediments, which
according to well 6205/3-1 (see Fig. 1 for location) should be
slightly lower than in the Vøring Margin (2375 kg m3 instead of
2400 kg m3 , at sea level), and of the Pre-Cretaceous basement,
which, for simplicity, is considered to be constant (2670 kg m3
instead of pressure dependent as 2400 3 5.2 3 1010 P kg m3 ).
To compare the calculated elevation, Bouguer anomaly, geoid
height variations and surface heat flow with the measured values,
we have averaged the regional observables within a 50 km wide
strip centred over the profile. Because of their scarcity, heat-flow
data have been averaged over a 100 km wide strip. Figure 3
shows the results obtained from the best fitting model, where the
LAB geometry has been imposed to fit the observables and the
crustal geometry has been modified within the range of accuracy
of the available seismic data (Fig. 4). The main trend of
elevation, Bouguer anomaly, and geoid is well reproduced and
only small misfits of short-wavelength component (less than
50 km) are observed. These misfits are due to the simplified
geometry of the sedimentary cover, which does not influence the
final result of this lithospheric-scale modelling.
The large scatter in the measured heat-flow values makes it
difficult to achieve a good fit with the modelling results. The
calculated values onland and for most of the Møre Basin are
around 50 mW m2 with a gentle increase towards the Marginal
High in general agreement with observations. In the oceanic
domain, however, the calculated values increase from 60 to
75 mW m2 , in clear contrast to observations that show lower
values (35–55 mW m2 ). These measured values are abnormally
low and although their origin is not well known (Sundvor et al.
2000) they could be related to recent giant landslides (e.g.
Bondevik et al. 2003). The sudden deposition of large masses of
sediments, which cool during the transport and mixing with
seawater, produces a transient decrease in the geothermal
gradient measured on the newly deposited sea floor. Near the
shelf break there is a narrow positive heat-flow anomaly
(.100 mW m2 ), the short wavelength of which indicates a
shallow origin probably related to warm water seepage produced
by overpressure (Sundvor et al. 2000).
The results from our model permit the construction of a
complete and continuous crustal and lithospheric cross-section of
the Møre Margin from the Caledonian mountains onshore to the
deep oceanic domain. The Moho depth varies from around
42 km under the Caledonides to about 20 km in the central part
1008
M . F E R NÀ N D E Z E T A L .
Fig. 2. Observables used to model the present-day lithospheric structure in the Møre and Vøring margins and location of the modelled lithospheric
profiles: (a) elevation, contours every 500 m; (b) Bouguer anomaly, contours every 25 mGal; (c) geoid height, contours every 1 m; (d) surface heat flow
(mW m2 ); d, locations of sea-floor heat-flow measurements.
of the Møre Basin. From there, it remains approximately at a
constant depth until the Faroe–Shetland Escarpment. The Moho
shallows again towards the oceanic domain, reaching a depth of
c. 13 km at the NW end of the profile. The pre-Cretaceous crust
of the Møre Margin shows nearly constant thickness for both
upper and lower crust in the SE sector of the profile beneath the
Caledonides (Fig. 4). Towards the NW, both crustal layers
decrease in thickness to reach minima below the deepest part of
the Møre Basin, where the upper crust thins to 5 km and the
lower crust completely thins out. The upper crust thickens again
to 7 km below the Møre Marginal High. In the northwestern half
of the profile, the base of the crust comprises a high-velocity
body, which, by similarity to the Vøring Margin, is possibly
related to magmatic underplating. This body is c. 5 km thick
with a maximum thickness of 7–8 km under the OCB. This
crustal structure roughly coincides with that inferred from ESP
data along a transect that runs slightly oblique to our profile
(Olafsson et al. 1992; see also Fig. 4). The main differences are
the high-velocity body (VP c. 7.1–7.5 km s1 ) at about 140–
170 km horizontal distance in our profile, and the high-velocity
D E E P S T RU C T U R E O F T H E M I D - N O RW E G I A N M A R G I N
Elevation (m)
NW
-100
1000
0
100
200
300
400
SE
OCB
0
-1000
-2000
B.A.(mGal)
-3000
200
100
0
Geoid (m)
52
48
S. H. F. (mW/m 2)
The lithospheric mantle thins from continental to oceanic
domain in a step-like manner (Fig. 3), reflecting the superposition of several tectonothermal events since the Palaeozoic
and the secular variation of the lithospheric thickness with age.
Under the Caledonides, which in the upper crustal levels are
formed by sequences of thrust sheets transported onto the underlying Precambrian basement (e.g. Gee et al. 1985; Hurich 1996),
the LAB is located at 160 km depth. This depth is in good
agreement with the expected thickness for a Mid- to Late
Proterozoic aged lithosphere (Artemieva & Mooney 2001).
Beneath the Møre Basin, developed on a Phanerozoic domain,
the lithospheric thickness is 125 km, decreasing to values of 80–
60 km beneath the Cenozoic rifted domain. These ‘lithospheric
steps’, which along the modelled profile have widths of c.
150 km, match well with lithospheric thickness values based on
global geochemical analyses (Poudjom-Djomani et al. 2001).
44
Discussion: comparison between the Møre and Vøring
margins
40
36
100
80
60
40
20
Møre Margin
OCB
0
Møre Basin
CL Caledonides
UC
30
Depth (km)
1009
Underplating
60
LC
Lithospheric Mantle
90
120
Asthenosphere
150
180
-100
0
100
200
300
400
Distance (km)
Fig. 3. Results of the present-day lithospheric structure modelling in the
Møre Margin. From top to bottom: (a) elevation; (b) Bouguer anomaly;
(c) geoid height; (d) surface heat flow. Bold continuous lines denote
calculated values. Grey strips in (a)– (c) and grey dots with bars and
wide strip in (d) denote measured values and associated scatter. (e)
Modelled geometry (see Fig. 4 for details). OCB, ocean–continent
boundary; CL, coastline; UC, upper crust; LC, lower crust.
lower crust (VP c. 8.5 km s1 ) at 230–300 km distance. The
velocity structure that delineates the first anomalous body was
recorded only by ESP L5 and is considered poorly constrained
by the authors. Neither ESP E40 nor E89 located some tens of
kilometres away from L5 implied such anomalous velocity
structure. The second anomalous body was recorded by ESP E88
and it is considered of good quality although its nature is unclear
(Olafsson et al. 1992). The projected position of this highvelocity lower crustal body onto our profile coincides with the
region where, according to our results, the lower crust thins out
and, therefore, the body could correspond to upper mantle rocks.
The main goals of this work were to obtain a continuous
lithospheric cross-section across the Møre Margin, from the
mainland to the deep oceanic domain, and to compare the
resulting crustal and lithospheric structure with that of the
adjacent Vøring Margin. To this end we used an integrated
modelling technique that combines several regional geophysical
observables (elevation, gravity, geoid and heat flow) with available seismic data. The interpolation or extrapolation of previous
seismic data by using geopotential fields, lithostatic equations
and thermal regime allowed us to define the major features of the
crust and the lithospheric mantle across the Møre Margin. This
approach is based on assumptions of local isostasy, thermal
steady state, and constant density beneath the level of compensation. The effects of possible departures from these assumptions
in the Northern North Atlantic region have been extensively
discussed by Fernàndez et al. (2004a), who applied the same
approach to the Vøring Margin.
One of the major differences between the Møre and the Vøring
margins is the absence in the Møre Margin of the crustal block
corresponding to the Trøndelag Platform, which makes the
geophysical signatures of both margins apparently very different
(see Fig. 2). However, the regional trends turn out to be similar
when plotting the observables and modelling results incorporating the gap corresponding to the Trøndelag Platform, as shown
in Figure 5. In this figure, the Møre and the Vøring margins
share a prominent crustal and lithospheric thinning from the
Norwegian Caledonian Belt to the deep oceanic domain but a
careful analysis of the geophysical observables reveals distinct
features around the OCB region and over the Caledonian
Mountains. The basin underneath the Trøndelag Platform was
developed during the Late Palaeozoic–Mid-Triassic rifting episode, which also affected, although to a lesser extent, the region
to the SE of the Møre Basin. Subsequent Late Jurassic–Early
Cretaceous rifting formed the Møre and Vøring basins, also
producing subsidence in the Trøndelag Platform. No later
tectonic activity is recorded in the Trøndelag Platform and
therefore the asymmetry between the Møre and the Vøring
margins is an inherited feature from the first post-Caledonian
rifting events.
The Vøring Marginal High region displays a shallower
bathymetry and lower amplitude Bouguer anomaly than in the
corresponding region of the Møre Margin (see Fig. 5 between
50 and þ100 km horizontal distance). These differences can be
explained by the larger thickness of oceanic crust and continental
1010
M . F E R NÀ N D E Z E T A L .
Fig. 4. Crustal structure along the Møre Margin profile as deduced from 2D modelling and available seismic data used to constrain the model. E46, L4,
etc., ESP data from Olafsson et al. (1992). Numbers indicate measured VP velocity (km s1 ).
underplating in the Vøring Margin. According to our results, the
maximum vertical thickness of underplated material beneath the
OCB in the Møre Margin is about 7–8 km (Fig. 4), whereas in
the Vøring Margin it reaches values of up to 15–16 km
(Fernàndez et al. 2004a; Fig. 5). These results are in agreement
with recent extensive OBS surveys, which show lateral variations
in underplating thickness beneath the Vøring Transform Margin
(e.g. Berndt et al. 2001; Mjelde et al. 2001, 2003). Berndt et al.
(2001) proposed that the spatial variations in melt production are
produced by lateral heat transport from the oceanic lithosphere in
the Møre Margin to the adjacent continental lithosphere of the
Vøring Margin and by local lithospheric thinning related to a
pull-apart setting generated at the NW segment of the transform
margin. Although the Møre transect runs parallel SW of the
Vøring Transform Margin, we suggest that the differences
relative to the Vøring transect are controlled by the same
mechanisms as proposed by Berndt et al. (2001).
It is worth noting that the crustal thickness in the oceanic
domain in the Møre Margin is mainly constrained by available
data from ESP E-45 and E46 (Olafsson et al. 1992; see Fig. 4),
and extended to the NW by geopotential modelling. Nevertheless, the Mohns Ridge is characterized by ultra-slow spreading
and thin oceanic crust of 4–5 km (Klingelhöfer et al. 2000).
Assuming that the low melting rate persisted since the decay of
the break-up thermal anomaly, i.e. from c. 50 Ma (e.g. Clift
1997), we should expect a Moho depth of about 10–11 km
beyond magnetic anomaly 24 instead of the 13–16 km predicted
by our model. Recent results from OBS modelling show that the
Moho depth is not greater than c. 10 km NW of the Møre
Marginal High (R. Mjelde, pers. comm.). Imposing a Moho
depth of 10 km in the oceanic domain would reduce the lithospheric thickness by about 15–20 km to keep the bathymetry,
although the gravity and the geoid signatures would differ
noticeably from observations. A way to reconcile our modelling
results with OBS modelling (R. Mjelde, pers. comm.) and very
low spreading rates is to consider that our layer 3 is mainly
composed of serpentinized mantle with P-wave velocities well
below 8 km s1 . This hypothesis has been proposed by Klingelhöfer et al. (2000) for the Mohns Ridge by comparing seismic
data and REE concentrations.
Another major difference is related to the geoid height and
average topography over the Caledonides, which show differences of 6–8 m and 250–350 m, respectively, between the Møre
and the Vøring transects (Fig. 5, between 450 and 600 km
horizontal distance). In consequence, the resulting lithospheric
thickness beneath this region is about 30 km thinner in the Møre
transect. This southward-directed lithospheric thinning is also
observed in deep seismic experiments and thermal models
carried out in the Fennoscandian Shield, which show that the
lithospheric thickness decreases from 150–200 km in the northern and central parts of the shield to values of 100 km in the
southern Baltic Sea (e.g. Ansorge et al. 1992; Balling 1995;
Gregersen et al. 2002). Furthermore, global thermal and chemical studies (e.g. Artemieva & Mooney 2001; Poudjom-Djomani
et al. 2001) indicate that the thickness of the continental
lithosphere generally decreases with age from .200 km in
Archaean cratons, to 200 50 km in early Proterozoic lithospheres (Vøring transect), and to about 140 40 km in mid- and
late Proterozoic cratons (Møre transect).
A question that deserves some discussion concerns the
differences found in the present-day lithospheric structure of the
Møre and Vøring basins and its significance in terms of
Cenozoic thermal evolution. The Møre and Vøring margins
followed a similar crustal evolution from the Early Jurassic to
the Late Cretaceous–Palaeocene, but with a slightly higher
stretching factor in the Møre Margin (e.g. Skogseid et al. 2000;
Gómez et al. 2004). As in the Vøring Margin, the lithospheric
thickness across the Møre Margin decreases from continental to
oceanic domain with a staircase geometry, the steps coinciding
with boundaries of major crustal tectonic units (see lithospheric
sections in Fig. 5, lower panels). The bathymetry of the oceanic
domain in both margins (3000–3200 m) is noticeably shallower
than that predicted by cooling models (c. 5000 m). Fernàndez et
al. (2004a) found that even after considering the buoyancy of the
sedimentary cover and the anomalously thick oceanic crust, the
Vøring Margin has a residual bathymetry of c. 600 m. This
anomalous bathymetry was interpreted by those workers as
produced either by a long-lived thermal anomaly related to the
early Cenozoic Icelandic plume or by mantle depletion related to
high mantle potential temperatures and partial melting during
break-up. The similarities in the oceanic crustal structure of the
two margins indicate that the residual bathymetry is not restricted
to the Vøring Margin but extends to the Møre Margin, thus
reinforcing the hypothesis of a deep-seated origin and very long
wavelength process. Despite the similar evolution of both
margins under the influence of a mantle plume, the lithospheric
thickness beneath the Møre Basin (125 km) is slightly greater
than beneath the Vøring Basin (115 km). In addition, the
D E E P S T RU C T U R E O F T H E M I D - N O RW E G I A N M A R G I N
Elevation (m)
NW
-100
1000
0
100
200
300
400
SE
OCB
0
-1000
Calculated Vøring
Observed Vøring
Observed Møre
Calculated Møre
-2000
-3000
B.A.(mGal)
500
200
References
0
Geoid (m)
48
44
40
S. H. F. (mW/m 2)
36
Observed Møre
100
Observed Vøring
80
60
40
20
OCB
30
Trøndelag P.
Vøring Basin
0
Depth (km)
We would like to thank R. Mjelde, R. Stephenson and T. Needham for
their thorough review and valuable comments and suggestions. R.
England and an anonymous reviewer revised an earlier short version of
the paper, and they are also thanked. This work has been supported by a
collaborative project between the Institute of Earth Sciences–CSIC of
Barcelona (Spain) and the NORSK-HYDRO Research Centre of Bergen
(Norway) and developed in the framework of the ‘Grup d’Estructura i
Processos Litosfèrics’ 2001 SGR 00339.
100
52
CL
Caledon.
UC
LC
Underplating
60
Lithospheric Mantle
90
120
Vøring Margin
150
Asthenosphere
180
OCB
0
CL Caledonides
Møre Basin
UC
30
Depth (km)
1011
Underplating
60
LC
Lithospheric Mantle
90
120
Møre Margin
Asthenosphere
150
180
-100
0
100
200
Distance (km)
300
400
500
Fig. 5. Results of the Møre Margin modelling and comparison with the
Vøring Margin. CL, coastline; UC, upper crust; LC, continental lower
crust; OCB, ocean–continent boundary (origin of the models). The gap
in the Møre profiles simulates the absent Trøndelag Platform structure.
calculated surface heat flow is about 5–15 mW m2 lower in the
Møre Basin, suggesting a faster thermal recovery and greater
degree of subsidence in the Møre Basin produced by a more
local process. The lateral heat transfer from the Møre lithosphere
towards the adjacent and thicker Vøring lithosphere proposed by
Berndt et al. (2001), together with the jump of the spreading
centre from the Aegir Ridge to the Mohns Ridge at 25 Ma
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Received 7 September 2004; revised typescript accepted 12 July 2005.
Scientific editing by Tim Needham