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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). 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