3-D density and magnetic crustal characterization of the southwestern Barents Shelf: implications...

Geophys. J. Int. (2011) 184, 1147–1166 doi: 10.1111/j.1365-246X.2010.04888.x

hysics

3-D density and magnetic crustal characterization of thesouthwestern Barents Shelf: implications for the offshoreprolongation of the Norwegian Caledonides

C. Barrere,1,2,∗ J. Ebbing1,2 and L. Gernigon1

1Geological Survey of Norway, 7491 Trondheim, Norway. E-mail: cecilebarrere@hotmail.com2Department of Petroleum Engineering and Applied Geophysics, Norwegian University of Sciences and Technology, 7491 Trondheim, Norway

Accepted 2010 November 10. Received 2010 November 9; in original form 2009 August 1

S U M M A R YThe presented study focuses on understanding the Barents Sea tectonic evolution. 3-D jointgravity and magnetic modelling of the southwestern Barents Shelf is based on a wealth ofoffshore seismic and onshore geological information. It allows characterizing the crust withrespect to its density and magnetic properties. The main outcomes of the study are (1) newinformation on key geological interfaces through the production of a new top basement mapand upper/lower crustal boundary and Moho maps. In addition, (2) a crustal units map basedon density and magnetic properties distribution is proposed and helps understanding of thetectonic evolution of the region. Finally, (3) the study has highlighted disparate basin evolutioneast and west of the Loppa high. To the east of the Loppa High, a combination of Timanianand Caledonian faults and weakness zones may have played an important role in the evolutionof the Mesozoic and Cenozoic sedimentary basins. To the west of the Loppa High, the basinalevolution seems mostly controlled by the reactivated Caledonian suture.

The integration of new interpretations leads to a new structural conception of the Caledonianorogen with a unique Caledonide branch propagating towards the north and a confirmationof Caledonian nappes emplaced asymmetrically in the western Barents Sea. The proposedgeometry is interpreted as linked to the palaeogeography of the Baltican Plate.

Key words: Gravity anomalies and Earth structure; Magnetic anomalies: modelling andinterpretation; Composition of the continental crust; Sedimentary basin processes.

1 I N T RO D U C T I O N

The southwestern Barents Shelf is situated to the north of theFinnmark region, which is the northernmost onshore area ofNorway. This part of the Norwegian shelf is tectonically complexand represents an ensemble of deep basins, basement highs andplatforms (Gabrielsen 1984; Gabrielsen et al. 1990) (Figs 1a andb). A large number of seismic profiles have been collected duringthe last decades south of 74◦N, but most of these seismic data donot allow imaging the top basement and below due to a lack of deepseismic penetration and the presence of salt and carbonates in thedeep basins. Skilbrei (1991, 1995) presented top basement mapscombining magnetic depth estimates and seismic profiles, whichuntil this study has been the best available compilation. Hitherto,the IKU seismic reflection data set has been the main resourcefor deep-crustal imaging (e.g. Gudlaugsson et al. 1987; Faleideet al. 1993; Gudlaugsson & Faleide 1994; Sanner 1995; Breivik

∗Now at: Beicip-Franlab, 232, Avenue Napoleon Bonaparte, P.O. Box21392502 Rueil Malmaison, France.

et al. 1998, 2005). More specifically, wide-angle data (Breivik et al.2002, 2003, 2005) and the IKU (the Norwegian Department ofContinental Shelf Research) reflectivity have previously been used(Ritzmann & Faleide 2007) to investigate the geometry and extentof the inferred prolongation of the Norwegian Caledonides acrossthe Barents Shelf.

Barrere et al. (2009) presented 2-D joint density and mag-netic modelling along the IKU A, B and C seismic profiles in-tegrated with the geological data and potential field maps thatwere available onshore and offshore. They presented a prelimi-nary basement unit map for the southwestern Barents Shelf. In thesouthwestern Barents Sea (Fig. 1), continental crust and a stripeof oceanic crust along the margin are present. An intermediatecrustal type may also be present along a sharp continent–oceantransition (COT), previously interpreted as a strike-slip systemalong the western Barents Sea margin (Ziegler 1988; Faleideet al. 1993).

Based on basement characterization, Barrere et al. (2009) sug-gested a new regional interpretation for the offshore prolonga-tion of the Norwegian Caledonides, which links the southern part

Geophysical Journal International

1148 C. Barrere, J. Ebbing and L. Gernigon

Figure 1. (a) Global location map. Barents Sea shelf and surrounding land masses: bathymetry-topography map. The study area for basement characterizationis the southwestern (SW) Barents Sea (69◦N–75◦N and 13◦E–30◦E). The black lines show the tectonic units as defined by the NPD (Norwegian PetroleumDirectorate). (b) Local location map. Tectonic units and main faults of the study area of the southwestern Barents Sea.

of the west Barents Shelf to an elbow-shaped Caledonian struc-ture propagating towards the north. In the present contribution wedescribe a new 3-D density/magnetic model, which was imple-mented to evaluate, enhance and extend the earlier interpretationregarding the distribution of crustal units and the offshore regionalgeology.

2 G E O L O G I C A L A N D T E C T O N I CS E T T I N G

The southwestern Barents Sea basement composition and structureis inherited from a complex geological history. The basins observedtoday originate from a succession of major rifting episodes, which

3-D joint modelling of the SW Barents Shelf 1149

occurred in the Late Palaeozoic and from the Late Jurassic to theEarly Cretaceous (Faleide et al. 1993). Most of the major rift basinson the Barents Shelf were formed during these episodes. A lastrifting episode in the latest Cretaceous-Palaeocene (Faleide et al.1996) led to continental breakup and onset of seafloor spreading inthe Norwegian Greenland Sea.

The basement in the Barents Sea is characterized by the diversityof the terranes involved in its composition. This diversity originatesfrom the successive orogenies that took place from the Palaeopro-terozoic to the Caledonian times. The neighbouring terranes to ourstudy area witness on this terranes assemblage.

2.1 Adjacent onshore geology to the study area

The outcropping and subcropping Fennoscandian (Baltic) Shieldcomprises several geological provinces. Six major units canbe distinguished (Gaal & Gorbatschev 1987): the Sveconorwe-gian (1.25–1 Ga), Svecofennian (1.9–1.8 Ga) and Lapland-Kola-Karelian (2.4–1.9 Ga) terranes constitute the substratum (Gee &Stephenson 2006) (Fig. 2). A belt of high magnetic granitoid, plu-tonic and extrusive rocks the Transscandinavian Igneous Belt (TIB)(1.8–1.6 Ga) occurs at the southwestern margin of the Svecofen-nides. The Caledonian fold belt (450–400 Ma) and sedimentaryrocks of the Palaeozoic platform cover the westernmost and east-ernmost parts of the Archaean to Palaeoproterozoic rocks, respec-tively. The various terranes of Lapland-Kola-Karelian block and theCaledonian fold belt are known to propagate offshore beneath theBarents Sea. The Lapland-Kola-Karelian block is characterized by acomplex association of geological terranes and structures of diverseorigin and age (Daly et al. 2006; Kostyuchenko et al. 2006). Theresult is a patchwork of greenstone belts and granitic and gneissiccomplexes, producing a set of characteristic magnetic and gravityanomalies in mainland Finmark (Olesen et al. 1990) and neighbour-ing areas of Russia and Finland (Kostyuchenko et al. 2006).

2.2 Post-Sveconorwegian tectonic events

Baltica’s northeastern margin is characterized by structures re-lated to the Late Neoproterozoic Timanian orogeny (Ivanova 2001;Roberts & Siedlecka 2002; Gee & Pease 2004; Roberts & Olovyan-ishnikov 2004; Siedlecka et al. 2004; Gee et al. 2006). On theVaranger Peninsula (Fig. 1a) the exposed part of the NW–SE-trending Timanide orogen occurs northeast of the dextral strike-slipTrollfjorden-Komagelva Fault Zone (TKFZ) (Roberts & Gee 1985;Roberts & Siedlecka 2002).

The Caledonian orogen formed as a result of thecontinent–continent collision between Laurentia and Baltica inSilurian to Early Devonian time, the so-called Scandian orogeny(Roberts & Gale 1978; Torsvik et al. 1996; Roberts 2003; Gee 2005;Gee et al. 2006). Onshore Norway, the stack of Scandian thrust-sheets emplaced onto the autochthonous Fennoscandian Shield arerecognized as four major groups of allochthons (Lower, Middle, Up-per and Uppermost) (Roberts 1983; Roberts & Gee 1985; Siedleckaet al. 2004; Gee 2005; Nystuen et al. 2008). The Lower and MiddleAllochthons consist of rocks derived from the Baltoscandian-marginshelf (Neoproterozoic to Silurian pericratonic deposits and conti-nental rise, respectively) and their underlying crystalline basement.Mafic intrusions characterize the uppermost units of the Middle Al-lochthon including even portion of nappes interpreted as belongingto the outermost part of the pericontinental Baltoscandian margin.The Upper Allochthon comprises thrust sheets of lithologies de-

Figure 2. Simplified subdivision of the Fennoscandian Shield (after Gaal& Gorbatschev 1987). The Fennoscandian Shield comprises six major ge-ological provinces: the substratum consists of the Sveconorwegian (1.25–1Ga), Svecofennian (1.9–1.8 Ga) and Lapland-Kola-Karelian (2.4–1.9 Ga)terranes. The Transscandinavian Igneous Belt (TIB) (1.8–1.6 Ga), occursalong the southwestern margin of the Svecofennides; this is an intrusivecomplex of mostly granitoid composition that is propagating northwardsunderneath the Caledonian nappes. The Caledonian fold belt (450–400 Ma)and the Palaeozoic platform cover occur in the westernmost and easternmostparts of the Fennoscandian shield, respectively.

rived from the Iapetus Ocean. Some others nappes of this grouphave Baltican affinities whereas some higher thrust sheets containfaunas of Laurentian origin. The Uppermost Allochthon derivesfrom the Laurentia margin and comprises shelf and slope rise suc-cessions, some ophiolites and major granitic batholiths (Stephens& Gee 1989; Gee 2005; Barnes et al. 2007; Roberts et al. 2007).

All along Norway, in many parts of the Caledonides there is ev-idence of late Scandian transverse and orogen-parallel extensionof the Caledonides (Roberts 1983; Hossack 1984; Andersen 1998;Braathen et al. 2002), interpreted as signs of late orogenic gravita-tional collapse with coeval rapid erosion of the mountains. Broaddetachment zones developed obliquely to the recorded Scandianthrust direction have been described (e.g. Braathen et al. 2002;Olesen et al. 2002; Osmundsen et al. 2002, 2003) and are asso-ciated with Devonian supra-detachment basins. Extensional col-lapse of the Caledonide orogen has been inferred to also have takenplace in the Barents Sea Caledonides (Gudlaugsson et al. 1998).Gudlaugsson et al. (1998) studied the Carboniferous–Permian rift-ing structures and suggested that weakness zones in the basement

were inherited from the Caledonian orogeny. They considered thatCarboniferous–Permian rift structures continued along the strike ofthe North Atlantic rift at least 600 km into the Barents Sea.

3 M E T H O D O L O G Y

We have modelled the crustal structure of the SW Barents Sea todetermine its main crustal characteristics. For this, we established a3-D forward modelling of potential field and integration of a wealthof seismic information.

3.1 3-D modelling

The IGMAS software (Gotze & Lahmeyer 1988) has been usedfor forward modelling of the potential field data. Within IGMASthe geometry is first defined along parallel vertical cross-sections(Fig. 3). In our model, the line spacing is ranging from 10 to 20 kmdepending on the complexity of the modelled structures. The geom-etry is automatically triangulated between the sections, thus definingthe 3-D geometry. The gravity and magnetic fields are then calcu-lated and the resulting field compared with the observed potentialfield. 3-D modelling requires constraints on the geometry (e.g. seis-mic) and petrophysical parameters and a large amount of additionaldata (e.g. well data, seismic horizons and profiles) was integratedto constrain our 3-D model. Note that the resolution of the regionalmodel (>10 km) is not high enough to allow an integration ofsmaller intrasedimentary features (e.g. salt domes).

To use absolute densities comparable with the petrophysicaldatabase in the modelling, a reference model has to be de-fined to model the Bouguer anomaly without an arbitrary shift.The densities in the model are defined with respect to referencedensities representing the ‘normal’ crustal column at the coast(Table 1).

Magnetization of crustal rocks is mainly related to the magnetitecontent in the rock. The Curie temperature of magnetite is 580 ◦Cand at this temperature rocks lose their ability to remain magne-tized. Assuming a normal thermal gradient, the Curie temperatureis located in the deep crust (e.g. Ebbing et al. 2009). Magnetic fieldcalculations require the definition of an external magnetic field. Thefixed magnetic field and the remanent field were modelled parallelto the induced magnetic field. We define the normal inducing mag-netic field with a constant field strength of 53 300 nT, and constantinclination of 79◦ and declination of 4.3◦.

4 DATA B A S E S

4.1 Potential field data

4.1.1 Bouguer anomaly

The Bouguer anomaly (Fig. 3a) was calculated from the freeair anomaly compilation by Skilbrei et al. (2000). The ap-plied bathymetric data are based on the International Bathymet-ric Chart of the Oceans (IBCAO) (Jakobsson et al. 2000) com-bined with the GTOPO30 grid (onshore data) (http://edc.usgs.gov/products/elevation/gtopo30/dem_img.html), with resolutionsof 2.5 and 1 km, respectively.

A simple Bouguer correction at sea was carried out using a bathy-metric grid with a resolution of 2 km and reduction densities of2200 kg m−3 and 2670 kg m−3 for offshore and onshore, respec-tively.

4.1.2 Magnetic anomaly

Aeromagnetic data are available from a magnetic compilation(Fig. 3b) of the western Barents Sea by Olesen et al. (2006). Thedata set is compiled from reprocessed aeromagnetic surveys andline spacing ranges from 0.5 to 2.5 km over mainland Norway andfrom 3 to 8 km over the continental shelf.

4.2 Petrophysical data (Table 2)

In our model the density values from the gravity modelling alongIKU profiles A, B and C by Barrere et al. (2009) were used as initialparameters. Densities of the sedimentary layers were based on welldata (Tsikalas 1992) and published tables based on velocity-densityrelationships of sedimentary units obtained from the seismic re-fraction and reflection/gravity studies. Bedrock densities are basedon direct onshore measurements (Olesen et al. 1990; Galitchaninaet al. 1995); deep-crustal densities are based on published valuesfrom refraction data models (Breivik et al. 1998, 2002, 2003, 2005;Mjelde et al. 2002) inferred from velocity-density relationships andgravity modelling. The errors from the velocity-density relations onthese densities are of the order of ±50 kg m−3 and ±100 kg m−3

(Olesen et al. 1990) for the upper-crustal layers and deep-crustallayers, respectively.

In the upper crust, the magnetic sources mainly relate to Cale-donian and Archaean to Palaeoproterozoic rocks and mafic intru-sions within the sedimentary basins (Am 1975; Olesen et al. 1990;Skilbrei 1995; Barrere et al. 2009). For the magnetic field, the mag-netic susceptibility and remanence from the magnetic modellingalong IKU profiles A, B and C by Barrere et al. (2009) were used asinitial parameters. Those values were derived from onshore samples(Troms and Finnmark regions) (Olesen et al. 1990; Slagstad et al.2008). Q-ratios were applied according to the samples and mod-ified during the modelling. For consistency with published worksrelated to the long-wavelength terrestrial magnetic anomaly, we seta homogeneous and low Q-ratio and magnetic susceptibility for thelower crust and mantle with Q = 0.4 and magnetic susceptibility =1000.10−5 (SI). Sedimentary rocks are set to 30.10−5 (SI) as theyhave very low magnetic properties in comparison with the basementrocks (e.g. Olesen et al. 1990).

4.3 Geometric constraints

To constrain the sedimentary layers we obtained access to three,industrial, depth-converted seismic horizons: top Tertiary, baseCretaceous and top Permian. These horizons were produced bydepth-conversion of seismic horizons using regional velocity lawscalibrated by well data. The sedimentary rocks were thus subdi-vided into four principal units: Quaternary, Neogene-Paleogene-Cretaceous, Jurassic-Triassic and Palaeozoic. In the southwesternBarents region, six wells (black crosses, Fig. 2) reach the top base-ment; they were used to calibrate the modelled top basement andcheck the reliability of the depth-converted seismic horizons.

We set up our initial model using the Barents50 model ofRitzmann et al. (2007) and the top basement reported by Skilbrei(1991, 1995). The former describes a crustal velocity model witha resolution of 50 km, which also provides information alongall available regional seismic profiles with 25 km sampling. Thetop basement of the Barents50 model is roughly similar to thedepth to magnetic basement established by the magnetic interpre-tation of Skilbrei (1991, 1995), which provides a locally higherresolution.

Figure 3. (a) Bouguer anomaly and (b) total magnetic field (reduced to the pole) with the main faults superimposed. The white lines indicate the location ofthe vertical planes defining the 3-D model; also represented: the IKU deep-seismic reflection data (thick black lines), the wide-angle data (thick grey lines)and the deep exploration wells reaching the basement (black crosses). Information available concerning exploration wells 7128/6–1, 7120/1–1, 7120/2–1,7120/12–1, 7226/11–1 and 7128/4–1 can be found at: http://www.npd.no/engelsk/cwi/pbl/en/index.htm. The modelling parameters are constrained by theonshore petrophysical database; the measured samples are located by white dots.

The IKU deep-seismic reflection profiles and the seismic refrac-tion data (Breivik et al. 2002, 2003, 2005; Mjelde et al. 2002) wereused to refine the crustal structure of our model. The boundarybetween the upper and lower crust varies between 20 and 22 km

depending on the reflectivity along the IKU profiles and the seismicvelocities from refraction seismic lines. In addition, a recent OceanBottom Seismometer (OBS) profile (Clark et al. 2009) has beenincluded in the final step of the modelling.

Table 1. Reference model description.

Depth to boundaries (km) Density (kg m−3)

Upper crust 0–15 2750Lower crust 15–32 2950Mantle 32–120 3265

5 M O D E L L I N G R E S U LT S

5.1 Comparison between observed and modelledpotential fields

5.1.1 Density modelling

The final differences between the measured and modelled gravityanomalies have a standard deviation of less than ±8.0 mGal. Thisvalue is slightly higher than the accuracy for the gravity data, butthe remaining mismatch can be explained largely as relating to localstructures below the resolution of our model (e.g. salt domes). Theshort-wavelength anomalies (<10 km) onshore have not been mod-elled and consequently create local deviations from the modelledBouguer anomalies.

5.1.2 Magnetic modelling

The observed and modelled magnetic anomalies show a compara-ble general pattern. The anomalies linked to basement topography(wavelengths: 100–200 km) are relatively well modelled comparedto the short-wavelength (<100 km) anomalies linked to intrabase-ment magnetic sources and/or shallow magnetic sources. Becauseof our simplified settings, the magnetic modelling represents thegeneral magnetic gradients but not the absolute amplitudes of themagnetic field.

5.2 Density and magnetic properties

Density and magnetic modelling is used to test different geody-namic and geotectonic concepts that would imply a certain densityconfiguration. Figs 4 and 5 show the set-up of the model and themain 3-D horizons. In Fig. 5, the profiles show a good regional fit,but local deviations are observed, especially in magnetic modelling.

The model densities and magnetic properties are summarizedin Table 3. The modelled values are used to distinguish differentbasement units, the spatial extensions of which are presented inFig. 4(A).

5.2.1 Modelled densities (Table 3)

On the Bjarmeland Platform (Fig. 1), the densities are slightlyhigher than the 2750 kg m−3 average values usually consideredfor the Fennoscandian Shield basement (BAS0) (Galitchanina et al.1995). Also, to produce a Moho depth compatible with the seismicMoho (Ritzmann et al. 2007), a lower crustal body (LCB) had tobe introduced over the central part of the SW Barents Shelf. TheLCB’s 3000 kg m−3 density value contrasts with the surrounding2950 kg m−3 density of the regular lower crust density.

Along the western margin, the basement units MB1, MB2 andMB3 are modelled with density values around 2850 kg m−3. Locally,blocks (MI) of 2900 kg m−3 density were introduced in the model.

5.2.2 Modelled susceptibilities (Table 3)

The tops of the magnetic sources are assumed to be the top basementand the top of the oceanic basalts obtained by density modelling, forthe continental and oceanic crust, respectively. Due to the resolutionof the model and lack of constraining data, no intrabasement mag-netic sources are distinguished. Therefore, the resulting magneticmodelling highlights the main changes in magnetic properties ofthe upper crust.

The final model shows a variation of the upper-crustal magneticsusceptibility values from 500.10−5 (SI) to 5000.10−5 (SI).

5.3 3-D crustal configuration

Our 3-D model allows us to define and present key elements of thesouthwestern Barents Sea crustal architecture. As a result we havecompiled maps of the depth to Moho (Fig. 4B), the top basement(Fig. 4C) and crystalline crust thickness (Fig. 4D) extracted fromthe 3-D model.

5.3.1 Depth to the crust-mantle boundary (Moho)

The Moho (Fig. 4B) is, in general, associated with a density con-trast of 350 kg m−3 between the lower crust and the upper mantle.Only across the LCB is this contrast slightly smaller (Fig. 4A). Theresulting Moho geometry reflects the Moho of the Barents50 modelat the continental margin and onshore. Over most of the margin, theMoho is similar to the Barents50 model, but varies significantly inthe trend of Moho undulations. Along the IKU profiles (Fig. 2) theMoho depths (Fig. 4B) are essentially the same with the exceptionof IKU-B where a new OBS interpretation suggests a deeper Mohoand provides an update of the Barents50 model (Clark et al. 2009).

The Moho (Fig. 4B) undulates over the continental shelf betweendepths of 20 and 35 km. In the central study area, an E–W shal-lowing correlates with the location of basins and highs. We alsoobserve a steep deepening of the Moho, from 20 to 30 km, be-tween the COT and the Ringvassøy-Loppa and Bjørnøyrenna FaultComplexes. Interestingly, the depth to the Moho is in the order of30–32.5 km below the Bjarmeland Platform and northwards andshows a gradual shallowing from north to south offshore.

5.3.2 Depth to top basement

The density contrast between the crystalline basement and Palaeo-zoic sedimentary rocks is at least 50 kg m−3. In our model, sedi-mentary rocks are considered to be relatively non-magnetic and thetop basement (Fig. 4C) was regarded as the upper limit of the mag-netic sources. Over large parts of the shelf, this interface is locatedat depths between 4 and 8 km. The shallowest crystalline basement(<2 km) occurs at the Gardarbanken High, north of the StappenHigh and on Bjørnøya, where it crops out (Fig. 1).

In the northern part of the Nordkapp Basin, where the depth tocrystalline basement reaches 12 km, there is a deep graben. Muchdeeper basins to the west of the Loppa High and south of the StappenHigh were modelled with a depth to basement locally reaching>15 km.

5.3.3 High-density LCB

We have modelled the LCB in 3-D (Fig. 4A), a body previouslyinterpreted in the deep lower crust at the western rim of the Loppa

(ms−

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30–3

Figure 4. (A) Crustal units are established by grouping blocks of comparable density and magnetic properties. LCB, lower crust high-density body; UCB,upper crust high-density body; BAS0, Onshore Fennoscandian Shield; BAS1, Archaean to Palaeoproterozoic rocks affected by the Caledonian orogeny;BAS2, Archaean to Palaeoproterozoic rocks weakly affected by the Caledonian orogeny, clearly with lower magnetization than the type BAS1; MB1, high-density/medium-magnetic crust; MB2, Vestbakken volcanic province; MB3, high-density/high-magnetic crust. (B) New Moho map from our 3-D model. (C)Depth to top basement taken from our 3-D model. The depth to top basement coincides over most of the area with the depth to the top of the Caledoniannappes. (D) Crystalline basment thickness and simplified structural map (solid grey lines). The crustal thickness map is computed from the modelled depthstop basement and Moho. The map shows the intense crustal thinning to the west of the alignment of the Ringvassøy-Loppa and Bjørnøyrenna Fault Complexes.(E) Crustal thinning factor map. The map provides a quantitative estimation of the thinning intensity and a qualitative estimation of the main directions ofextension. (F) Five ‘crustal zones’ consisting of one or several basement units are distinguished: (1) onshore zone, (2) offshore coastal zone, (3) marginal zone,(4) central zone and (5) zone covering the eastern and northern areas.

Figure 5. Location map: integrated modelling cross sections (A–D) (red solid lines) parallel to existing seismic data: (A) P3 and IKU D, (B) IKU A, (C) IKUF and (D) IKU B. Cross sections: the two top frames show the modelled (dash line) and observed (solid line) Bouguer (red) and magnetic (blue) anomalies.The bottom frames display the model; black numbers represent densities in kg m−3 and basement units are interpreted. The model is overlain by the reflectivityof the IKU data. The triangles represent Vp velocities extracted from the Barent50 model (Ritzmann et al. 2007). In panels (A) the fourth frame presents acomparison of our model with the model by Breivik et al. (2002) along P3. In panels (D), the model does not respect the Barents50 constraints (Vp velocities)but rather a recent seismic refraction data set named Petrobar-07 (Clark et al. 2009).

High along profile IKU B (Barrere et al. 2009). The presence of thisdeep buried high-density body was necessary to model correctly theBouguer high at the location of the Loppa High. The 3-D modellingallowed us to evaluate the northward and southward extension ofthis LCB (Fig. 4A), but the lack of good seismic constraints did notallow us to determine its precised thickness.

5.3.4 High-density bodies in the upper crust (UCB and MI)

A basement strip of high-densities (UCB) about 2800 kg m−3 andmagnetic susceptibility of 1000.10−5 (SI) has been modelled alongmost of the coast of Finnmark (Fig. 4A, UCB). Local high-densitybodies (Fig. 4A, MI) with a density of 2900 kg m−3 and a magneticsusceptibility of up to 2500.10−5 (SI) have also been modelledbetween two profiles at the Norsel High and northeast of the LoppaHigh.

5.3.5 Crystalline crust thickness and thinning factor map

The crystalline basement thickness map (Fig. 4D) is computed fromthe difference between the modelled Moho and top basement. The20 km isopach contour separates a narrow thin crust (10–20 km) be-tween the continental margin and the alignment of the Ringvassøy-Loppa and Bjørnøyrenna Fault Complexes from a large eastern areawith a crustal thickness between 20 and 28 km.

An estimation of the apparent crustal thinning through a crustalthinning factor (β-factor) (McKenzie 1978) was computed (Fig. 4E)from the crustal thickness grid inferred from our 3-D model. The33 km thickness of the Bjarmeland Platform is considered as thereference crystalline basement thickness before basin formation.

βfactor = 33/crystalline basement thickness. (1)

Although the calculated β-factors map integrates the superposedsignal of several tectonic phases, the map is used to estimate thelocation and intensity of extension with respect to what is esti-mated to be the untouched crustal thickness. β-factors greater thanor equal to 2 are computed west of the Ringvassøy-Loppa andBjørnøyrenna Fault Complexes (at the location of SørvestsnagetBasin >3; Harstad Basin >3; Tromsø Basin 2 to 3 and BjørnøyaBasin 0.5 to 3.5). The North Nordkapp Basin shows a maximumβ-value of 2.5 and in the Hammerfest Basin β-values <1.5. Apartfrom along the COT, the maximum crustal thinning seems to followthe trend of two fault alignments: (1) the ENE–WSW alignment ofthe Finnmark, Masøy and Thor Iversen Fault Complexes and (2) theN–S alignment of the Ringvassøy Loppa and Bjørnøyrenna FaultComplexes.

In addition to the strong crustal thinning along the margin, thezone between 74◦N and the Finnmark and Masøy Fault Complexesshows β-values of about 1.5. One can observe that this area corre-lates with a shallower Moho (30–32 km) compared to the platformareas. Towards the east the crustal thickness increases to 32 km,

Figure 5. (Continued.)

and is clearly associated with the transition from the Eastern to theWestern Barents Sea.

6 I N T E R P R E TAT I O N S A N DD I S C U S S I O N

6.1 Comparison with previous top basement estimates

In Fig. 6, the new top basement model is compared with the compi-lations by Ritzmann et al. (2007, Fig. 6A) and the depth to magneticbasement maps published by Skilbrei (1991, 1995, Fig. 6B). Northand east of the Loppa High, differences greater than 5 km are ob-served, where the low-amplitude magnetic anomaly and lack ofseismic data prevented Skilbrei (1991, 1995) from obtaining mag-netic depth estimates. North of 74◦N our model shows a shallower

top basement than Skilbrei (1991, 1995). Here, the low amplitudemagnetic anomalies have been interpreted by him as a consequenceof a deepening of the top basement. In our model, the magneticsusceptibility in the basement changes to lower values, which con-sequently leads to a shallower top basement.

The Barents50 model integrates the refraction data north of 74◦Nand we expected the correlation between our modelled top base-ment and the Barents50 top basement to be reasonable. However,the difference map between the two (Fig. 6A) shows a signifi-cant underestimation (i.e. 6–10 km) of the top basement in theBarents50 model despite seismic constraints. The misfit comesfrom a different crystalline basement density definition. Indeed,comparing the seismic refraction model and our geological model(Fig. 5A), it appears that a layer with a density of 2750 kg m−3 is in-terpreted as sedimentary rocks in the refraction model. In our model,

sedimentary rocks have densities <2750 kg m−3 while crystallinerocks have densities greater than 2750 kg m−3. From the densitytables established by Tsikalas (1992) for the sedimentary rock sam-ples and from measurements of onshore basement samples (Olesenet al. 1990; Galitchanina et al. 1995), we regard a density higherthan 2750 kg m−3 to be more appropriate for basement rocks.

6.2 Interpretation of the crustal units map

The densities allow us to distinguish different basement units butresolve the Caledonian nappes only in general terms. In fact, exceptfor exotic terranes the Norwegian Caledonian nappes correspond toa low-magnetic basement on top of an Archaean to Palaeoprotero-zoic basement with higher magnetization properties (Olesen et al.1990). The northward extension of the Caledonian nappes is basedon the assumption of the offshore propagation of the nappes thatoccur in northern Norway (Am 1975; Olesen et al. 1990; Skilbrei1995; Siedlecka & Roberts 1996; Gernigon et al. 2007). Estima-

tions of their extension and thicknesses from our model are difficultas the density and magnetization contrasts between the nappes andthe Archaean to Palaeoproterozoic basement are low.

After interpretation and correlation between basement units ofcompatible density and magnetic properties, five crustal zones con-sisting of one or several units (Fig. 4F) have been distinguished:(1) an onshore zone, (2) an offshore coastal zone, (3) a zone alongthe COT, (4) a central zone and (5) a zone covering the eastern andnorthern parts of the study area.

6.2.1 Onshore zone

For the BAS0 onshore (Fig. 4A), the Fennoscandian Shield wasdivided into two bodies, one of 2750 kg m−3 density interpretedas Archaean to Palaeoproterozoic, high-grade metamorphic rocks(potential granulites), and the second with a slightly lower density(2700 kg m−3) which is interpreted as lower grade metamorphicrocks such as Archaean to Palaeoproterozoic granitic gneisses.

6.2.2 Coastal zone

Offshore, along the coast, the high-density upper crustal body(UCB) (Figs 4 and 5D) with intermediate magnetic properties isconsidered to be related to rocks within the Middle and Upper Al-

lochthons intruded by major mafic-ultramafic, plutonic complexessimilar to the onshore Vendian-age (570–560 Ma) Seiland IgneousProvince (Roberts et al. 2006) and the Early Silurian, HonningsvagIgneous Complex (Robins 1998; Roberts et al. 2003; Corfu et al.2006). In compliance with the onshore observations and the

Table 3. Modelling parameters: the different crustal units are defined by a combination of petrophysical values obtained by density and magnetic modelling.The association of neighbour crustal units defines the zones geologically interpreted.

Density (kg m−3) Magnetic properties

Q-ratioSusceptibility (.10−5

SI) max. min.

Mantle Continentalmantle

3300 0.4 800

Oceanicmantle

Lower crust Standardlower crust

2950 0.4 800

LCB (lowercrustal body)

3000 0.4 800

Oceanic crust Basalt 2900 1 2000Upper crust Onshore zone BAS1 onshore

FennoscandianShield

2700–2750 0.5 1500 1000

CN Caledoniannappes

2750 2 500

Coastal zone UCB high-densitybody

2800 0.5 1000

Loppa Highzone

BAS1 Loppa High(south & west)

2750 2800 0.5 4500

BAS1 Loppa High(east & north)

BAS1 StappenHigh (south)

COT zone MB1 Hornsundarea SørvestnagetBasin

2850 2880 0.5 2500 1000

MB3 HarstadBasin

2860 1–0.5 5000 3000

MB2 Vestbakkenvolcanic province

2800 0.5 3000

Eastern zones BAS2 north ofNordkapp BasinBAS2 south ofNordkapp Basin

2790 2770 0.5 2000 1000

MI (Norsel High,N-E Loppa High)

2900 0.6 2500

Northernzones

BAS2 StappenHigh north centralarea

2750 0.5 3000 1000

Sedimentary rocks Quaternary 2300 0.3 30Neogene-Palaeogene-Cretaceous

2450 0.3 30

Jurassic-Triassic

2550 0.3 30

Palaeozoic 2600 0.3 30

samples of Caledonian nappes samples taken from drillcores(Slagstad et al. 2008), we have modelled a Caledonian nappes bodyat the top of the UCB.

6.2.3 External margin

In the western part of the study area, four bodies are distinguished.They correlate with distinctive structural elements: (1) the HarstadBasin, (2) the Vestbakken Volcanic Province, (3) the SørvestsnagetBasin and (4) the Hornsund Area west of the Stappen High. The fourbodies have high densities of around 2850 kg m−3 and very variablemagnetic susceptibilities from 1000.10−5 to 5000.10−5 (SI). Thegood correlation between basement units and tectonic units (i.e.

basin or high) reflects the strong relationships between basementcomposition and crustal architecture.

Over the Sørvestsnaget Basin, the Bouguer anomaly high andthe low magnetic signature may be comparable with a ‘quiet zone’that has been described from the vicinity of some margins (Gunn1997). This ‘quiet zone’ could be interpreted either as extremelythinned crust or as attenuated crust with an intermediate characterbetween true continental and true oceanic crust that developed closeto the COT. Alternatively, it could possibly be due to a specificchronostratigraphic period of reverse polarity. Whatever the case,both a better seismic imaging and a more focused study of theSørvestnaget Basin are necessary to understand this very complexarea.

Figure 6. Difference map between our new top basement estimates (Fig. 4C)and the top basement (A) from the Barents50 model (Ritzmann et al. 2007)and (B) from the basement grid based on estimates of depth to magneticbasement. (Skilbrei 1991, 1995).

6.2.4 Central zone

This elongate zone encompasses the Loppa High, the BjørnøyaBasin and the southern part of the Stappen High. Several units withthe same density value (2750 kg m−3) and high susceptibility weredistinguished in the upper crust. Compared to the onshore geology,the relatively high susceptibility is interpreted as indicating a crustconsisting of magnetic gneisses comparable to the ones mappedand sampled onshore Norway (Olesen et al. 1990). They are heregrouped under the label BAS1.

In the lower crust, a high-density body (LCB) of 3000 kg m−3 ismodelled to the west of the Loppa High (Figs 4 and 5D). The mod-elling indicates its approximate extension along the Ringvassøy-Loppa and Bjørnøyrenna Fault Complexes. Locally, for example at180 km, it reveals the existence of a shallowing of the upper/lowercrustal boundary. The elongation of the high-density body sug-gests a close genetic link to the development of these major faults.It suggests that the crustal thinning was accommodated along theRingvassøy Loppa and/or the Bjørnøyrenna Fault Complexes in amanner comparable to the major detachments documented onshore(Braathen et al. 2002; Osmundsen et al. 2002, 2003) and offshoreNorway (Olesen et al. 2002). In addition to changes in the reflectiv-ity and density, the LCB modelled along profile IKU-F correlateswith a steep jump in Moho depth (Ritzmann & Faleide 2007) belowthe central Loppa High (Fig. 5C, 85 km). The structural and geo-physical characteristics of this LCB strengthen our interpretation ofit as a core complex (Barrere et al. 2009) but better seismic imagingis needed to understand how the structures are linked to each other.

6.2.5 Eastern and northern zones

East and north of the Loppa High the upper crust (BAS2) consistsof two bodies that are different from the upper crust type BAS1.On a regional scale, the BAS1/BAS2 (Figs 4A and 5) boundaryclearly separates a northeastern zone of platforms from a deeplyrifted southwestern zone. The BAS2 crust appears to have a slightlylower magnetic susceptibility (<3500.10−5 SI) to the east and northof the Loppa High and a little higher density (2790 kg m−3) in thenorthern areas of the study area.

In the East Barents Sea, the NW–SE striking trends have beeninterpreted as related to Timanian structures formed in Late Neo-proterozoic times (Ivanova 2001), but the northwestern limit of theTimanides, as well as the interactions between Timanian and Cale-donian structures remains unclear. Although the western boundaryof the BAS2 crustal unit (Fig. 4A) is schematic in its definition ofthe geometry along the vertical sections, the seismic profiles P3(Fig. 5A) and IKU A (Fig. 5B) confirm the presence of both a re-flectivity change and a possible structural boundary coinciding withcontrasting density/magnetization values. Local basement units (MIbodies, Figs 4A and 5B) are interpreted as mafic intrusions; theycould be sheets emplaced between the Caledonian nappes or bodieslinked to the formation of the Mesozoic basins.

6.3 Interpretation of the crustal thinning factor map

Breivik et al. (1998) showed a previous crustal thinning factor maphighlighting the complexity of the β-ratio pattern over the Tromsø,Bjørnøya and Sørvestsnaget basins, which led them to the theoryof a margin formed by continental transform faulting rather thanby rifting. Our new map (Fig. 4E) shows the composite patternof the β-ratio over the entire southwestern Barents Sea. High β-factors and an extension mostly N–S to NNE–SSW are confirmedfor the basins initiated in Palaeozoic and Cenozoic times along themargin and a E–W to ENE–WSW extension and lower β-factorsare mapped for the North Nordkapp and Hammerfest basins thatwere initiated in Palaeozoic times (Rønnevik & Jacobsen 1984;Gudlaugsson et al. 1987, 1998; Faleide et al. 1991, 1993, 1996;Breivik et al. 1998). The trends of the extension, as well as theintensity of crustal thinning, do not correlate with the ages of thebasins. This mismatch is an argument in favour of pre-existingweakness zones locally controlling the development of the basin

architecture. It may also be due to interplay of successive, complex,crustal thinning episodes.

6.4 The Laurentian/Baltican suture

No evidence of a suture between Laurentia and Baltica has beenobserved on Svalbard and the Svalbard Caledonian terranes are rec-ognized as the northerly continuation of the Caledonides of easternGreenland (Gee et al. 1995; Witt-Nilsson et al. 1998). In addition,all of Svalbard’s terranes from west to east are generally consideredto be Laurentian (Fortey 1975; Gee & Tebenkov 2004; Cocks &Torsvik 2005; Gee 2005; Torsvik & Cocks 2005), and since theBillefjorden Fault Zone N–S strike-slip system (n◦1, Fig. 7) oc-curred between two Laurentian-affinity terranes we disagree withthe interpretation of a Caledonian suture along the BillefjordenFault Zone as suggested by Ritzmann & Faleide (2007). Further-more, recent studies on Nordaustlandet (eastern Svalbard) (n◦2,Fig. 7) (Tebenkov et al. 2002; Johansson et al. 2004; Johanssonet al. 2005) have reported an increasing metamorphic gradient andintensity of deformation from west to east (Tebenkov et al. 2002).High-grade complexes with widespread migmatization have provento be Caledonian, high-temperature, low-pressure terranes (Harland1997) and Caledonian migmatization has been documented as farnortheast as Kvitøya (n◦3, Fig. 7) (Gee 2004).

For these reasons, we placed the Caledonian suture between Nor-daustlandet and Franz Josef Land, in agreement with studies byGee et al. (2006) and Mazur et al. (2009) (Figs 1a and 7). Inaddition, we observe a NNE–SSW alignment of strongly focusedmagnetic anomalies correlating with positive Bouguer anomalieseast of Svalbard (n◦4, Fig. 7). Despite the fact that these focusedmagnetic anomalies most likely relate to Late Mesozoic intrusionslinked to the significant magmatic event (Grogan et al. 1998) at theorigin of a large igneous province (Maher 2001), we consider theseintrusions to be possibly controlled in depth by an older weaknesszone which may coincide with the Caledonian suture.

Consequently, we believed the offshore path of the Caledoniansuture (n◦5, Fig. 7B) to occur along the outer part of Lofoten, west ofthe Hammerfest Basin, the Loppa High and the Gardarbanken High(Figs 1a, 7B and C) and then to propagate north-northeastwardstowards Kvitøya (n◦3, Fig. 7). The N–S alignment of the RingvassøyLoppa, Bjørnøyrenna and Fingerdjupet Fault Complexes and theproposed link to the Billefjorden Fault Zone on Svalbard could beconsequently interpreted as associated with a deep-seated weaknesszone (Skilbrei 1991; Barrere et al. 2009) instead of a suture (Breiviket al. 2005; Ritzmann & Faleide 2007).

The location of the Caledonian suture between the Nordaust-landet and Franz Josef Land (Gee et al. 2006) (Fig. 1a and n◦5, Figs7B and C) would involve the existence of Caledonian thrust sheetsin this area. Because the Svalbard Caledonian terranes are directnortherly continuations of the Caledonides of East Greenland (Gee& Tebenkov 2004; Higgins et al. 2004), the westward thrusting ofthe Nordaustlandet Terrane (Gee 2005) is in agreement with theexpected general geometry. If the Caledonian suture lies east ofSvalbard and if no other mega structure separates the suture and theBFZ it is most likely that the BFZ originated from mechanisms ofterranes extrusion linked to an oblique collision of Laurentia andBaltica in that region.

6.5 Interaction between Timanian and Caledonianstructures

A branch of the Caledonian thrust belt propagating towards thenorth from NE Finnmark with nappes emplaced asymmetrically in

the western Barents Sea (n◦6, Fig. 7) was first proposed by Barrereet al. (2009) and today is supported by the results of the presentedstudy.

We had a close look at empirical and numerical modelling offold-thrust belt geometry (Macedo & Marshak 1999) testing the re-lationship between thrust geometry and geological setting in whichthe fold belt formed. These works show that the hypothetic geome-try combining a unique northward branch and asymmetric nappesemplacement would imply an oblique convergent model and thepresence of an asymmetric basin at pre-Caledonian times in thesouthwestern Barents Sea. In this concept of oblique convergentmodel, a strike-slip fault forms parallel to the direction of back-drop movement at the vicinity of the fold belt long limb forelandboundary (Macedo & Marshak 1999). Following this model, weconsider the distribution of the nappes as inherited from the Balticaplate geometry. We consider that the geometry of the Caledonianthrusts could be due to the existence of an asymmetric Neoprotero-zoic basin or at least a relatively low area through the southwesternBarents Sea with respect to the NW–SE-trending Timanian struc-tures. In addition, we found several elements arguing in favour ofthe structural schema modelled:

(1) First, the presence of Neoproterozoic pericratonic depositsand deep-water basinal successions in the Parauchthon and LowerAllochthons in Finnmark (Figs 1 and 7) (Roberts & Siedlecka 2002;Siedlecka et al. 2004; Nystuen et al. 2008) have demonstrated theexistence of a Neoproterozoic basin along the northeastern Tima-nian margin.

(2) Also, on the Varanger Peninsula, the ENE–WSW-trendingfrontal Caledonian thrust is mapped overriding the TKFZ (n◦9,Fig. 7) and truncates the NW–SE-trending Timanian structuresthus demonstrating interactions between Timanian and Caledonianstructures.

(3) In addition, at present, the major NW-trending BAS1/BAS2boundary modelled in the centre of the southwestern Barents Shelf(n◦7, Figs 7B and C) challenges the concept of a Caledonian branchpropagating north-eastward (Breivik et al. 2002) or a collision fanwidening towards the NE (Ritzmann & Faleide 2007). This bound-ary may be interpreted as the approximate position of the Protero-zoic basin border. Due to its position, the immediate surroundingsof the basin border were likely of Timanian nature. This boundarycould be interpreted as a contact between the Caledonian collisionprism and Baltica terranes accreted at Late Proterozoic implying apropagation of the Timanides northwestward until the Caledoniansuture (n◦7, Fig. 7).

(4) To finish the picture, the alignment of the Finnmark (FFC),Masøy (MFC) and Thor Iversen Fault Complexes (TIFC) (Fig. 1B)could be inherited from a strike-slip fault dextral (n◦8, Fig. 7) de-veloped parallel to the direction of oblique convergence and thiselement would agree with the modelled prisms.

Previous works highlighted the fact that the trends of the offshoreprolongation of the Caledonian thrusts correlate with the segmenta-tion of the Nordkapp Basin, suggesting a direct link between ancientCaledonian weakness zones trending NNW–SSE and changes in theshape of the Nordkapp Basin (Gernigon et al. 2007; Barrere et al.2009). The new 3-D modelling suggests that the Nordkapp Basindeveloped within both the BAS1 and the BAS2 crustal units. Thus,the Nordkapp Basin appears located at the meeting point of theTimanian and Caledonian trends, which may have facilitated rift-ing. In that concept, the NW–SE magnetic trends east of the LoppaHigh (Fig. 7B) have an uncertain origin; (1) they may be linkedto susceptibility contrasts between Caledonian nappes or (2) to

Figure 7. Map A shows results of this study. Map B presents locations and alignments commented in the paragraph 6.4. Map C puts together the interpretedlineaments drawing the new structural concept. (A) The map shows interesting correlations between the known structural elements, the modelled crustalthickness and the interpreted basement units. (B) The map presents the interpreted structural lineaments after integration of our 3-D density/magnetic modelwith the geophysical and geological information available on top of the magnetic data modelled. We show the suggested locations of a Caledonian suture,the offshore prolongation of the Caledonian thrusts, main Caledonian weakness zones as well as the delimitation of a Palaeoproterozoic basin part of theBaltica Plate. The map also shows that the magnetic anomalies are closely related to both basement lithology and structural elements. For further explanationand reference to numbers, see text. (C) The structural map shows the interpreted lineaments and offshore prolongations of the Caledonian nappes. This mapsummarizes our regional geological interpretation presenting our vision of the distribution of the Caledonian orogenic prism and associated weakness zones.Our study also suggests the existence of a palaeoproterozoic basin that controlled the Caledonian trend of the thrusts and the later suture geometry.

susceptibility variations within the Timanides similar to the stronglinear magnetic anomalies observed in association with the Tima-nian terranes in the Timan Range and Pechora Basin farther south-east.

7 C O N C LU S I O N S

In this contribution, we present a new 3-D model for the SW Bar-ents Shelf that provides new insights into the complex 3-D crustalarchitecture.

(1) The new top basement map highlights the regional differ-ences between the platforms, the deep basins and the transitionto the North Atlantic Ocean. The resulting crustal thickness andcrustal thinning ratio maps show the occurrence of significant thin-ning processes in the western part of the Barents Sea. Furthermore,the trends of the extensions, as well as the intensity of crustal thin-ning, do not correlate with the ages of the basins, a feature thatfavours the pre-existence of crustal weakness zones, which con-trolled the initiation of the basin architecture and ensuring complexbasin evolution. On one hand, we suggest that pre-existing Cale-donian and Timanian weakness zones exerted a strong control onbasin evolution east of the Loppa High. On the other hand, forma-tion of the western basins (i.e. the Tromsø and Bjørnøya Basins)was controlled mostly by the reactivation of the Caledonian su-ture, which coincides with the alignment of the Bjørnøyrenna andRingvassøy-Loppa Fault Complexes.

(2) Our new crustal units map proposes a notable NW–SE-trending upper crustal boundary interpreted as the contact betweenterranes inherited from the Caledonian fold and thrust belt andBaltican terranes only weakly affected by the Caledonian orogeny.

(3) The regional interpretation integrating the 3-D model with theinterpreted weakness zones suggests a very asymmetric Caledoniancollisional prism with a unique Caledonian arm, and a Caledoniansuture to the west of the Loppa High propagating northwards be-tween Svalbard and Franz Josef Land. East of this suture, therewould be a fan of nappes thrusted eastwards in the southwesternBarents Sea and bounded to the south by a fault zone parallel to theoblique convergence between Baltica and Laurentia. This strike-slip fault system would lie along the alignment of the Finnmarkand Masøy and Thor Iversen Fault Complexes. West of the suture,we consider the development of a complex fault system involvingthe transport of Laurentia terranes along strike-slip systems such asthe Billefjorden Fault Zone. These transported terranes would bethe origin of the Svalbard assemblage.

A C K N OW L E D G M E N T S

The study was carried out as part of the projects ‘Basement HeatGeneration and Heat Flow in the western Barents Sea – Importancefor hydrocarbon systems’ (Petromaks project 169438) funded bythe PETROMAKS programme of the Research Council of Nor-way and StatoilHydro. We thank Jan Reidar Skilbrei for initiatingthe project and our project leader Christophe Pascal for adminis-trating it. Laurent Gernigon’s contribution is part of the Petrobarproject, also funded by the PETROMAKS programme of the Re-search Council of Norway. We also thank StatoilHydro and particu-larly Peter Midbøe and Trond Zakariassen for providing us with theseismic depth converted horizons. We also thank Oliver Ritzmannand Stoney Clark from the University of Oslo for information on theBarents50 model and new OBS data. We are very grateful to OdleivOlesen and David Roberts for discussions and comments on earlier

versions of the manuscript. We are very grateful to David Robertsfor editorial review before paper submission. Finally, we warmlythank Christine Fichler and Manel Fernandez, both contributed toimprove this paper during the assessment of the thesis of CecileBarrere.

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