The deep structure of the Scandes and its relation to ...lithosphere.info/papers/2013-Scandes-Maupin-Tecto.pdf(e.g. Roberts, 2003; Torsvik and Cocks, 2005) and whose remnants have

Tectonophysics xxx (2013) xxx–xxx

TECTO-125829; No of Pages 23

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Tectonophysics

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The deep structure of the Scandes and its relation to tectonic history andpresent-day topography

V. Maupin a,⁎, A. Agostini b, I. Artemieva c, N. Balling d, F. Beekman b, J. Ebbing e, R.W. England f,A. Frassetto c,g, S. Gradmann e, B.H. Jacobsen d, A. Köhler a, T. Kvarven h, A.B. Medhus d, R. Mjelde h, J. Ritter i,D. Sokoutis a,b, W. Stratford c,j, H. Thybo c, B. Wawerzinek i,l, C. Weidle a,k

a Department of Geosciences, University of Oslo, Oslo, Norwayb Department of Earth Sciences, Utrecht University, Utrecht, The Netherlandsc Department of Geography and Geology, University of Copenhagen, Copenhagen, Denmarkd Department of Earth Sciences, University of Aarhus, Aarhus, Denmarke NGU, Geological Survey of Norway, Trondheim, Norwayf Department of Geology, University of Leicester, Leicester, UKg IRIS, Washington DC, USAh Department of Earth Sciences, University of Bergen, Bergen, Norwayi Geophysical Institute, Karlsruhe Institute of Technology, Karlsruhe, Germanyj Department of Earth Sciences, University of Durham, Durham, UKk Institute of Geosciences, Christian-Albrechts-Universität, Kiel, Germanyl Leibniz Institute for Applied Geophysics, Hannover, Germany

⁎ Corresponding author at: Department of Geosciences,Blindern, N-0316 Oslo, Norway.

E-mail address: [email protected] (V. Maup

0040-1951/$ – see front matter © 2013 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.tecto.2013.03.010

Please cite this article as: Maupin, V., et al.,Tectonophysics (2013), http://dx.doi.org/10

a b s t r a c t
a r t i c l e i n f o
Article history:Received 13 July 2012Received in revised form 18 February 2013Accepted 5 March 2013Available online xxxx

Keywords:ScandesSeismic tomographyContinental lithosphereCrustal structureIsostasyIntracontinental topography

We review the results of the TopoScandiaDeep project, a component of the TOPOEUROPE project, in which wehave studied the crustal and upper mantle structures of southern Norway in relation to its high topography.The Scandinavian Mountain Chain (the Scandes) is an intracontinental mountain chain at the western edge ofthe Baltic shield, and has its southern part located in southern Norway. The timing as well as the processescausing the formation of the Scandes are disputed. We bring new geophysical constraints to this issue by pro-viding crustal and mantle seismic models for the area and by integrated modeling of the lithosphere and itspotential deformation.New maps of Moho depth and crustal seismic velocities have been compiled using data from refraction lines,P-receiver functions and noise cross-correlation. These results show a thickening of the crust from southwestto northeast and a small crustal root not directly located below the topographic high. P-, S- and surface wavetomography infer seismic mantle velocities lower than in normal shield structure, with a possible sharpboundary close to the Oslo Graben. These low velocities are imaged in the lithosphere and in the underlyingmantle down to the 410 km discontinuity.Integratedmodeling of seismicmodels and gravity data shows that the low velocities below southern Norway arecompatible with a change in lithosphere thickness from c. 100 km under southern Norway to nearly 200 kmunder southern Sweden, with possible additional differences in composition. The study also indicates that thetopography can be isostatically sustained by the density distribution in the crust and lithospheric mantle.We argue that the lithospheric lateral variation has been present for at least 300 My and has had a significantinfluence on the localization of the topography, independently of the mechanism for uplift.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

This paper summarizes the main results of the TopoScandiaDeepproject, in which we have analyzed the crust and upper mantle struc-tures of the southern Scandinavianmountain range (southern Scandes)in order to bring new constraints in the study of the link between

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The deep structure of the Sca.1016/j.tecto.2013.03.010

the deep crustal and upper mantle properties and the present-daytopography.

TopoScandiaDeep is part of TOPOEUROPE, the vision of which isto study the relation between Earth's topography and surface, mantleand plate tectonics processes (Cloetingh et al., 2007). It is wellestablished that the highest topography on Earth is located close toplate boundaries and results from the interaction between tectonicplates. The Alpine–Caucasus system in Europe is the result of such anorogenic process and it is the subject of many studies in TOPOEUROPE(ref to articles in same issue). Uplift and high topography areas are

ndes and its relation to tectonic history and present-day topography,

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2 V. Maupin et al. / Tectonophysics xxx (2013) xxx–xxx

however also found away from plate boundaries, as for example in theTransantarctic mountains (van Wijk et al., 2008), the Southern Rockymountains (Eaton, 2008), southern Africa (Partridge and Maud, 1987),Brazil (Gallagher et al., 1994), Urals (Puchkov and Danukalova, 2009),and the Scandes (this study, Fig. 1). High topography far away fromplate boundaries can be a remnant of ancient mountain belts or mayhave been created by epeirogenic uplift not related to a classical plateboundary setting. Most continental regions, and in particular cratonicareas, have a long and complex geological history. They have beenformed by the accretion of plates and show the imprint of successivecollisional, orogenic and rifting processes, leading to a complex struc-ture and a non-uniform rheology. The consequences of forces actingon systems with a complex rheological structure may not be simpleandmodelingmay bemisleading if the modeled rheology is too simpli-fied (Burov et al., 2007).

Pronounced topography is observed far from plate boundariesaround the northeast Atlantic. High topography along the ScandinavianPeninsula, including most of Norway, with surface elevation up toaround 2000–2500 m is among the most impressive topographic fea-tures of this region. Eastern Greenland also features high topographyin the Palaeogene North Atlantic Igneous Province, which today hasan average elevation of 2000 m with the highest peak (GunnbjørnFjeld) at around 3700 m altitude, higher than the maximum elevationof the ice cap. However, there is no general agreement about theprocesses of formation or the age of this topography.

Fig. 1. Topography and bathymetry of Northwestern Europe. Note the high elevations in soframes of the tomographic maps in Figs. 12 and 14.

Please cite this article as: Maupin, V., et al., The deep structure of the ScaTectonophysics (2013), http://dx.doi.org/10.1016/j.tecto.2013.03.010

The areas of high topography in Scandinavia are located mainlyalong or close to the trace of the Caledonides, a major mountainbelt that formed at around 430 to 410 Ma (Mid Silurian to EarlyDevonian) due to the collision of Laurentia with Baltica/Avalonia(e.g. Roberts, 2003; Torsvik and Cocks, 2005) and whose remnantshave been separated by the opening of the Atlantic Ocean and arepresently located in eastern North America, Greenland, Scandinavia,Scotland and Ireland. A debate persists (see Anell et al., 2009 and ref-erences therein) whether high topography is old and basically a rem-nant of the mountain building processes of the Caledonian orogeny asrecently argued by Nielsen et al. (2009), or whether the topography isgeologically much younger, as is widely believed (e.g., Anell et al.,2010; Bonow et al., 2007; Dorè, 1992; Japsen and Chalmers, 2000;Japsen et al., 2012; Lidmar-Bergström et al., 2000; Stuevold andEldholm, 1996). Faleide et al. (2002) provide evidence for strong re-cent rotation of sedimentary sequences offshore southern Norwayand Anell et al. (2010) observe a recent increase in sedimentationrate from the sedimentary record offshore Norway which is indicativefor uplift or increased erosion during the latest 5 My. In easternGreenland, the highest topography is found within the area coveredby the Paleogene igneous province in which a pile of basaltic lavasup to 7 km thick was extruded close to sea level at around 60 Ma(Brooks, 2011). The presence of the lavas provides a maximum age,and perhaps a cause, for the observed topography. However, there areno comparable lavas on the Scandinavian peninsula. Severalmechanisms

uthern Norway. The black rectangle indicates the area studied and corresponds to the



Fig. 2. Simplified geological map of Fennoscandia after Gorbatchev (2004). TIB is acommonly used acronym for “Transscandinavian Igneous Belt”.

3V. Maupin et al. / Tectonophysics xxx (2013) xxx–xxx

of uplift have been proposed in order to explain the presence of thetopography, most of them being related to the rifting event and openingof the North Atlantic at around 55 Ma. Among the most importantsuggestions are asthenospheric diapirism, lithospheric delamination,magmatic underplating, intraplate compression and rift flank uplift (seemodel suggestions and reviews in Chery et al., 1992; Clift et al., 1998;Cloetingh et al., 1990; Gabrielsen et al., 2005; Nielsen et al., 2002, 2009;Rohrmann and van der Beek, 1996). In the model of Nielsen et al.(2009) climatic forcing is responsible for a time varying and long erosionhistory and uplift is explained as due mainly to isostatic response toerosional unloading.

The different hypotheses imply different types of isostatic supportfor the topographic load: e.g. crustal isostasy, lithospheric isostasy,dynamic support, and flexural support. In order to distinguish betweenthe different models and potential mechanisms, detailed knowledge ofthe crustal and upper mantle structure is needed.

Here, we present an overview of the results of the TopoScandiaDeepproject so far. A major part is the detailed mapping of the seismic struc-ture of the crust and upper mantle of southern Norway. This has beendone using both controlled-source seismic data onshore and partlyoffshore and passive seismic data. The passive data have been analyzedwith various methods: P- and S-receiver functions, P- and S-body wavetomography, surfacewave dispersion analysis fromnoise interferometryand from earthquake data. The results of these investigations from thesurface down to 410 km depth will be presented. The implicationsof the seismic structure in terms of possible models of lateral variationsin composition and temperature have been analyzed quantitativelyby geophysical–petrological modeling that also constrains the modelby its fit to the observed gravity, and the results of this modeling willbe presented. Analog modeling used to test the implication of laterallithospheric variations on uplift due to intracontinental stress willalso be presented, before we summarize by discussing the possibleimplications of our results for the presence of the topography insouthern Norway.

2. Tectonic setting

The Scandes are located at the western edge of the FennoscandianShield (also referred to as the Baltic Shield). The main structure of thestudy region can be seen in Fig. 2, where the main blocks that amal-gamated to form the shield from Archean to Middle Proterozoic aredisplayed together with the Caledonian nappe structures. In south-ernmost Scandinavia, we find crust with a younger Phanerozoiccover.

Archean rocks with ages of up to more than 3 Ga are located in thenortheastern part of the shield and there is a general age decreasefrom northeast to southwest (Gorbatschev and Bogdanova, 1993).The main area that has been studied here, the southern Scandes, islocated in Southern Norway, in the Sveconorwegian province. Thispart was accreted to the shield during the 1.64–1.52 Ga Gothianand 1.52–1.48 Ga Telemarkian accretionary events and during theSveconorwegian orogeny, dated to 1.14–0.97 Ga (Bingen et al.,2008). A major unit located both within and to the east of our studyregion is the Transscandinavian Igneous belt (TIB), a large granitoidbelt that formed about 1.85 to 1.65 Ga (Lahtinen et al., 2008). Thisbelt is a prominent feature in magnetic maps and has a significantinfluence on the gravity field due to its generally reduced densityin the shallow crust (Henkel and Eriksson, 1987; Olesen et al., 2002;Pascal et al., 2007).

There is evidence for magmatic activity in the area at about580 Ma (Dahlgren, 1994). Ultramafic dikes with carbonatite affinitieswere emplaced to the west of the present-day location of the OsloGraben but the lateral extension of the area subject to magmatic ac-tivity may have been larger than indicated by the present-day surfacetraces. The formation of the Hedmark Basin at that time north of the


Oslo Graben suggests an extensional regime in the Late Proterozoic(Dahlgren, 1994).

This phase of extension was followed by the crustal collision thatresulted in the Caledonian orogeny that affected the western borderof the Baltic Shield. This major orogeny, with a main phase between430 and 410 Ma (Roberts, 2003; Torsvik and Cocks, 2005), resultedfrom the closure of the Iapetus Ocean and collision of Laurentia(incl. Greenland) with Baltica. Extensive nappes cover the westernedge of Baltica (Fig. 2). These nappes usually form the upper few kilo-meters of the crust, and the crust and mantle beneath are consideredas autochthonous units. The Western Gneiss Region, along the north-west coast of southern Norway (Fig. 2), has been strongly reworkedduring the orogeny and is composed of crustal and mantle materialsthatwere deeply buriedduring the orogeny and subsequently exhumed(Austrheim et al., 1997).

Slightly before the closure of the Iapetus Ocean, at about 440 Ma,Baltica was affected by the closure of another sea, the Tornquist Sea,on its southwestern margin (Cocks and Torsvik, 2006). The remnantof the closure of this ocean is probably located in the southeasternpart of the North Sea (Balling, 2000; MONA LISA Working Group,1997) and in the south of Denmark where dipping structures identifythe suture zone (Abramovitz et al., 1997; Abramovitz and Thybo,2000). To the east, south of Sweden this suture intersects the verywell-marked boundary between the East-European platform and theyounger Phanerozoic terranes of Central and Western Europe, knownas the Tornquist–Teisseyre Zone (TTZ). A clear step in lithosphericthickness and thermal state, as well as a sharp transition in mantle ve-locity and density have been imaged along this transition zone, whichmakes it one of the major features of the European lithosphere(Berthelsen, 1988; Grad et al., 2002; Zielhuis and Nolet, 1994).

From about 300 to 240 Ma, southern Norway and Denmark weresubject to rifting and magmatism, which formed the north–south ori-ented Oslo Graben in the southeastern part of South Norway (Fig. 2)and a series of other rift grabens in the North Sea region. These rifts




are part of a larger system of rifts that affected northwest Europe atthat time (Neumann et al., 1992). The extensive magmatism associ-ated with the rifting is an important source of information about thecomposition and thermal state of the mantle in the area at thattime.

The last major tectonic event that affected the area is the opening ofthe North Atlantic Ocean; more precisely the Norwegian–GreenlandSea. This occurred in the early Eocene (55 Ma) after a long period ofextension and rifting that lasted for the entire Mesozoic (Osmundsenand Ebbing, 2008 and ref. therein). The rifting took place parallelto the strike of the Caledonian suture, to form the rifted continentalmargin that we observe today.

3. Seismological models

3.1. Data and methodology

Location of most of the active and passive seismic data collected forthis study is summarized in Fig. 3. A large part of the seismological datawas acquired during the MAGNUS experiment: the temporary deploy-ment, from 2006 to 2008, of a network of broadband seismological

Fig. 3. Geographic overview of topography and main seismological stations deployedfor this project. Note that some studies used additional stations; see Medhus et al.(2012) in particular.


stations in southern Norway (Weidle et al., 2010), supplemented byrecordings at permanent stations. Data were also acquired for a shorterperiod of time in Sweden (DANSEIS experiment). Tomographic andreceiver function studies also took advantage of the existence of record-ings from previous temporary deployments whenever possible (seeMedhus et al., 2012 for a complete list). In addition, three controlled-source seismic profiles (MAGNUS-REX) were acquired over southernNorway in October 2007 in order to add information about the crustalstructure and sub-Moho seismic velocities in this region (Stratfordet al., 2009). One of these lines was supplemented to the west bythree OBS-profiles on the continental platform offshore in order tolink the continental part to the well-studied offshore basins. The studyalso includes the acquisition of broadband data over two lines whichtraverse the mountain range further north (SCANLIPS 1 and 2, Englandand Ebbing, 2012). The seismological data are complemented by poten-tial field data (Ebbing et al., 2012, and references therein).

The seismological data are analyzed by several teams using differenttechniques in order to map different characteristics of the subsurface,and to compare the independently obtained results. The controlled-source lines were used to constrain the P-wave and S-wave velocitiesin the crust and refine Moho depth in the study area (Stratford andThybo, 2011a, 2011b; Stratford et al., 2009). Dispersion characteristicsof short-period surface waves were extracted from ambient seismicnoise (Köhler et al., 2011). Combined with earthquake surface wavedata, these dispersion relations were used to constrain S-wave velocity(Vs) and anisotropy in the crust and upper mantle, as well as Mohodepth (Köhler et al., 2012b). Earthquake surface wave data were alsoused to generate a model of the average upper mantle S-wave velocitybelowsouthernNorway (Maupin, 2011).Mohodepth andmantle discon-tinuities were analyzed using P-wave (Frassetto and Thybo, 2010,submitted for publication) and S-wave receiver functions (Wawerzinek,2012). In addition, receiver functions were analyzed along the SCANLIPSlines further north (England and Ebbing, 2012). A tomographic inversionof the arrivals of the P-waves from teleseismic earthquakes was usedto constrain both the relative variations in mantle P-wave velocities(Vp) and their absolute values (Medhus et al., 2009, 2012). A 3-Dmodel of the mantle S-wave velocity contrasts was determined byteleseismic S-wave tomography (Wawerzinek et al., 2013). Analysisof SKS-splitting was also performed to analyze seismic anisotropy(Roy and Ritter, 2013).

The different methods were applied by the different researchgroups to generate individual models independently of each other. Aswe will show, these different models show a remarkable coherency.This gives us confidence that the different data sets give robust informa-tion. Still, it is important to keep in mind that the different techniquesused here have different sensitivities and resolutions and consequentlydifferent strengths and weaknesses. In order to benefit fully from thecomplementarity of the different data sets, it is desirable in a secondstage to combine all the data and seek a model that can explain all thedata together. Considering the coherency that we have obtained sofar, we are confident that such an analysis will be fruitful and willhelp in constraining variations in the physical properties of the crustand upper mantle, in particular in a better evaluation of the magnitudeof the velocity anomalies. The magnitude is a key parameter requiredfor a quantitative geodynamical interpretation of the models put for-ward to explain the topography but is often difficult to ascertain inindividual studies. In this article, we concentrate on summarizing andcomparing the results of the individual studies with emphasis on themodels obtained. We refer to the individual articles for details con-cerning the analysis procedures, the resolution tests and for moredetails about the models.

3.2. Crustal velocities

P- and S-wave crustal velocities as well as Poisson's ratio weredetermined from the controlled-source data along three profiles



Fig. 4. Crustal S-wave velocities along the three Magnus-Rex profiles depicted in Fig. 3(from Stratford and Thybo, 2011a). The elevation along the profile is shown at the top, to-getherwith the shot locations. The origins of the horizontal axes are at the positions of thefirst shot points for each profile.


(Fig. 3, Stratford and Thybo, 2011a, 2011b; Stratford et al., 2009).The location of the interfaces between layers found by analysis of theP-wave data was used without modification in the S-wave models. TheS-wave velocity models (Fig. 4) are here compared with correspondingprofiles in the 3-D S-wave velocity model obtained from teleseismicand ambient noise surface wave data (Köhler et al., 2012b) (Fig. 5).The exceptional quality of the S-wave arrivals in the MAGNUS-Rexdata made it possible to construct S-wave profiles along the three linesdown to Moho depth, providing an excellent opportunity to directlycompare the models derived from the controlled-source experiment tothose derived from the surface waves. The strong S-waves are mainlyobserved for explosions in the area with Caledonian structures, and weascribe their generation to the conversion of P to Swaves due to strongerimpedance contrasts in the extensive series of thin nappes in the area.The vertical sections through the 3D surface-wave model are straightlines following closely, but not exactly, the refraction lines (see Fig. 3).Line 2 in particular follows the maximum topography and its southernpart is further east than the refraction profile to remain in a well re-solved part of the surface-wave model. An additional EW section, calledline 4 and situated in the center of the study area, is shown in Fig. 5. Inaddition, we show a horizontal cross-section at 5 km depth of the 3Dmodel in Fig. 6.

The twomethods give very consistent Vsmodels, showing a crustalstructure without strong lateral variations. A large portion of the crustconsists of a layer with velocities increasing from 3.65 km/s at the topto about 3.85 km/s at the bottom. This layer is bound at the top by alayer with velocities of around 3.4 to 3.6 km/s and thickness of a fewkilometers and at the bottom by a thin lower crustal layer with veloc-ities of about 4.0 km/s. This last layer has not been observed directlyby seismic refraction analysis, suggesting a thickness not larger than2 km. The Poisson's ratio increases from 0.25 in the upper half of thecrust to 0.27 in the thin bottom layer (Stratford and Thybo, 2011a,2011b). This result is consistent with similar observations at presentlyactive rift zones in Siberia and east Africa (Thybo and Nielsen, 2009;Thybo et al., 2000). The relatively high velocities in the upper part ofthe crust are consistent with the absence of sediments in the area.The lower crust is particularly thin. It thickens below the Oslo Grabenwhere we do not have an indication of a significant Moho shallowingbut rather of a thickening of the lower crust. The velocities in the OsloGraben upper crust are lower than in the surrounding areas, as can beseen in the horizontal section (Fig. 6). Upper crust S-wave velocitiesare also lower in western Norway. We have verified that the disper-sion of the surface waves is not biased by the large relief present inthis area (Köhler et al., 2012a) and that these reduced velocities aretherefore not an artifact of the relief. Low velocities are also found inthe controlled-source profiles, where the low-velocity region has aslightly higher Poisson's ratio of 0.255 as compared to the values of0.25 further east (Fig. 4). A similar transition from west to east isalso seen in line 4 where we observe a sharp transition, located at hor-izontal distance 300 km on Fig. 5, from a thick low-velocity upperlayer in the west to a very thin layer in the east, overlying a middlecrust of low velocity compared to the west. The low S-wave velocitiesand high Poisson's ratio in the upper crust in western Norway couldresult from reworking of the basement at the time of the Caledonianorogeny (Stratford and Thybo, 2011b).

The seismic structure of the on- to offshore transition into thethinned crust of the Møre basin along controlled-source line 1(Fig. 7) shows that the transition from normal crustal thickness ofabout 40 km to a thinned crystalline crust (light blue layer labeled“Crust” in Fig. 7) of no more than 8 km occurs over an approximately150 km wide transition zone. The zone of strongest thinning islocated west of the coastline. Beneath and landward of the coast-line, evidence is found for a two-layered crystalline crust, with Vpof 5.5–6.0 km/s and 6.6 km/s, respectively. These velocities areindicative of granitic–granodioritic rocks. The average crystallineVp beneath the Møre Basin is 6.4 km/s, suggesting that these felsic


rocks extendwestward to theMøreMarginal High. The thinned crust inthe Møre Basin is overlaid by a sedimentary package of up to 10 kmthickness, similar to the observations in the Vøring Basin further north(e.g. Mjelde et al., 2009). The sedimentary succession is dominantlyof Cretaceous age, but also contains significant amounts of Devonian–Jurassic and Cenozoic rocks.



Fig. 5. Vertical sections in the SV-wave velocity model obtained from the analysis ofsurface wave dispersions (Köhler et al., 2012b). Lines 1, 2 and 3 follow approximatelythe locations of the Magnus-Rex profiles (see Fig. 3). Line 4 is EW at 62°N. See insets forthe locations of the sections. The origins of the horizontal axes coincide with theorigins in Fig. 4.

Fig. 6. Horizontal slice in the range 4 to 6 km depth through the SV-wave velocity modelobtained from the analysis of surface wave dispersions (Köhler et al., 2012b). Geologicalunits: Fennoscandian Basement (B), Caledonian nappes (C) including Jotun nappes(J) and Hedmark group (H), Oslo Graben (O), and Western Gneiss Region (W). Noisedata are used in the whole area; earthquake data are used only within the area delimitedby the white dashed line due to growing errors at the boundaries.


3.3. Moho depth

Moho depth is constrained by the controlled-source profiles andsurface wave velocities discussed in the previous sections. Additionalinformation is obtained fromP- and S-wave receiver functions (Frassettoand Thybo, submitted for publication; Wawerzinek, 2012). Fig. 8 isa comparison of the Moho depths found independently by controlled-source seismics, P-receiver function and surface wave dispersion analy-sis. The map based on the controlled-source experiments results frominterpolation between the three MAGNUS-REX lines and a series ofolder refraction profiles summarized by Kinck et al. (1993). The locationof the complete set of lines (Stratford et al., 2009) shows that the map isbased on a good coverage of data except for two spots: the high topogra-phy area at 7–9°E and 60–61°N; an area of the Western Gneiss Regionthat will be filled when the analysis of the data from the profilesshown in Fig. 7 will be finalized. The P-receiver function analysis wasperformed with data from all MAGNUS stations and other temporaryand permanent stations in the area (Frassetto and Thybo, submitted forpublication) and covers thewhole area. The results provide an areal cov-erage that extends theMoho depth models obtained by P-wave receiverfunctions along the two CENMOVE lines (Svenningsen et al., 2007).

In southern Norway, in the southwestern part of the study region,there is good agreement between the Moho depths found by thethree methods. The seismic refraction map is smoother than the two


others due to its construction by interpolation between profiles andthis does not mean that it infers a smoother Moho discontinuity. Ingeneral, the receiver functions indicate a 2–3 km deeper Moho thanthe refraction seismic model and the surface wave model shows inter-mediate values. The uncertainty forMoho depth estimates along the re-fraction lines is ±1 km or±2 km, depending on the location (Stratfordet al., 2009). The values derived from receiver functions have an uncer-tainty estimated at±2.5 km,mainly due to uncertainty inmean crustalvelocities. The uncertainty is more difficult to evaluate for surface wavestudies due to larger trade-off with velocity. Moho depth is at around30 km depth along the coast and deepens to about 40 km on averagetowards the northeast. The crustal “root” at 42–43 km depth in the re-ceiver function model at 8–10°E and 60–61°N cannot be confirmed byrefraction studies due to a lack of data in this region. Its continuationfurther north, at about 62°N, is however located in an area well coveredby seismic refraction lines and there seems to be a discrepancy of about5 km in this region that cannot be simply ascribed to the uncertaintiesof bothmethods. Allmaps show thinning of the crust in the Oslo Grabenarea with some variation possibly related to a misinterpretation of thephases related to the top of the lower crust in the receiver functions. Thelimited crustal thinning associated with the Oslo Graben (b3 km crustalthinning), is similar to the findings in other rift grabens (Kirschner et al.,2011; Thybo and Nielsen, 2009; Thybo et al., 2000).

The correlation between the three maps is poorer in Sweden andin the northeastern corner of our study area where Moho depth is45 km in the models derived from refraction and surface wave data,whereas the receiver functions infer an up to 60 km thick crust. Theanalysis of the receiver functions shows however that the converted



Fig. 7. P-wave velocity model across the Møre continental platform along profile 1. See inset for the location of the profile. This profile is the continuation of Magnus-Rex profile 1shown in Figs. 4 and 5.


S-waves are weak in the area of strong discrepancy, indicating a lackof strong impedance contrast at the base of the crust. The presence ofa very high velocity layer in the lower crust (e.g. BABEL WorkingGroup, 1993) or an extended gradient from the lower crust to themantle (Olsson et al., 2008) could reduce the Moho to a second-order discontinuity that does not generate significant phase conver-sions. We notice that the sharp increase in Moho depth eastwardsin the receiver function model corresponds to the boundary betweenthe Sveconorwegian domain to the west and the TransscandinavianIgneous Belt (TIB) in the Baltic Shield proper in the east. In thenortheastern-most corner of the area covered with the receiver func-tions, eastward of the TIB, Moho depths are again around 40 km, inaccordance with the results obtained with P-receiver functions byOlsson et al. (2008). Our results are also in general agreement withthose from Olsson et al. (2008) in the small geographical overlaparea to the east of the Oslo Graben. In conclusion, in the areas withhigh resolution, the Moho depth maps obtained by the three methods

Fig. 8.Mohodepthsmaps obtained froma) refraction-seismic profiles (Stratford et al., 2009) coand Thybo, submitted for publication) and c) surfacewavedispersion analysis (Köhler et al., 201few points in panel b) are at depths between 55 and 60 km. (For interpretation of the referen


match on average and show a deepening towards the north-east. Inthe TIB area, the Moho is a weak converter and therefore less wellconstrained. This region is poorly covered by controlled source seis-mic data. Receiver functions indicate that Moho depth might varylocally more than shown in the interpolated map based on seismicrefraction data. The Moho depths obtained by S-receiver functions(Wawerzinek, 2012) are not shown since they are not as well re-solved as in the three methods presented here, but they indicate anaverage depth of 37 km, in good agreement with the other methods,and a slight deepening towards northeast.

The thickening of the crust towards the NE does not correlate wellwith the maximum topography, and the small root is shifted by about60 km eastwards of themaximum topography (see Fig. 4 line 1 for ex.).This implies that the topography in Norway is not simply compensatedbyAiry type isostasy. However, the Bouguer anomaly lowover southernNorway correlates strongly with the high topography, which indi-cates a high degree of isostatic compensation (e.g. Balling, 1980).

mbinedwith the compilation of Kinck et al. (1993), b) P-wave receiver functions (Frassetto2b). The color scale has been cut at 55 km in order to give appropriate contrasts although aces to color in this figure legend, the reader is referred to the web version of this article.)



Fig. 10. SV-wave velocitywith depth resulting from the inversion of the average dispersioncurve of the Rayleigh waves in southern Norway (blue lines; Maupin, 2011). The modelswith Moho depth fixed at 35, 37 and 39 km are shown here. The profiles are comparedto the ak135 profile (blue dashed line; Kennett et al., 1995), to 3 SV-wave velocity profilesin Proterozoic fold belts (green lines; Lebedev et al., 2009) and to the range of velocities forcratonic areas (gray area; Pedersen et al., 2009). Resolution is poor below 200–250 kmdepth. (For interpretation of the references to color in this figure legend, the reader isreferred to the web version of this article.)


Lateral variation within the crust is a key to resolve this apparentcontradiction. A significant contributor to the gravity in this regionis a high-density lower crust located to the east of the Scandes,which tapers out beneath the high topography, and which ensuresthat isostatic calculations for the Fennoscandian shield show a highdegree of isostatic compensation (Ebbing, 2007; Ebbing et al., 2012;Frassetto and Thybo, submitted for publication; Stratford and Thybo,2011a). Ebbing et al. (2012) show that an Airy–Pratt isostatic modelwith a laterally varying crust and lithospheric thickness may largelyexplain the gravity field.

Further north, the presence of the high-velocity lower crust wasconfirmed along the central Norway SCANLIPS profile (profile SCANLIPS1 on Fig. 3) (England and Ebbing, 2012). By forward and inverse model-ing and migration of the receiver functions a model was establishedwhich shows the crust thickening from about 32 km at the Norwegiancoast to about 43 km beneath the central Scandinavian mountainrange and then remaining constant beneath Sweden (Fig. 9). Themodel shows furthermore good evidence for a high-velocity lowercrust underlying much of Sweden which thins to the west beneathNorway. Preliminary results for the northern SCANLIPS 2 profileshow a similar change in Moho depth from the coast towards Sweden.A high-velocity lower crust is not detected as a discrete layer as forthe southern profile, but is also required here on the basis of isostaticand gravity considerations.

These results stress the importance of taking several factors intoaccount for assessing the isostatic balance of an area. The 60 km east-ward shift of the maximum of the topography compared to the max-imum Moho depth indicates that flexural rigidity of the lithospheremay play a role in sustaining the topography, or, alternatively, thatother mechanisms sustain the asymmetric topography.

3.4. Upper mantle seismic velocities

The seismic velocities in the uppermantleweremostly derived fromteleseismic P-wave tomography (Medhus et al., 2012), teleseismicS-wave tomography (Wawerzinek et al., 2013), and surface wave anal-ysis (Köhler et al., 2012b; Maupin, 2011). Sub-Moho seismic velocitiesare also constrained by the controlled-source experiments that infervelocities of 8.05 to 8.15 km/s for P-waves and relatively high velocitiesof 4.65 to 4.70 km/s for S-waves in the center of southern Norway.A larger range of sub-Moho seismic velocities is observed from the sur-face wave dispersion tomography (Fig. 5) but the trade-off with Mohodepth in dispersion analysis makes it difficult to interpret these valueswith confidence, independently of other information. This trade-off isalso apparent in the models derived from the analysis of the averagedispersion of the Rayleigh waves in southern Norway (Maupin, 2011):

Fig. 9. P-wave velocity-depth model for the SCANLIPS 1 profile derived from constra


for averageMoho depth varying from 35 to 41 km, the inferred averagesub-Moho S-wave velocity varies from 4.45 to 4.6 km/s. Although themodel with Moho depth fixed at 41 km has the best agreement withthe average lower crustal and mantle sub-Moho velocities found by thecontrolled-source experiments, we show in Fig. 10 the model obtainedby fixing Moho depth from 35 to 39 km, in better agreement with theMoho depth maps presented above.

The most notable feature of the model in Fig. 10, and which is notstrongly affected by a trade-off with Moho depth, is a low velocityzone in the depth range 100 to 200 km. The depth-averaged S-wavevelocity in this depth range is estimated with an uncertainty of about2% (Maupin, 2011). Shear-wave velocities of 4.4 km/s are inferred,lower than the velocities in model ak135 (Kennett et al., 1995) by 2%,

ined forward modeling of receiver functions (from England and Ebbing, 2012).




and notably lower (almost 4%) than the range expected for Proterozoicfold belts of similar age (4.50 to 4.75 km/s in the depth range between100 km and 150 km (Lebedev et al., 2009)) or in older cratonic areaswhere S-wave velocities between 4.60 km/s and at least 4.80 km/sare expected (Lebedev et al., 2009; Pedersen et al., 2009). This aver-age S-wave model (Fig. 10) has been built with emphasis on usingsurface-wave recordings of up to 200 s periods to obtain the best possi-ble depth resolution at the expense of the lateral resolution. The lateralvariations are analyzed in a tomographic study using surface waves upto 67 s period (Köhler et al., 2012b). This tomographic model alsoshows low velocities at 110 km depth (Fig. 11), albeit the anomaly isweaker than in the averaged model. It shows S-wave velocities varyingfrom about 4.50 km/s under southern Norway (at the upper limit ofuncertainty of the average model) to 4.60 km/s across the Swedishborder at 110 km depth.

The teleseismic S-wave tomography model also contains lowervelocities below southern Norway than further east beneath the BalticShield (Figs. 12 and 13). The velocity contrast reaches about 3 to 4%and dips downwards from west to east (Fig. 13). At shallow depths(35–120 km), the reduced S-wave velocities are located in the westernhalf of southern Norway and to the north. The location of this lowS-wave velocity corresponds very well with an area of lowmantle den-sity (Artemieva et al., 2006) and to where Ebbing et al. (2012) find thelargest imbalance in the isostatic equilibrium of their model and pro-pose the presence of a low-densitymantle material to explain the grav-ity field in addition to the lithospheric thickness variation. The generalvelocity increase fromwest to east is in overall agreementwith findingsfrom surface wave data in the central area where the surface-wavetomography has good resolution (Fig. 11).

The low S-wave velocity anomaly increases in amplitude withdepth (Figs. 12 and 13) and reaches its maximum amplitude at about400 km depth. Resolution tests (Wawerzinek, 2012; Wawerzineket al., 2013) support this finding which may indicate an increased tem-perature at the top of themantle transition zone andwhich then would

Fig. 11. Horizontal slice at 110 km depth of the SV-wave velocity model obtained fromthe analysis of surface wave dispersions (Köhler et al., 2012b).


predict a deeper 410 km discontinuity compared to average Earthmodels (Bina and Helffrich, 1994).

Depth slices of the velocity model from the P-wave teleseismicbody wave tomography (Fig. 14; Medhus et al., 2012) also show atransition from lower velocities in the west below southern Norwayto higher velocities to the east. Recordings from additional stationsin Denmark compared to the S-wave tomography provide resolutionfurther south and show that the low upper mantle velocities extendfurther south, beneath the deep basins in Denmark. The teleseismictomography inversion method used here (Medhus et al., 2012) in-cludes both relative and absolute travel-times, and therefore providesboth the usual Vp model relative to an unknown regional average andan absolute model with respect to the ak135 model (Kennett et al.,1995). The relative and absolute velocity models are not fundamen-tally different and lead to similar interpretations. It is shown that therelative model has best resolution in the upper part of the model(100–200 km depth range) and that the absolute model, shown inFig. 14, has best resolution at a depth of 300–400 km (Medhus et al.,2012). The difference in resolution between the two models resultsfrom the absolute traveltime method being able to include less steeprays and therefore additional crossing points at depth. The major con-trast across the Tornquist–Sorgenfrei transition zone is well imaged inthe 100 to 300 km depth range, in addition to a low-velocity anomalywest of the Oslo Graben, where the velocity is lower than in the ak135reference model by 1–2%. Amplitude variation of up to about ±2%persists down to at least 400 km depth, where the model shows anincrease in velocity from west to east in the southern part of southernNorway and northern Denmark.

There is a clear indication that the mantle below southern Norwayhas lower seismic velocities than expected beneath a Proterozoicfold belt. This is a very consistent result that we obtain with all thetechniques used to analyze the seismological data and is valid forP-waves as well as for S-waves. The exact amplitude and location ofthe low velocity are however not completely consistent betweenstudies. The surface wave average model infers low velocities in thedepth range 100 km down to at least its maximum resolution depthof 200–250 km. In combination with the results from the controlledsource experiments, this model also suggests that the low velocitiesdo not extend up toMoho depth. The P- and S-body-wave tomographicmodels suffer from lack of resolution above 100 km depth and inferlow velocities from 100 km depth down to at least 400 km depth. Thelocations of the anomalies centered below southern Norway are inagreement between 200 and 350 km, depth but the shallowest part ofthe anomaly is shifted eastwards in the P-wave model compared tothe S-wave model and the deeper parts of the P-models show highestanomaly amplitudes shifted to the southwest. The overall amplitudesof about 3% of the relative variations are very similar in the twobody-wave tomographic models, an unexpected feature for thermallyinduced anomalies (e.g. Goes et al., 2004), suggesting the possibility ofadditional compositional variations. The amplitude of the velocityanomalies does not decrease with depth and even increases in theS-wave body-wave tomographic model (Fig. 12). The discrepancy be-tween these models may be related to different resolutions and a jointdata analysis is necessary to study if surface wave dispersions andP- and S-delay times are compatible with a unique model. In consider-ing the fit of amplitudes, it is noted that the P-wave velocity (Fig. 14)is particularly slow in the 100–200 km depth range below southernNorway, and that this property correlates with the low-velocity zonein the average surface wave model (Fig. 10). The lower contrast of 1%in the S-wave body-wave tomography model (Fig. 12) in this depthrange is on the other hand in better agreement with the smaller con-trast found in the surface wave tomographic model (Fig. 11).

Although not documented as clearly and unequivocally before, alow-velocity mantle anomaly in southern Norway is not unique toour models. Low P-wave velocity in the sub-crustal mantle belowsouthern Norway was proposed by Bannister et al. (1991) although



Fig. 12. Four horizontal cross-sections through the S-wave tomographic model of Wawerzinek et al. (2013) at 95, 215, 290 and 409 km depth. The circles show nodes of the modelparameterization. The dark line in upper left panel shows the location of the vertical section shown in Fig. 13.


their method did not permit a precise depth constraint on the lowvelocities. A strong upper mantle low-velocity anomaly is also presentin the area in the regional surface wave tomographic model of Weidle


andMaupin (2008). This low velocity anomaly can also be traced in sev-eral recent global models (Ritsema et al., 2011 (in particular at 400 kmdepth); Amaru, 2007; van der Meer et al., 2010) and in large-scale



Fig. 13. East–west vertical cross-section in the S-wave tomographic model of Wawerzinek et al. (2013) at 60° North, corresponding to profile 3 in Figs. 4 and 5. The location of thesection is also shown in Fig. 12.


regional models (Bijwaard et al., 1998; Marquering and Snieder, 1996;Pilidou et al., 2005; Schivardi and Morelli, 2011). In the recentsurface-wave based regional model of Legendre et al. (2012), theS-wave velocity below southern Norway is also at about 4.4 km/s, butis a transition region from lower velocities to the north to higher veloc-ities to the south. A low-velocity develops in this model below southernNorway only from 200 km down to 330 km depth. The results of thepresent project have constrained a sharp pronounced transition be-tween the low velocities in southern Norway and normal shield veloci-ties further east. This first order observation is crucial for interpretationof the tectonothermal history of the lithosphere and possible implica-tions for the evolution of high topography. We should also note thatsimilar low velocity zones have previously been identified in thewhole Baltic Shield (Abramovitz et al., 2002; Perchuc and Thybo,1996) and even globally (Rychert and Shearer, 2009; Thybo, 2006).

3.5. Upper mantle seismic discontinuities

The P- and S-wave receiver functions (Frassetto and Thybo, 2010;Wawerzinek, 2012) address discontinuities in the entire upper man-tle. Besides Moho, the S-wave receiver functions image mostly nega-tive discontinuities down to a depth of 250 km (Fig. 15). From 58°Nto 60°N, we observe a major negative discontinuity (decreasingvelocity with depth) at 70 to 90 km depth with a decreasing velocitycontrast dipping towards north and another one at 100 to 150 kmdepth. These discontinuities have a tendency to deepen towards theeast. The P-wave receiver functions show a similar trend of two setsof negative discontinuities, deepening to the north east from 50 to100 km depth and from 150 to 200 km depth (Fig. 16), perhapswith a weak positive discontinuity in between. It is not straightfor-ward to associate any of these discontinuities with the lithosphere–asthenosphere boundary (LAB). In addition, detailed analysis of thecompatibility of the two data sets requires a common data inversionthat has not been carried out yet. Still, the receiver functions clearlyindicate the prevalence of negative discontinuities, in agreement withthe profiles obtained from the surface wave dispersions (Fig. 10) thatshow a low-velocity zone in the depth range of 100 to 200 km, and adeepening of the pattern towards the northeast. This is also in agreementwith previous inferences of the thickening of the lithosphere in thisdirection (Artemieva et al., 2006; Calcagnile, 1982) and negative discon-tinuities found further east beneath Sweden (Olsson et al., 2006).

The P-receiver functions also image the 410-km discontinuity(Fig. 16), which shows a deepening of about 20 km in the center ofthe study area that correlates quite well in location and amplitudewith the maximum of low-velocity in the teleseismic S-wave model(Fig. 12). The P-wave velocity model also shows a low-velocity inthis area, but further southwest (Fig. 14). The 410-km discontinuity


has been found to be at a normal depth of about 410 km in northernGermany (Alinaghi et al., 2003; Grunewald et al., 2001) as well as inthe northern North Sea, just to the west of our study area where adepth of 414 ± 5 km has been inferred (Helffrich et al., 2003).

A 20 km downward shift of the 410-discontinuity correspondsto a temperature increase of about 200 °C (Helffrich, 2000). Sincemigration velocities have been taken from the tomographic models,this shift is not related to errors caused by too high migration veloci-ties. A 200 °C temperature contrast at that depth is predicted to pro-duce velocity variations of ±1.0% for S-waves and of ±0.5% forP-waves (Goes et al., 2004). The velocity variation will be slightlylarger at 200 km for the same temperature contrast: ±1.1% forS-waves and ±0.7% for P-waves. In the asthenosphere, the S-wavevelocity variations (of ±1.5% to 2%) that are observed are mostlyconsistent with rather large temperature variations of the order of200 to 300 °C. The observation that Vp varies more than Vs, however,is not compatible with purely thermal variations and would requireadditional elements such as variations in composition.

3.6. Summary on seismic structure

Summarizing the results above, we may say that southern Norwayhas

• No major lateral variation within the crust• A small root shifted about 60 km eastward from the location ofmaximum topography

• Crustal thickness increasing generally eastwards• Lithospheric thickness increasing eastwards• Low P and S-wave velocities in the mantle from about 100 to 400 kmdepth.

4. Temperature, density and composition of the mantle

An interpretation of seismological models in terms of thermo-chemical anomalies of the mantle requires knowledge of the densitystructure given different sensitivities of seismic velocities and densityto variations in temperature, mineral composition, fluid phase andmelt. In order to study how the upper mantle velocity variationsrelate to the thermal structure and composition of the lithosphere,thermo-isostatic modeling and geophysical–petrological forwardmodeling were performed. The main ingredients in these modelingmethods are observed topography and gravity and/or geoid anoma-lies that are used together with the seismic velocities to constrainheat-transfer and thermodynamically-consistent models of the com-position and temperature of the lithosphere. In a 1-D inversionstudy by Kolstrup et al. (2012) subsurface temperature, temperature-



Fig. 14. P-wave tomographic model of Medhus et al. (2012). This figure shows the absolute variations with respect to model ak135 in 4 different depth ranges.


dependent density and seismic velocities were calculated from a set ofinput parameters (topography, Moho depth, geoid height and heatflow) assuming isostatic and thermal equilibrium. Their results suggesta thickening of the thermal lithosphere below southern Norway fromwest to east. The western part is found to have higher temperatures,lower densities and lower synthetic S-wave velocities than the eastern


part, compatible with results from the P-wave travel time tomographicstudy (Medhus et al., 2012). Comparison of the synthetic S-velocity pro-files beneath southwesternNorwaywith velocity profiles inverted fromRayleigh wave dispersion data suggests that the higher temperaturesassociated with a thinner lithosphere can explain parts of the seismiclow-velocity anomaly.



Fig. 15. Vertical profiles of Common-Conversion-Point stacks of S-receiver functions from Wawerzinek (2012). Insets show the locations of the profiles. The receiver functions areshown as black lines and as a color-coded background with red and blue corresponding respectively to positive and negative amplitudes. (For interpretation of the references tocolor in this figure legend, the reader is referred to the web version of this article.)


This studywas subsequently extended to 3Dusing a directmodelingapproach and a self-consistent 3D subsurface model was developedto investigate the observed velocity variations between Norway andSweden in conjunction with gravity and geoid observations (Gradmannet al., submitted for publication). This modeling approach makes itpossible to analyze the thermal and compositional properties of thelithospheric mantle that are compatible with an isostatic state ofequilibrium, observed topography, gravity, and seismic velocities.The model was built using combined geophysical–petrological for-ward modeling of the lithosphere and sublithospheric upper mantle(LitMod3D: Afonso et al., 2008; Fullea et al., 2009) and covers south-ern Norway and southern Sweden (Fig. 17). TheMoho depth is basedon the results of the MAGNUS-REX survey (Stratford et al., 2009)as well as the compilations by Kinck et al. (1993). The geometry ofthe Lithosphere–Asthenosphere Boundary (LAB), defining the posi-tion of the 1300 °C isotherm, is adjusted during the modeling sinceit is not well constrained from previous studies (Artemieva, 2006;Calcagnile, 1982; Plomerová et al., 2008). Thermophysical propertiesof the mantle are derived from several possible characteristic chem-ical compositions (Afonso et al., 2008). The chemical compositionsare adjusted during the modeling to fit the observed data. In themodel presented here (Fig. 17), the isostatic state and gravity fieldcan be explained by the crustal architecture and long-wavelengthtrends in uppermost mantle structure and composition.

A major temperature difference between southern Norway andsouthern Sweden is required in order to reproduce the variations inseismic velocity observed in the area, in particular in the P-wavetomographic model of Medhus et al. (2012) and in the velocity-depthprofiles of Maupin (2011) compared to profiles further east (Cotteet al., 2002; Pedersen et al., 2009). Considering the lithosphere as athermally conductive layer in steady-state equilibrium, temperaturedistribution in the lithosphere is directly controlled by its thickness.The observed velocity variation requires a major change in lithosphere


thickness from approximately around 100 km under southern Norwayto nearly 200 km under southern Sweden (Fig. 17b).

In addition, in order to satisfy isostatic equilibriumand tomatch geoidand gravity data, a change in mantle composition from Phanerozoic-type (in the sense of Griffin et al., 1998) under southern Norway toProterozoic-type (more depleted, lighter) under southern Sweden isrequired. The seismic velocity is here primarily controlled by the temper-ature field and only to a small degree by the composition, i.e. by changesin density and elastic moduli (Fig. 16). We do not consider the effectsof compositional variations in minor elements or water content (Goeset al., 2000). Minor changes in the thermophysical properties of theSubContinental LithosphericMantle (SCLM) are considered, as they addi-tionally control the temperature field (Hieronymus et al., 2007). Thegravity data and topography signature also suggest that the lateral transi-tion from thick Proterozoic to thin Phanerozoic SCLM is not subvertical,and can be represented by older SCLM overlying younger SCLM in an~100 km wide transition zone (Fig. 17).

The present model has been constrained to satisfy the S-wave ve-locities obtained by the analysis of surface wave dispersion (Fig. 10).The two body-wave tomographic models (Figs. 12 and 14) also showlow velocities in southern Norway although with some differencescompared to the surface-wave-based model. The P-wave tomographyof Medhus et al. (2012) (Fig. 14) shows a velocity variation underNorway and Sweden in the 100–200 km depth range which is ingood agreement with the integrated 3D model, both in the locationof the maximum gradient and in amplitude. The S-wave tomographydepicts a sharp, eastward dipping velocity contrast, extending fromthe base of the crust to 400 km depth (see Fig. 13). The location ofthe largest velocity gradient is located 200 km westward comparedto the integrated 3D. Both tomographic models show low-velocityanomalies which continue to at least 400 km depth, a feature thatcannot be reproduced with a uniform sub-lithospheric mantle asassumed in the present modeling. This however does not rule out



Fig. 16. Vertical profiles of Common-Conversion-Point stacks of P-receiver functions from Frassetto and Thybo (2010). Station location and topography are shown at the top.Insets show the locations of the profiles. The plotting conventions are the same as in Fig. 15 but the color scale saturates at ±0.05. The depth of 410 km is indicated with ablack line. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)


variations in density of the underlying asthenosphere contributing tothe gravity signal. Such a sub-lithospheric gravity signal is generallyof low amplitude compared to the lithospheric field and its uncer-tainties, and only affects the ultra-long wavelength of the gravityfield which is filtered out from the present data. The comparison ofthe seismologically and numerically derived velocity models is onlyconsidered qualitatively here. A quantitative approach is not feasiblewith the unresolved differences in magnitude of P- and S-waveanomalies (see Section 3) as well as the simplification of verticallyhomogeneous SCLM composition employed in the numerical models.

The temperatures derived here correlate with the temperaturesderived for the upper mantle across Europe by Goes et al. (2000).Their study is also based on the transformation of P- and S-wave seis-mic tomographic velocities into mantle temperatures. While most ofthe Baltic Shield shows low temperatures, with for example 1000 °Cat 100 km depth and a lithospheric thickness of 200 km for southernSweden, southern Norway stands out with high temperatures of 1200to 1300 °C at 100 km depth, in good agreement with our results.

A step in the lithosphere implies large contrasts in viscosity andtectonothermal history of the adjacent domains. The region of themodeled boundary zone in the sub-lithospheric mantle largely coin-cides with the Permian Oslo Graben, with the main deformationzone during the Neoproterozoic Sveconorwegian orogeny, and with


the edge of the Paleoproterozoic Svecofennian domain. While thedifferent tectonic ages suggest independent evolution, their spatialcorrelation together with today's lithospheric step suggests multiplereactivation events. This indicates that the presence of the litho-spheric step observed today resulted, possibly indirectly, from structuralheterogeneities inherited from earlier Phanerozoic and Proterozoicevents.

5. Experimental modeling of Scandes mountain building

The presence of an anomalous mantle below southern Norway,as indicated by models derived from seismological data and obtainedby integrated geophysical modeling, could be important for its defor-mation style. In order to test this, analog experiments were carried outto investigate the effect of different lithospheric structures on the styleof lithospheric deformationwhich could be comparedwith present-daysouthern Norway (Agostini et al., submitted for publication). Thehypothesized (abrupt) lateral variations in composition, tempera-ture and thickness of the crust and lithosphere across southernFennoscandia imply related variations in rheological strength. Abruptcontrasts in rheological strength and/or the presence of anomalouslyweak or strong zones can have a strong influence on the localization



Fig. 17. Results of integrated 3D geophysical modeling. a) Overview of model domain. b) Observed and calculated elevation and Bouguer anomaly along a cross section at 61°N. c) Cross section at 61 N showing model geometry and densities.d) Cross section at 61°N showing seismic S-wave velocities. e) Comparison of modeled (red, blue) and observed (gray) velocity-depth plots. f) Cross section at 61°N showing temperature. (For interpretation of the references to color in thisfigure legend, the reader is referred to the web version of this article.)

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and mode of deformation of lithosphere that is subjected to horizontaltectonic loading (i.e. Willingshofer et al., 2005).

4D lithosphere-scale analog experiments were carried out toinvestigate the role of a weak lower lithosphere for the localizationof mountain building (Agostini et al., submitted for publication).The experimental models incorporate three distinct domains fromwest to east (Fig. 18): (1) an offshore continental shelf between theoceanic-continental lithosphere boundary (COB) and the shoreline;(2) a continental area about 300 km wide overlying variable thick-ness lithosphere; and (3) a cratonic area, about 200 km wide, whichcould be considered as analogs for the continental shelf, the areain which the southern Scandes mountains have formed and theFennoscandian craton respectively. As the width of the continentalshelf (from the COB to the coastal area) is not uniform in the studyarea, an average value of about 350–400 km is adopted. In eachdomain the experimental lithosphere consists of two or three differ-ent layers floating on a viscous fluid representing the asthenosphere.The top layer is composed of quartz sand as an analog of the brittleupper crust, whereas the deeper layers consist of mixtures of varioussilicon putties as analogs of the ductile lower crust and mantle litho-sphere. The models are characterized by a complex geometry withstrong lateral transitions in thickness and brittle–ductile strengthvariations (Fig. 18) designed as analogs for the northeastern Atlanticmargin with and without a weak lower lithosphere beneath theScandes. The analog models are scaled following the principles ofgeometric and dynamic-rheological similarity (Agostini et al.,submitted for publication; Weijermars and Schmeling, 1986).

Experimentswere performed in a transparent tankwith onemovingwall (acting as an analog for the oceanic lithosphere), which inducescompression on the analog lithosphere (Fig. 18). The applied compres-sion simulates the intracontinental horizontal stress arising from mid-Atlantic sea-floor spreading,which is considered as one possible drivingmechanism responsible for the inversion structures characterizing theNorwegian Sea, as well as for the building of the Scandes (e.g. Anellet al., 2009; Fejerskov and Lindholm, 2000; Pascal and Gabrielsen,2001; Vagnes et al., 1996).

In summary, the 4D analog experiments show that deformationin the relatively strong domains tends to be dominated by long-wavelength lithosphere folding, whereas deformation in the weakerdomains is characterized by localized uplift and subsidence (e.g. Fig. 19)(Agostini et al., submitted for publication). The main conclusion fromthe analog modeling experiments is that only models with a weakupper mantle rheology successfully predict the development of amountain range at the correct location of the present-day ScandinavianMountain Chain (Fig. 19, right), and with comparable morphologicalcharacteristics such as steep slopes dipping towards the coastlineand gentle slopes dipping towards the Fennoscandian inland areas(e.g. Lidmar-Bergström et al., 2000). Model results indicate that thelargest part of the deformation at the boundary between shelf andScandes is distributed in a narrow belt (in the order of 4–6 cm, scaledto natural values of ~80–120 km). However, the observed upliftedareas of the Scandes are 2–3 times wider. A possible explanation ofthis discrepancy is that unlike the actual lithosphere the model didnot incorporate or inherit a long and complex deformation history.The structures developed during the different tectonic phases, whichled to the present configuration, allowedwider distribution of deforma-tion. In nature, deformation is not randomly distributed but avoidsthe strongest regions and concentrates along pre-existing weakness,reactivating and focusing on them. The high number of preexistingstructures that could be reactivated may have resulted in the observedwide uplifted areas.

6. Implications for tectonic history and discussion

There are two main questions that arise in connection with theresults presented here: what is the origin of the lithospheric and


sublithospheric lower velocities below southern Norway? What istheir relation to the present-day topography?

6.1. Mantle diapir intrusion in the lithosphere?

The presence of low seismic velocities below southern Norway haspreviously been used as an argument in favor of a mantle diapirmodel for the uplift of the Scandes (Rohrmann and van der Beek,1996; Rohrmann et al., 2002). Following this model, uplift would becreated by intrusion into the lithosphere of some anomalouslywarm asthenospheric material flowing from the Icelandic hotspot30 to 20 My ago. Pascal and Olesen (2009) showed that the proposeddiapir would have to be too shallow and too warm to be solely respon-sible for the gravity low observed in southern Norway. Gravity model-ing (Ebbing et al., 2012; see below) taking into account the laterallyvarying crustal structure, shows however that the short-wavelengthpart of the gravity low is well explained by the crustal structure andthat only the long-wavelength component of the gravity low needs tooriginate from the upper mantle. The gravity field is shown inSection 4 to be consistentwith a composition and temperature anomalybelow southern Norway. The gravity field alone cannot therefore beused as an argument against the diapir model.

The underlying assumption behind the diapir model is that thetransit of the Icelandic hotspot from below Greenland to the west ofthe Atlantic Ocean, about 30 My ago, resulted in a light warm as-thenospheric material flowing eastwards, reaching the base of theScandinavian continental lithosphere and that a Rayleigh–Taylor in-stability allowed this buoyant material to rise into the lithosphere.The understanding of the consequences of impingement of hotmantle material below continental lithosphere has greatly improvedin recent years due to better modeling of the complex rheology ofthe continental lithosphere (Burov and Guillou-Frottier, 2005; Burovet al., 2007). It has been shown that the hot material from a plumehead can create Rayleigh–Taylor instabilities if the lithosphere iswarm (equivalent to a thermal age of 100 My) and the viscosity con-trast between lithosphere and asthenosphere is not too large. In thecase of lower asthenospheric viscosity, the plume head erodes thelithosphere but is not able to destabilize it, and in the case of a colderlithosphere (equivalent to 400 My age or more), the asthenosphericmaterial ponds at the bottom but does not erode the lithospheresignificantly. In addition, the consequence of such Rayleigh–Taylorinstabilities in terms of topography is more complex than the domeshape observed in southern Norway. Jurine et al. (2005) have empha-sized the role of the buoyancy ratio between the lithosphere and theplume material in the behavior of the plume and also have shownthat lithospheric penetration is unlikely for Archean and Proterozoiclithospheres unless an exceptionally strong mantle plume is involved.It seems therefore unlikely that some warm asthenospheric materialfrom Iceland could have destabilized an initially cold lithospherebelow southern Norway and caused uplift, and that the low velocitiesobserved today in the lithosphere are a result of this process.

6.2. Mantle diapir below the lithosphere?

However, even if intrusion of the lithosphere by a mantle diapirdoes not seem likely, it does not exclude the possibility that the lowvelocities observed below the lithosphere and down to 400 km depthcould be related to a general large area of low velocities below thenorthern Atlantic and that this area is related to the Iceland plume. Inthe tomographic model of Amaru (2007), the region of low seismic ve-locity located below Iceland has a branch towards the Baltic craton inthe depth range 100 to 400 km that correspondswell with the anomalythat we image in the body-wave tomography. The P-tomographymodels indicate that also a deep connection to the North Sea region tothe southwest may exist (Fig. 14). A similar asthenospheric featurehas been proposed to explain the nearby combination of moderate



Fig. 18. (upper) Cartoon showing the initial set-up for an analog experiment investigating lateral variations in crustal and lithosphere thickness, composition and rheology acrosssouthern Fennoscandia. UC = brittle upper crustal layer; LC = ductile lower crustal layer; UM = upper mantle layer. The black arrow indicates the direction of compression. Bottompanel shows rheological strength profiles at locations a, b and c. (lower) Same analog experiment, however, with a weaker upper mantle layer (green mix 2) for the central Scandesdomain. (after Agostini et al., submitted for publication).


Please cite this article as: Maupin, V., et al., The deep structure of the Scandes and its relation to tectonic history and present-day topography,Tectonophysics (2013), http://dx.doi.org/10.1016/j.tecto.2013.03.010


Fig. 19. Experimental results of (upper panels) model with a homogeneous upper mantle composition across southern Fennoscandia, and (lower panels) model with an anomalousweak upper mantle for southern Norway. (a) Top views of the model after 1% and 4% of bulk shortening. The black arrows indicate the direction of compression. (b) Shaded reliefpictures of the surface topography after 1% and 4% of bulk shortening. (c) Topographic evolution profiles taken from the center of the model parallel to the compression directionafter different amounts of bulk shortening. (d) Cross section of the model (trace is the dotted black line in panel a).


Please cite this article as: Maupin, V., et al., The deep structure of the Scandes and its relation to tectonic history and present-day topography,Tectonophysics (2013), http://dx.doi.org/10.1016/j.tecto.2013.03.010



topography and crustal structure in the northernUK (Davis et al., 2012),although the connection to the Iceland plume has less support in thisarea than in southern Norway according to regional tomographies(e.g. Pilidou et al., 2005).

It is however unlikely that a potential sublithospheric diapir hascreated the present-day topography in the Scandes. According toBurov and Guillou-Frottier (2005), sublithospheric material belowan old lithosphere (from 400 Ma) produces surface topography oflarger wavelengths (1500 km in their model) and smaller amplitude(400 m) than observed in southern Norway.

In addition, the lithospheric structure is likely to be close to iso-static equilibrium, reducing the need for a deep mantle componentin sustaining the topography. Ebbing (2007) and Ebbing et al. (2012)showed that, although Moho topography alone cannot isostaticallyexplain the topography of the Scandes, a more complex compensatedAiry–Pratt model that takes into account the presence of a laterallyvarying high-density lower crust, varying lithospheric thickness andlow-density crustal material in the Transscandinavian Igneous Belt(TIB) can together explain the gravity field very well. The additionalconstraints on Moho depth and crustal structure that have beenachieved in the present study, and that are largely already accountedfor in Ebbing et al. (2012), do not contradict these conclusions. Themore detailed knowledge of crustal structure that we have achievedin this and other recent studies enables us to state that the area is likelyto be close to isostatic equilibrium, a result suggested earlier based onthe anti-correlation between regional topography and Bouguer gravityanomaly (e.g. Balling, 1980).

6.3. Relation between metasomatism and the lateral variation inlithospheric structure

The gradual thickening of the lithosphere from Norway to Swedenin the model of Ebbing et al. (2012), which explains the long wave-lengths of the gravity data, is taken from the early studies of Rayleighwave dispersion along a few paths in Fennoscandia (Calcagnile, 1982).This lateral variation has been confirmed by the present study andmodeled with an improved horizontal and vertical resolution (Figs. 12,13 and 14). Surfacewave and gravity data have been shown to be consis-tent with a relatively sharp increase of 100 km in lithospheric thicknessclose to the eastern border of the Oslo Graben associated with a contrastin elemental depletion (Section 4, Gradmann et al., submitted forpublication). A change from fertile (zero-depletion)mantle below south-ern Norway to Proterozoic mantle below southern Sweden (0.6–0.9%depletion) is also required by buoyancy analysis (Artemieva, 2007).The geometry and location of the contrast proposed here fit rather wellwith the results of the P-wave tomography but overpredict the ampli-tude of the contrasts in the S-wave tomographic model by about a factorof 2, which we can consider as satisfactory considering the limitations oftomographic models and lithospheric modeling.

A contrast of 4% in S-wave velocity between southern Sweden andsouthern Norway may be explained by a temperature difference ofthe order of 400 °C in the 100 to 200 km depth range. The S-wavetomography implies a contrast of 200 °C over a lateral transitionzone of 100 to 200 km. It is commonly argued that large temperaturecontrasts over small distances cannot survive in the lithosphere overlong periods due to the equilibration effect of thermal conduction andthat compositional variations or the presence of melt is necessaryto explain abrupt seismic velocity variation in old lithospheres. Thedirect effect of compositional variations on seismic velocities has,however, been shown to be able to produce variations of about 1%only (e.g. Goes et al., 2004) and temperature is in many cases farfrom the solidus, making it unlikely that partial melt will be presentand significantly affect seismic velocities. Regardless of other possibleeffects, temperature variation is therefore expected to play a signifi-cant role in producing the seismic velocity contrasts that we observe.Compositional variations may however have an indirect influence on


seismic velocities by affecting either viscosity contrasts (Hieronymuset al., 2007) or concentration in radioactive elements (Hieronymusand Goes, 2010), that have a direct influence on the resultingtemperature.

Several mechanisms can produce increased mantle temperatures.Whereas ponding of hot mantle material (such as produced by man-tle plume, convection instability or delamination) would increasemantle heat flow supplied to the lithosphere, infiltration of magmasinto the lithosphere may produce its reheating from inside by thesupply of material enriched in radioactive elements. (Hieronymusand Goes, 2010). The timescales of the two processes are fundamen-tally different, the reheating by increased heat flow being transientwhile reheating by radioactive sources being basically steady-state due to the long half-life of the radioactive elements involved.Hieronymus and Goes (2010) showed that different degrees of litho-spheric depletion are associated with mild variations in the concen-tration of radioactive elements that suffice to maintain steady-statetemperature differences of 100 °C to 300 °C at mid-depth in the lith-osphere. Their model can explain an S-wave velocity contrast ofalmost 2% over a distance of 200 km by juxtaposing a depleted Archeanlithosphere with a less depleted Proterozoic lithosphere. They infer ab-solute S-wave velocities that fit well with those observed in Archeancratons and Proterozoic fold belts in the depth range 100 to 200 km(see Fig. 10).

In our case, the tectonic history and the results presented inSection 4 show that a juxtaposition of a Proterozoic-type and aPhanerozoic-type lithosphere would probably be more appropriate.Considering that this implies an overall lower degree of depletion, weanticipate an overall reduction in the absolute velocities but a similaramplitude of lateral variation. The velocities observed here may there-fore partially be the result of different degrees of depletion and thesharpness of the contrast does not require that the Norwegian litho-sphere has been locally replaced or heated recently. Hieronymusand Goes (2010) show that the surface heat-flow resulting from theirmodel is hardly affected by the deep temperature variations due tothe insulating effect of the surface layers. An interpretation of theupper mantle velocities in terms of temperature differences does nottherefore contradict the rather uniform and moderate surface heat-flow in southern Norway (Slagstad et al., 2009).

The formation of the Oslo Graben within the Baltic Shield has beenproposed to be associated with a localization of the deformation dur-ing Permian extension by the presence of a step in lithospheric thick-ness of at least 45 km just east of the Graben (Pascal and Cloetingh,2002). Albeit smaller, this step is remarkably similar to the one wesee in tomographic models today, suggesting that the step is olderthan 300 Ma. Enrichment in radioactive elements in southern Norwaycould be native or related to one or several subsequent geologicalevents. One candidate is local metasomatism in the lower lithosphererelated to the magmatic activity in the area at about 580 Ma. Metaso-matism has been proposed to explain the geochemical and petrologicaldata of the Oslo Graben magmatic rocks (Neumann et al., 1992) thathave been shown to derive from twomantle sources with different de-grees of depletion. Although the metasomatism inferred by Neumannet al. (1992) was recorded in the area of the Oslo Graben, it mighthave affected a larger area but be visible at the surface only in areaswhere volcanism brought some material close to the surface. On theother hand, geochemical analysis of mantle-derived rocks indicatesthat the lithosphere is depleted in the whole Baltic Shield, includingsouthern Norway (Andersen and Sundvoll, 1995). Very high degreesof depletion have also been inferred in the Western Gneiss Region(Beyer et al., 2006), where peridotites also bear evidence for the severaltectono-magmatic events that have formed the lithosphere and itscomplex history (Beyer et al., 2006; Lapen et al., 2009). There istherefore no unequivocal geological evidence for an enrichment ofthe lithosphere below southern Norway that could contribute to in-crease its temperature.




6.4. Metacratonization of the lithosphere

Another candidate for the rejuvenation of the lithosphere is theformation of the Caledonides and the associated subduction of thewestern margin of the Baltic craton, presently southern Norway. Ithas been proposed that the margins of cratons involved in interconti-nental collisions may partly subduct but still preserve major cratoniccharacteristics, and that southern Norway is a good candidate forsuch a metacratonic area (Liégeois et al., 2012). The consequence of“metacratonization” is reduced seismic velocities at the bottom of thelithosphere, as shown in the Saharan Craton where reduced velocitiesoccur below 100 km depth (Adbelsalam et al., 2011). These authors pro-pose several possible mechanisms at the origin of this velocity modifica-tion, in particular lithospheric delamination and convective removal.

6.5. Comparison with other intracontinental mountain chains

Several other regions worldwide show high topography awayfrom a recent plate boundary. We have already mentioned Greenland,located on the margin conjugate to Scandinavia. The Urals Mountainsare also a recent (0–5 My) topographic feature (Puchkov andDanukalova, 2009) situated at the location of an ancient Hercynianorogenic structure, and away from any present-day plate boundary.Their well-preserved crustal root (Carbonell et al., 1996) distinguishesthem from the Scandes and could be a remnant of the Hercynian struc-ture (Leech, 2001), but the present-day topography is inferred to resultfrom intraplate deformation (Puchkov and Danukalova, 2009). TheTransantarctic Mountains have a seismic structure more similar tothat of southern Norway. They are located at the outer edge of the East-ern Antarctic craton, next to the younger tectonic area of the Ross Sea.They display a well-developed topography of 3500 m associated witha small crustal root of about 5 km displaced towards the cratonic areacompared to the maximum topography, and low seismic velocities inthe lithospheric mantle. The topography, together with the seismicstructure and the gravity field, can be explained by buoyancy loads, inparticular mantle buoyancy, deflecting an 80 km thick elastic platebroken at the edge of the craton (Lawrence et al., 2006). The modelfor the formation of the Transantarctic Mountains may however notapply to the southern Scandes, due to the smaller elastic thickness inScandinavia and the anticorrelation between topography and Bougueranomaly, which is not observed in Antarctica, suggesting a lesser degreeof compensation in Antarctica. A recent receiver function study hasshown that another intraplate mountain range in Antarctica, theGamburtsev mountains, is underlain by a ca. 10 km thick crustal keel(Hansen et al., 2010), which may indicate that crustal isostasy main-tains the high topography. The Colorado Plateau also has similaritieswith southern Norway: high elevation (1800 m) in intraplate settingof a Proterozoic lithosphere. The Plateau is also associated with con-trasts in seismic velocities of up to 12%, but the lowvelocities are locatedat the edge of the high topography and not directly below, and are asso-ciated with Neogene magmatism at the surface. Edge convection thaterodes the lithosphere of the Colorado Plateau is able to explain theobservations well (van Wijk et al., 2010).

Although there are several intracontinental high elevation areaswith some features similar to the ones of the southern Scandes, itturns out that each mountain chain has its own characteristics, makingit unique and leading to different preferred mechanisms for their tec-tonic evolutions. This diversity probably reflects the diversity of theevolution of the continental lithosphere and the need to account for tec-tonic inheritance and a combination of differentmechanisms to explainthe presence of high topography in intracontinental settings.

7. Conclusion

Several high-resolution independent seismological models indi-cate the presence of low seismic wave velocities in the mantle


below southern Norway. P- and S-body wave tomographies as wellas a small depression of the 410 discontinuity suggest that the anom-aly extends down to the transition zone. The anomaly is locatedapproximately in-between the coast in the west and the Oslo Grabenin the east, underlying the highest topography of the southern Scandes.A major issue in interpreting the low velocities is the relation betweenthe lithospheric part of the velocity anomaly and the sublithosphericpart. Establishing a connection between the sublithospheric anomalyand the Icelandic plume and comparing to the lithospheric and sub-lithospheric structure in the northern part of the Scandes could shedlight on this issue. This would require extension of the tomographicand geophysical studies to a larger regional scale. New high-resolutioncrustal models suggest a crustal root that is not located right belowthe region of high topography. The seismic models combined with thegravity data show that crustal isostasy is not sufficient to explain themountain chain but that it contributes, together with lateral variationsin the lithospheric thermo-chemical structure, to a system that is likelyto be close to isostatic equilibrium. As such, dynamic support from themantle is not needed to explain the presence of the topography today.However, the presence of low velocities in the asthenosphere points tothe possibility for a minor dynamic mantle contribution.

Independently of the process that led to the formation of thetopography, it is clear that the geological history of the area has ledto major lateral variations in crustal and mantle structures that haveto be taken into account when searching for the mechanisms thatcreated the topography.

Acknowledgments

This work has been done in the framework of the ESF EUROCORESTOPO-EUROPE Programme07-TOPO-EUROPE-FP-014: The ScandinavianMountain Chain — deep processes (TopoScandiaDeep; www.geo.uio.no/toposcandiadeep). We acknowledge financial support from the ResearchCouncil of Norway, the Danish National Science Research Council,Deutsche Forschnungsgemeinschaft and Statoil. MAGNUS wave-forms were recorded with the mobile KArlsruhe BroadBand Array(KABBA) of the Karlsruhe Institute of Technology, Germany as wellas with permanent stations of the NORSAR array and the NorwegianNational Seismological Network. Financial support for the MAGNUSexperiment was provided by the Universities of Aarhus, Copenhagen,Karlsruhe and Oslo as well as NORSAR. Acquisition of the SCANLIPSdatawas supported by the Geological Survey of Norway and equipmentloans from the UK Natural Environment Research Council's GeophysicalEquipment Facility. We thank two anonymous reviewers for detailedand very constructive reviews.

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The deep structure of the Scandes and its relation to ...lithosphere.info/papers/2013-Scandes-Maupin-Tecto.pdf(e.g. Roberts, 2003; Torsvik and Cocks, 2005) and whose remnants have