Deep structure of the Vøring Margin: the transition from a continental shield to a young oceanic...

Deep structure of the V ring Margin: the transition froma continental shield to a young oceanic lithosphere

M. Ferna¤ndez a;�, M. Torne a, D. Garcia-Castellanos a;1, J. Verge¤s a,W. Wheeler b;c, R. Karpuz d

a Department of Geophysics, Institute of Earth Sciences (J. Almera), CSIC, Sole¤ Sabar|¤s s/n, 08028 Barcelona, Spainb Norsk Hydro Research Center, Sandsliveien 90, 5020 Bergen, Norway

c Geological Institute, University of Bergen, Bergen, Norwayd Norsk Hydro Saga Petroleum Iran, Bokharest Bld. Bokharest St., 15137 Tehran, Iran

Received 22 September 2003; received in revised form 14 January 2004; accepted 23 January 2004

Abstract

The present-day lithospheric structure across the Norwegian Margin in the V ring region is presented. The V ringMargin is characterized by the presence of large volume of magmatic underplating, thick Mesozoic basins, andprominent crustal thinning. Results from recent deep seismic experiments in the V ring Basin and V ring MarginalHigh and 3-D gravity modelling have been implemented in a regional 2-D lithospheric transect that runs from theNorwegian Caledonian Belt to the oceanic domain, crossing the transition between the Precambrian Fennoscandian/Baltic shield and the Cenozoic northern North Atlantic oceanic lithosphere. The modelling approach integrateselevation, gravity, geoid and heat flow data under the assumptions of thermal steady-state and local isostasy. Theresults confirm and refine the major trends of the crustal geometry which is characterized by a Moho depth varyingfrom about 45 km beneath the Caledonian thrusts, to 30 km beneath the Tr ndelag Platform, to 20^14 km beneaththe V ring Basin and to 13 km in the oceanic domain. The lithosphere thins from the Norwegian Caledonian Belt(190 km thick) to the oceanic domain (6 60 km) in a stepwise manner reflecting the progressive seawards migration oflithospheric deformation since the Paleozoic. Our lithospheric thickness results are different from those predicted bypost-rift lithospheric cooling models. In the oceanic domain, observed residual bathymetry (V600 m) is interpreted asthe remaining effect of a deep seated thermal perturbation during the upper Cretaceous which would also produce aheat flow anomaly of V15 mW m32. Alternatively, residual bathymetry can be interpreted in the absence of thermalanomalies as produced by depletion of the sublithospheric mantle extending down to 130^230 km depth depending onthe density contrast (10^30 kg m33).@ 2004 Elsevier B.V. All rights reserved.

Keywords: crustal structure; lithospheric structure; North Atlantic; continental margin; mantle depletion

0012-821X / 04 / $ ^ see front matter @ 2004 Elsevier B.V. All rights reserved.doi:10.1016/S0012-821X(04)00092-5

* Corresponding author. Tel. : +34-93-409-5410; Fax: +34-93-411-0012. E-mail address: [email protected] (M. Ferna¤ndez).

1 Present address: Faculty of Earth Sciences, Vrije University, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands.

EPSL 7028 25-3-04 Cyaan Magenta Geel Zwart

Earth and Planetary Science Letters 221 (2004) 131^144

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1. Introduction

The V ring rifted passive margin is located atthe central segment of the Norwegian Margin andforms part of the northern North Atlantic regionthat is recognized as one of the largest igneousprovinces of the world [1]. The present-day crustalcon¢guration is mainly a consequence of the Me-sozoic extension that followed the late PaleozoicCaledonian Orogeny resulting in the emplacementof large volumes of breakup-related magma, bothsubaerial and at crustal levels in the Paleocene.The continental breakup (V56 Ma) separatedGreenland from Fennoscandia and hence themargin represents the transition region betweenan old Proterozoic cratonic continental litho-sphere and a relatively young Cenozoic oceaniclithosphere. The evolution of the northern NorthAtlantic region (north of the Charlie^Gibbs Frac-ture Zone) is associated with deep mantle process-es which are seen as responsible for the majorfeatures observed in the area: seaward dippingre£ectors, magmatic underplating, major segmen-tation, the Iceland hotspot, and an anomalouslyshallow bathymetry (e.g. [1,2]).Several studies have been carried out recently to

image the crustal structure of the V ring Margin,showing that it is characterized by the presence oflarge volume of underplated material, thick Creta-ceous basins, and a signi¢cantly thinned continen-tal crust (e.g. [3^6]). Deep seismic experimentsand thermal modelling performed in the Fenno-scandian shield show that the lithospheric thick-ness attains values of up to 200 km [7^9]. In con-trast, the young age of the oceanic lithosphere (56Myr), implies a lithospheric thickness of 95 kmaccording to the half-space cooling model, oreven less, if the observed shallow bathymetry isattributed to thermal e¡ects [10]. The recently ac-quired seismic data allow for a well-constrainedcrustal model of the V ring Margin making theregion an excellent target to study the deep litho-spheric structure across a continental margin thattransits from continental-cratonic to young-oce-anic domains.In this context, the goals of this paper are two-

fold: (1) to determine the present-day lithosphericstructure from the old and thick cratonic conti-

nental domain to the young and thin oceanic do-main, and (2) to discuss the origin and present-day in£uence of the deep thermal perturbationthat a¡ected the northern North Atlantic region.The study has been performed along a 2-D, 750-km-long lithospheric transect in which we haveintegrated seismic, heat £ow, regional elevation,gravity and geoid data.

2. Geological setting

The Fennoscandian/Baltic region has under-gone a complex tectonic evolution characterizedby an overall NE^SW decrease in age of Archeanand Proterozoic deformation. Over these NW^SEArchean^Proterozoic belts is superimposed aroughly orthogonal trend of Paleozoic to Recentstructures (Fig. 1). The Precambrian Fennoscan-dian/Baltic shield consists of three tectonic do-mains: the northeastern Archean Domain (3.5^3.1 Ga) made up of island arc complexes thatcollided 2.0^1.9 Ga; the central Svecofennian Do-main formed by the growth and collision of juve-nile arcs (1.9^1.7 Ga) intruded by upper mantleand lower crust magmas developed in an exten-sional tectonic setting (1.7^1.5 Ga); and theSouthwestern Scandinavian Domain which devel-oped during the Gothian Orogeny (1.7^1.5 Ga)and was later reworked by the SveconorwegianOrogeny (1.2^0.9 Ga). Details on the Precambrianevolution of the Fennoscandian/Baltic shield canbe found in, among others, [11,12] and referencestherein.The Caledonian Orogeny (430^390 Ma) initi-

ated with the closure of the Iapetus ocean andthe consequent collision of Laurentia and Baltica.The Baltoscandian Margin consists of sequencesof thrust sheets transported several hundred kmto the east onto the Fennoscandian Precambrianbasement [13]. Seismic re£ection pro¢les showthat the stack of thrust sheets in the Central Ca-ledonides extend to 15^20 km depth well into thePrecambrian basement [14,15]. During the mid-dle^late Devonian, gravitational collapse pro-duced the extensional reactivation of thrust faultsresulting in the formation of half-graben basins.This phase was followed by a succession of exten-


M. Ferna¤ndez et al. / Earth and Planetary Science Letters 221 (2004) 131^144132

sional episodes that initiated in the late Carbon-iferous and culminated with the late Paleocene^early Eocene continental breakup and sea£oorspreading between Norway and Greenland (e.g.[16]). The late Paleozoic^early Cenozoic extensionwas responsible for the individualization of thethree geological provinces forming the V ringmargin: the Tr ndelag Platform to the SE, the

V ring Basin in the central region, and the V ringMarginal High to the NW which separates thecontinental domain from the oceanic domain ofthe Lofoten oceanic basin (e.g. [16,17]) (Fig. 1).The early Tertiary lithospheric breakup was ac-companied by the emplacement of subaerial andsubmarine basalts and abundant intrusive com-plexes along the ocean^continent transition in

Fig. 1. Geological map of the V ring Margin with the main structural units and the modelled pro¢le. Abbreviations inset: VM,V ring Margin study area; A, Archean Domain; B, Svecofenian Domain (Early Proterozoic); C, Southwestern Scandinavian Do-main (Middle-to-Late Proterozoic); D, Norwegian Caledonian Belt.


M. Ferna¤ndez et al. / Earth and Planetary Science Letters 221 (2004) 131^144 133

the whole northern North Atlantic region [1].From the early Eocene the margin evolution wasdominated by thermal subsidence and some local-ized tectonic inversion. More details on the Me-sozoic and Cenozoic geological and tectonic evo-lution of the V ring Margin can be found in[16,2,17^19] among many others.

3. Methodology and available data

This study reports the modelling of the present-day lithospheric structure of the V ring Marginalong a SE^NW trending transect perpendicu-lar to major structures. The transect initiates inthe onshore continental domain and crosses theTr ndelag Platform, the V ring Basin and theV ring Marginal High, and ends in the oceanicdomain, near the Jan Mayen fracture zone (Fig.2). The study requires the combination of variousobservables (seismics, elevation, Bouguer anom-aly, geoid height and surface heat £ow) that giveinformation on the density and temperature dis-tribution at di¡erent depth ranges. Calculationsare performed by using a ¢nite element code[20] which solves simultaneously the geopotential,lithostatic, and heat transport equations. The lith-ospheric section is divided into a mesh of trian-gular elements and a number of bodies with dif-ferent material properties: thermal conductivity,heat production and its variation with depth,

and density and its variation with pressure andtemperature (Table 1). The top of the model cor-responds to the Earth’s surface and the bottomto the lithosphere^asthenosphere boundary. Thedensity of the lithospheric mantle is assumed tobe temperature dependent and given by bm =ba(1+K(Ta3T(z)) according to [21], where ba isthe density of the asthenosphere, K is the thermalexpansion coe⁄cient, and Ta is the temperature ofthe asthenosphere. Temperature distribution andsurface heat £ow are calculated by solving thesteady-state heat transport equation assuming noheat £ow across the lateral boundaries and a ¢xedtemperature at the top and bottom of the modelof 5‡C and 1350‡C, respectively. The Bouguergravity anomaly is calculated using Talwani’s2-D algorithm [22] for each element of the meshwhich allows for density variations depending onpressure, temperature, and lithology. Regionswith negative topography (below see level) havebeen ‘¢lled’ with a material with the same densityvalue used in reducing Bouguer anomaly fromfree-air gravity data (2670 kg m33 in this study).The geoid anomaly is calculated using a 3D algo-rithm based on the resolution of the gravity po-tential for a regular prism and the Brun’s formula[23], modi¢ed for a 2-D ¢nite element grid (H.Zeyen, pers. commun.). Elevation is calculatedon every column of the mesh under the assump-tion of local isostasy and following [24]. Thedepth of isostatic compensation for calculating

Table 1Rock parameters used in present-day lithospheric modelling

Lithology Thermal conduct. Heat production Density(W m31 K31) (WW m33) (kg m33)

Tertiary/Quaternary sediments 2.2 1.2 2200Tertiary volcanics 2.1 0.5 2500Cretaceous sediments 2.4 1.2 2400*5.2 10310 PTriassic sediments 2.4 1.2 2650Pre-Cretaceous basement 2.5 1.6 exp(-z/15000) 2400*5.2 10310 PHigh-density crust 3.2 1.6 exp(-z/15000) 2850Upper crust 3.2 1.6 exp(-z/15000) 2650*1.4 10310 PLower crust 2.1 0.2 2950Oceanic crust 2.1 0.2 2850Underplating 3.0 0.02 3000Lithospheric mantle 3.2 0.02 3200a

a The density of the lithospheric mantle is temperature-dependent following bm =ba (1+3.5 1035 (Ta3Tz)) being ba = 3200 kgm33. ‘P’ denotes pressure (Pa) and ‘z’ depth (m).



the elevation and the gravity and geoid anomaliesis taken at the maximum depth reached by thelithospheric mantle, and the space between thisdepth and the base of the model is ¢lled withasthenospheric material with a constant density.A preliminar crustal and lithospheric structure is

introduced to the model according to the avail-able data. The resulting elevation, Bouguer anom-aly, geoid height variation, and surface heat £oware compared with the measured values and thegeometry of the crust and the lithospheric mantleis modi¢ed in those regions where they are poorly

Fig. 2. Observables used to model the present-day lithospheric structure of the V ring margin (see text for data sources). (a) Ele-vation, contours every 500 m. Filled circles and squares indicate OBS and ESP data, respectively. (b) Bouguer anomaly, contoursevery 25 mGal. (c) Geoid height, contours every 1 m. (d) Surface heat-£ow (mW m32) superposed on bathymetry, solid circlesdenote the locations of sea£oor heat-£ow measurements. Solid line indicates the location of the modelled lithospheric pro¢le.



constrained until the best ¢t is obtained. For amore detailed description of this modelling tech-nique, the reader is referred to [20,25].The crustal structure has been initially con-

strained from seismic data (Fig. 4). In the oceanicdomain, we have used ocean bottom seismograph(OBS) data [5] and expanded spread and wideaperture CDP pro¢ling data [26,27]. Wide-angledata show a high velocity layer (Vp= 6.8^7.8km/s) at lower crustal levels with a thickness vary-ing from 3 km (OBS-16) to 8 km (EPS-13) atmagnetic anomaly 22, and 16 km underneaththe V ring Marginal High (magnetic anomaly24, OBS-13 and ESP-12). The shallow structurehas been taken from [17,19] (cross section F^FP).For the V ring Marginal High and V ring Basin,we have used the crustal structure obtained froma combined 3-D seismic and gravity analysis [6] towhich we have added a body representing Terti-ary volcanics related to seaward dipping re£ectors(OBS-25, [5]). According to this model, under-neath the Cenozoic and Cretaceous sedimentarycover (up to 10 km depth) there is a Pre-Creta-ceous layer laying on top of a relatively high-den-sity upper^middle crust. At deeper crustal levels, ahigh-velocity/high-density lower crustal body withsigni¢cant lateral thickness variations is recog-nized along the V ring Marginal High extendingthroughout most of the V ring Basin. In theTr ndelag Platform and the o¡shore section ofthe pro¢le we have taken seismic results from[28,29], respectively. These analyses indicate thatthe Moho depth increases sharply from about 25km underneath the Tr ndelag Platform to morethan 45 km in the central part of the Caledoniandomain.Regional data have been compiled from di¡er-

ent sources. Elevation and free-air gravity datawere obtained, respectively, from the EuropeDTM2.5 model and [30]. Both datasets wereused to calculate the simple Bouguer anomaly o¡-shore with a density reduction of 2670 kg m33,whereas onshore we used the NGU (GeologicalSurvey of Norway) Bouguer anomaly map [31].Geoid height data were obtained from theEGM-96 model expanded up to degree and order360 [32]. Heat £ow data have been compiled from[33,34] o¡shore, and [35] onshore. Fig. 2 shows

the maps of the observables used and the locationof the modelled pro¢le.Elevation reaches average values of 800 m in

the Caledonide front, decreasing very rapidly tosea level. The Tr ndelag Platform, which is about250 km wide, shows bathymetries of 0^400 m,whereas the V ring Marginal High is character-ized by water depths of about 1400 m. The deep-est parts of the oceanic domain remain at depthsof 3000^3500 m. The Bouguer anomaly mapshows negative values onshore, around 340mGal, increasing up to 250 mGal in the oceanicdomain. Superimposed on this regional trend, theV ring Margin is characterized by a succession of

Fig. 3. Results of the present-day lithospheric structure mod-elling. (a) Elevation. (b) Bouguer anomaly. (c) Geoid height.(d) Surface heat-£ow. Thick solid lines denote calculated val-ues. Gray strips (a^c) and gray dots with bars and wide strip(d) denote measured values and associated scatter. (e) Mod-elled geometry (see Fig. 4 for details).



relatively short-wavelength gravity highs andlows, which re£ect the irregular structure of thebasement [6]. Unlike elevation and Bougueranomaly, which show a NW^SE oriented gra-dient, the geoid height map shows an E^W ori-ented gradient with values increasing from 36^40m onshore to 48^52 m in the oceanic domain. Thesurface heat £ow map shows onshore valuesaround 50^60 mW m32, whereas o¡shore themeasurements, typi¢ed by high scatter, rangefrom 30^40 mW m32 to more than 90^100 mWm32. The heat £ow in the V ring Basin andV ring Marginal High ranges from 50 to 75mW m32 [34]. These values have not been cor-rected for recent sedimentation (around 1 kmthickness since Pliocene) and, after correction,could increase about 10%.

4. Results

The modelled pro¢le runs perpendicular to the

V ring Margin with a total length of about 750km. To compare the results with geophysical ob-servations we have averaged the grided regionaldata (elevation, Bouguer anomaly and geoidheight, Fig. 2) within a 50-km-wide strip centeredover the pro¢le. Heat £ow data have been aver-aged over a 100 km width because of their scar-city. The rock parameters we used in the modelare summarized in Table 1. For the V ring Basincrust we use the density values proposed by [6],whereas in the Tr ndelag Platform and in the o¡-shore section we use an average pressure-depen-dent density according to the Vp vertical gradientobserved in seismic refraction data [36]. Theremaining model parameters are taken from com-mon values in literature. Fig. 3 shows the ob-tained results with the best ¢tting model. Cal-culated elevation, Bouguer anomaly and geoidheight are in generally good agreement with ob-servations with the exception of short-wavelengthmis¢ts, which can be attributed to local variationsin shallow crustal structures. The calculated heat

Fig. 4. Modelled crustal structure and seismic constraints. Solid lines correspond to the crustal geometry deduced from 3-D grav-ity and seismic analysis [6]. Dashed lines correspond to the crustal structure derived from [28] and dotted-dashed from [29]. Ar-rows indicate the position of OBS and their respective number following [5]. Abbreviations: se, bottom of sediments; uv, bottomof upper volcanics; lv, bottom of lower volcanics; pos, bottom of pre-opening sediments; cc, bottom of continental crust; Mh,Moho. Corresponding model parameters are listed in Table 1.



£ow shows a gentle seaward increase from valuesof 45^50 mW m32 on land to around 75Y 10 mWm32 in the oceanic domain, although the highscatter of measured data makes it di⁄cult to eval-uate the apparent match.The lower panel in Fig. 3 shows the modelled

lithospheric structure. The most outstanding fea-ture is that the lithospheric thickness varies fromabout 190 km beneath the Caledonide front toless than 60 km beneath the 50 Myr old oceaniccrust (near magnetic anomaly 21). This thinning ismainly tuned by the simultaneous ¢tting of theabsolute elevation and the geoid height whichare particularly sensitive to the density distribu-tion at subcrustal levels. Therefore, the step-likelithosphere^asthenosphere boundary is related tothe observed geoid undulations of about 200 kmwavelength. We infer that the ‘steps’ re£ect thesuperposition of di¡erent tectonothermal eventssince the Paleozoic.Fig. 4 shows the crustal structure from our

model which con¢rms and re¢nes the resultsfrom previous seismic studies. Major discrepan-cies are found in the oceanic domain, wherethe thickness of the lowermost layer (underplat-ing or anomalous oceanic layer 3) obtained fromour model is of about 5 km instead of the3 km found from OBS-16 [5] or the 8 km foundfrom ESP-13 [26]. However, OBS-16 is locatedat the end of the OBS pro¢le and hence the res-olution is not very good, and ESP-13 has beenprojected onto the pro¢le from a distance of 100km and lateral variations of the crustal structureare likely to occur (Fig. 2a. The northwesternthinning of the anomalously thick layer 3 ismainly constrained by the Bouguer anomaly,which requires a more gentle decrease in thicknessthan that proposed by [5]. In the V ring Basinregion, there is a good agreement with the crustalstructure proposed by [6] from 3-D seismic andgravity modelling, and minor discrepancies atdeep levels are expected given the smoothing en-forced in our 2-D approach. In the Tr ndelagPlatform, our results show a roughly £at Moholying at 30 km depth, which deepens sharplydown to 45 km in the emerged section of thepro¢le, in agreement with regional seismic data[28,29].

5. Discussion

The obtained lithospheric structure is mainlybased on the depth-density distribution obtainedfrom the integrated modelling of di¡erent geo-physical observables (seismics, gravity, geoid, ele-vation, and surface heat £ow). Major assumptionsof the model are: (1) the density of the lithospher-ic mantle is only temperature dependent, (2) ther-mal steady-state, and (3) the density of the sub-lithospheric mantle is constant everywhere.The ¢rst assumption presumes that the density

of the lithospheric mantle is governed by the ther-mal expansion coe⁄cient and neglects possiblecompositional changes related to chemical mantledepletion, which may a¡ect the bulk mantle den-sity. A recent study [37] shows a secular variationin lithospheric composition and the subsequentchange of average mantle density which, at stan-dard conditions, increases from Archean (3310Y16 kg m33) to Proterozoic (3340Y 20 kg m33) toPhanerozoic (3360Y 20 kg m33). Although the SEend of our pro¢le probably crosses the transitionfrom the Phanerozoic to early^middle Proterozoicdomains, the geometry and boundary of thedi¡erent-aged lithospheres and the variations inaverage mantle density are highly uncertain.Hence we have not included the e¡ects of seculardensity variation in our model. On the otherhand, the choice of the thermal expansion coef-¢cient value may a¡ect the results since highervalues will result in higher average mantle den-sities and then lower lithospheric thicknessesto keep the observed elevation and geoid anom-aly. The values reported in the literature for thethermal expansion coe⁄cient of the lithosphericmantle range between 3.1^3.2U1035 K31 and3.94^4.1U1035 K31 ([21,38,25], respectively). Fol-lowing [39], the thermal expansion coe⁄cientis commonly underestimated and these authorspropose a value of 3.85U1035 K31 based on ex-perimental data. In the present study we haveused an intermediate value of 3.5U1035 K31

and have calculated the in£uence of varyingthe thermal expansion coe⁄cient by Y 0.5U1035

K31 on the lithospheric thickness. The resultsshow that the depth of the lithosphere^astheno-sphere boundary varies Y 10 km beneath the Ca-



ledonian domain and Y 5 km beneath the oceanicdomain.The assumption of thermal steady-state condi-

tions is strictly valid along most of the continentalsection of the pro¢le particularly in the Tr ndelagPlatform and the Caledonian Domain. Near theV ring Marginal High and along the oceanic do-main, however, the temperature distribution has anoticeable transient component. Nevertheless, thelithospheric structure obtained from our model-ling approach is mainly constrained by the simul-taneous ¢tting of density-depending observablessuch as elevation, gravity, and geoid. In this sense,the steady-state temperature distribution allows usto calculate in ¢rst approach the lithosphericmantle density and the present-day surface heat£ow. The departures from simple lithosphericcooling models are discussed below. A more pre-cise temperature distribution would require a de-tailed knowledge of the evolution of the marginand additional constraints as the tectonic subsi-dence through time, which are beyond the scopeof this study.The third assumption is much more debatable

particularly in settings corresponding to largeigneous provinces where the excess of magmatismcan result in the depletion of heavy elements inthe sublithospheric mantle and hence in a densityreduction of 10^35 kg m33 depending on the de-gree of melting [40]. Since this density reductionmay extend down to 200 km depth [41,42], itse¡ect on elevation and geoid height and there-fore in the resulting lithospheric structure can benoticeable. We will treat this question further inmore detail.Bearing in mind these considerations, the re-

sults obtained in this study reveal some majorcharacteristics of the transition from the Fenno-scandian/Baltic shield to the Cenozoic oceaniclithosphere in the V ring Margin region. Further-more, the inferred crustal and lithospheric struc-ture gives some additional constraints on thegeodynamic evolution of this portion of the Nor-wegian Margin.

5.1. Crustal structure

According to our results, the V ring Margin

crust thins regionally from ca. 45 km beneaththe Caledonian Front onshore to ca. 10 km inthe oceanic domain. However, the thinning aswell as the internal structure of the crust is farfrom homogeneous. The most relevant featuresare: (1) the apparent lateral density variations atmid-crustal levels landward from the V ring Ba-sin, and (2) the progressive thinning of the lowercontinental crust beneath the Tr ndelag Platform.The 3-D seismic and gravity study carried out

in the V ring Basin [6] shows that the density ofthe crust underlying the Pre-Cretaceous sedimentsmust be about 2850 kg m33. The lack of detailedseismic information made us to use a pressure-de-pendent density for the whole upper^middle crustin the Tr ndelag Platform and the onshore re-gion. According to this P-b relationship (Table1), an average density of 2850 kg m33 is obtainedfor the depth interval between 15 km (b=2800 kgm33) and 25 km (b=2900 kg m33), whichstrongly suggests that the high-density mid-crustalbody in the V ring Basin is extended Precambrianmiddle crust similar to that existing beneath thepresent-day Caledonian thrust sheets. Neverthe-less, our model cannot di¡erentiate between alter-native interpretations on the origin of the highdensity mid-crustal body in the V ring Basinwhich could imply either Cenozoic massive mag-matic intrusions, or a separated crustal block ex-humed during the Caledonian Orogeny [6].High Vp values (s 7.0 km/s) at deep crustal

levels are observed beneath the V ring Basin,the Tr ndelag Platform, and the ScandinavianCaledonides (e.g. [36,29,28,5]). However, whetherthese three areas of high-velocity lower crust arethe same in nature and origin, and whether theorigin is related to the opening of the northernNorth Atlantic or to Precambrian rifting process-es, is a controversial question. Here, our interestis to constrain the lateral extent of the early Ce-nozoic underplating and how far it penetratedbeneath the continental domain. We observethat the lower crust thins progressively alongour modelled pro¢le, and virtually thins out be-neath the Tr ndelag Platform; also that mag-matic sills are restricted to the V ring Basin [6].These two relations give support to the interpre-tation that the high velocity body beneath the



V ring Basin is distinctly di¡erent from that be-neath the Tr ndelag Platform^Caledonian do-main, both in nature and origin. Hence we followthe interpretation that the high velocity body be-neath the V ring Basin was emplaced during thelate Paleocene^early Eocene rifting as a conse-quence of continental breakup, whereas the ma¢ccharacter of the high-velocity lower crust beneaththe Tr ndelag Platform and the Caledonian do-main would have been inherited from Archeanrifting and later collision of the Fennoscandian/Baltic craton at 2.0^1.5 Ga [29].

5.2. Lithospheric mantle structure

The V ring Margin lithosphere mantle is char-acterized by the pronounced rising of the base ofthe lithosphere from 190 km depth beneath theNorwegian Caledonian Belt to less than 60 kmin the oceanic domain. This pattern is very similarto that obtained along a pro¢le from the BalticSea to the North Sea at 60‡ N latitude, derivedfrom teleseismic P-residuals [43]. Moreover, esti-mations from seismic experiments and thermalmodels carried out along the northern segmentof the European GeoTraverse in the Fennoscan-dian shield [7^9,35] show that the lithospherereaches a thickness of 150^200 km in the northernand central parts of the shield, decreasing to val-ues of 100 km in the southern Baltic Sea. Similarvalues are found from a global thermal study [44]in which the authors deduce that the thickness ofthe continental lithosphere generally decreaseswith age from s 200 km in Archean cratons, to200Y 50 km in early Proterozoic lithospheres, toabout 140Y 40 km in middle and late Proterozoiccratons.The base of the lithosphere re£ects a step-like

geometry with lithospheric thickness of about 190km beneath the Caledonide front, 140 km beneaththe Triassic^Jurassic Tr ndelag Platform, 115 kmbeneath the Cretaceous V ring Basin, and 60 kmbeneath the early Cenozoic oceanic lithosphere.The inferred values coincide well with those pro-posed for di¡erent tectonothermal regions [37].These authors ¢nd, based on global geochemicaland thermal analyses, that the lithospheric thick-ness varies with the tectonic setting, being about

150^180 km in Proterozoid cratonic areas, 100^140 km in Phanerozoic ares, and 60^100 km incontinental rifted areas. Therefore, we infer thatthe geometry of the lithosphere^asthenosphereboundary is re£ecting the successive tectonother-mal events occurred in the region.In the oceanic domain, lithospheric cooling

models (e.g. [21,45]) predict a t1=2 dependence ofthe ocean-£oor topography and heat £ow, andde£ection of isotherms away from the oceanicridge for lithospheres younger than 70 Myr. Asimple calculation yields a predicted lithosphericthickness of around 90 km for the modelled oce-anic section, whereas the values obtained fromour model vary from 50 km in the youngest oce-anic crust at 80 km in the oldest part. Similardiscrepancies in expected vs. observed lithosphericthickness have been reported along a lithosphericpro¢le that crosses the Hatton Bank and ends atthe mid-Atlantic ridge [10] where the authorsfound, based on a combined seismic and gravityanalysis, a lithospheric thickness of 65^80 km be-tween magnetic anomalies 21 and 24.

5.3. Mantle depletion vs. remnant thermal anomaly

Discrepancies between our results and the pre-dicted values from lithospheric cooling models arenot restricted to lithospheric thickness but arealso evident in bathymetry and sea£oor heat£ow. The observed bathymetry in the oceanic do-main of the V ring margin ranges from 1400 to3200 m in old^young direction, which clearly con-trasts with the predicted values of 5100^4900 mfrom cooling models. Similarly, the modelled heat£ow ranges between 65 and 85 mW m32, whereasthe predicted values are around 60 mW m32.Lithospheric cooling models predict ocean-£oorsubsidence with age, disregarding sediment load-ing and assuming that crustal thickness is roughlyconstant. Thereby, departures from predicted ba-thymetry can be partly attributed to the sedimen-tary cover and to oceanic crustal thickness varia-tions. We have calculated the contribution ofsediments by replacing the sedimentary layer bywater in each column and considering that thedensity of sediments depends on the total sedi-mentary thickness [46]. The contribution of crus-



tal thickness variations has been computed bysubstituting the anomalously-thick layer III (witha density of 3000 kg m33 in our model) by uppermantle with a density contrast of 300 kg m33.After these calculations, the resulting bathymetryslightly increases with age following a trend sim-ilar to that expected from lithospheric coolingmodels, but lying 600 m shallower. The shallowbathymetry would correspond to a lithosphericage of 30 Myr, and not to the 56 Myr age inferredfrom magnetic anomalies (Fig. 5). This residualbathymetry (600 m) as well as the heat £owanomaly (about 10^15 mW m32) in the oceanicdomain indicates that a large and persistent ther-mal anomaly acted in the region, the e¡ects ofwhich would be still noticeable.An alternative explanation to the residual ba-

thymetry is that the sublithospheric mantle hasshelted lateral density variations related to mag-matism and chemical depletion. The density de-crease produced by depletion of heavy elements inthe sublithospheric mantle during melting andmagma migration is quite uncertain [47]. Hence,we have calculated the two end-member densitycontrast in the sublithospheric mantle (310 kgm33 and 330 kg m33, respectively), assuming a‘normal’ thickness for the lithospheric mantle inthe oceanic domain. The depleted region reachesdepths of 130^230 km, depending on the consid-ered density contrast, and extends landwards until

the Gjallar Ridge in the V ring Basin to maximizeits radius of in£uence. Fig. 6 compares the di¡er-ent lithospheric structures and the calculated sur-face heat £ow which is the only ‘observable’ thatwe allow to vary due to the lack of data. Calcu-lated elevation, gravity and geoid keep matchedwith the observed values. Lowering the densitycontrast below 310 kg m33 produces mis¢ts inBouguer anomaly and geoid height since thedepth of the compensation level must be in-creased. A model in which depletion is restrictedto the lithospheric mantle with an average densitycontrast of 320 kg m33 can also ¢t the observ-ables (not shown in Fig. 6).

5.4. Geodynamic implications

The most widely proposed mechanisms to ex-plain the evolution of the northern North Atlanticregion involve either the presence of a mantleplume (e.g. [1,2,47,48]) or, alternatively, second-ary mantle convection (e.g. [49^51]). Althoughfrom our analyses we cannot elucidate which ofthese mechanisms is the most suitable to account

Fig. 5. Comparison between the observed and reduced ba-thymetry obtained after subtracting the e¡ects of sedimenta-tion and underplating (see text). Thick lines indicate the pre-dicted bathymetry according to the half-space cooling modelfor ages of 55 and 30 Ma.

Fig. 6. (Top) Calculated surface heat £ow for models consid-ering depletion in the sublithospheric mantle (solid thick line)and model without mantle depletion (dashed thick line);measured values as in Fig. 3d. (Bottom) Geometry of modelswith depleted regions in the asthenosphere where densitycontrasts relative to ‘normal’ asthenosphere of 310 and 330kg m33 have been considered, respectively. In both modelsdepletions extends downward from the base of the litho-spheric mantle. Thick dashed line denotes the base of thelithosphere corresponding to model without depletion (Fig.3e).



for the Cenozoic evolution of the V ring Margin,arguments to favor a deep seated large-scale pro-cess as demanded by mantle plume or secondarymantle convection come from: (1) an anomalouspresent-day ocean bathymetry which is also rec-ognized over the whole northern North Atlanticregion; (2) a pronounced lithospheric thinningfrom the Fennoscandian shield to the Cenozoicoceanic domain which is likely to occur also inthe conjugate Greenland margin; and (3) a cor-rected ocean bathymetry which coincides with thesubsidence experienced by an oceanic lithosphereaged 30 Myr.The presence of a deep seated thermal anomaly

impinging the base of the lithosphere during theUpper Cretaceous would have reduced the mantleviscosity, allowing for a uniform thinning in thecentral part of the rift as proposed for warmlithospheres [52]. Under these conditions, mostof the lithospheric strength resides in the crust,which is dominated by a brittle/plastic rheologyand can fail abruptly. We infer that the persis-tency of the thermal anomaly until the continentalbreak-up prevented mantle cooling and the ‘run-away lithospheric thinning’ proposed for someareas of the North Atlantic (e.g. the Orphan ba-sin; [52]) and is partly responsible for a sharpocean^continent transition as exhibited by manyvolcanic continental margins [48]. Whether or notthe thermal e¡ects of such a deep-seated large-scale process have completely extinguished orare still remaining is a controversial question [1].One possibility is that the thermal e¡ects of an

Upper Cretaceous mantle plume (or mantle con-vection) could persist still presently and be re-sponsible for the shallow bathymetry observedin the region and an anomalously high surfaceheat £ow which, according to our modelling,would exceed the expected value by 10^15 mWm32. A recent study based on the total tectonicsubsidence variation and the regional gravity ¢eldcarried out in the Hatton continental margin [10]indicates that the e¡ects of a large end deep ther-mal anomaly are not restricted to the oceanic do-main but extend to the Rockall Trough and Hat-ton Bank continental domains. In this sense,Saunders et al. [1] state that the thermal in£uenceof the Iceland plume may be as widespread as it

was during early Tertiary times, at least to thesouth of Iceland.A long lived thermal perturbation is refuted by

several authors who claim for a very rapid de-crease of the anomalous potential temperature inthe mantle after the continental breakup [47,53]or for a very low remaining temperature anomaly(about 5^6 K in the V ring Margin [54]). If this isthe case, the only candidate to explain a regionalresidual bathymetry of about 600 m in the ab-sence of thermal anomalies is mantle depletion.However, far from active melting regions, large-scale mantle convection tends to mix the formerdepleted mantle with undepleted material reduc-ing the density anomaly and the ability of thismechanism to cause permanent uplift [54].It seems therefore clear that the vertical move-

ments experienced by the margin(s) after the con-tinental breakup have been in£uenced by a sub-lithospheric thermal anomaly whose intensity andduration are not well known yet. In particular,the depth to which the rifted cratonic lithospheremust thermally recover and the feedback betweenlarge lateral thermal gradients and mantle convec-tion may have played an important role in theevolution of northern North Atlantic region.

Acknowledgements

This work was supported as a collaborativeproject between the Institute of Earth Sciences^CSIC of Barcelona (Spain) and the NORSK^HY-DRO Research Centre of Bergen (Norway) andwas developed in the framework of the ‘Grupd’Estructura i Processos Litosfe'rics’ 2001 SGR00339. K. Louden, C. Lowe and J. Skogseid pro-vided very constructive comments on an earlierversion of this paper.[SK]

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