Dynamic models for mantle flow and seismic anisotropy in ...geo.mff.cuni.cz/~oc/ggg07.pdf · 3]...

Dynamic models for mantle flow and seismic anisotropy inthe North Atlantic region and comparison with observations

Gabriele MarquartSRON and Institute of Earth Science, University of Utrecht, Postbus 80.021, 3508 TA Utrecht, Netherlands([email protected])

Harro SchmelingInstitute of Earth Sciences, Geophysics Section, J. W. Goethe University, Feldbergstrasse 47, D-60323 Frankfurt,Germany

Ondrej CadekDepartment of Geophysics, Faculty of Mathematics and Physics, Charles University, V Holesovickach 2, 180 00 Praha 8,Czech Republic

[1] Dynamic mantle flow and temperature models for the North Atlantic based on a regionalized P-waveand a global S-wave tomography model were derived under the constraint of a maximum fit to theobserved gravity field. For the regional flow model Cartesian geometry, temperature- and depth-dependentviscosity and a free slip surface were assumed, while the global model assumed a radially dependentviscosity and kinematic plate velocity boundary conditions. Both models show pronounced upwellingswithin the upper mantle beneath the Iceland area and the lower mantle beneath, the regional modelcontaining a lateral shift associated with a horizontal flow near 660 km depth. The upper mantletemperature field of the regional model shows two distinct anomalies, one beneath Iceland and the westerlyadjacent regions with a connection to a deep mantle root and an excess temperature of 200�C, and a secondone below 300 km at the Kolbeinsey Ridge with an excess temperature of 120�C. These anomalies do notappear to be connected. An essentially radial flow pattern is found south of Iceland with ridge parallel flowalong Reykjanes and divergent flow at the Kolbeinsey Ridge. The long-wavelength global model does notshow such details but is characterized by a NE-SW elongated upwelling flow beneath Iceland and a ridgeperpendicular flow within the upper mantle. From the modeled flow fields, lattice preferred orientations(LPO) of olivine are calculated. For the regional model, azimuthal seismic anisotropy is predicted with fastdirections diverging away from Iceland and the Kolbeinsey ridge. The global model predicts roughly ridgeperpendicular fast directions. Comparison of predicted with observed seismic anisotropy models showsregions of good agreement north, east, and SE of Iceland, as well as for Iceland. No agreement is foundbeneath the Reykjanes Ridge area, leading to the speculation that the fast directions are perpendicular tothe flow due to a change in the LPO generating mechanism. Regarding geochemical findings, the regionalflow model can explain plume-related geochemical signatures observed on the Reykjanes Ridge andpredicts a deep, hot melt zone beneath the Kolbeinsey Ridge without plume tracers.

Components: 14,393 words, 10 figures, 1 table.

Keywords: North Atlantic; Iceland; Reykjanes Ridge; Kolbeinsey Ridge; mantle flow field; seismic anisotropy.

Index Terms: 8121 Tectonophysics: Dynamics: convection currents, and mantle plumes; 8137 Tectonophysics: Hotspots,

large igneous provinces, and flood basalt volcanism; 8180 Tectonophysics: Tomography (6982, 7270).

Received 5 May 2006; Revised 6 September 2006; Accepted 27 October 2006; Published 20 February 2007.

G3G3GeochemistryGeophysics

Geosystems

Published by AGU and the Geochemical Society

AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES

GeochemistryGeophysics

Geosystems

Article

Volume 8, Number 2

20 February 2007

Q02008, doi:10.1029/2006GC001359

ISSN: 1525-2027

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http://dx.doi.org/10.1029/2006GC001359

Marquart, G., H. Schmeling, and O. Cadek (2007), Dynamic models for mantle flow and seismic anisotropy in the North

Atlantic region and comparison with observations, Geochem. Geophys. Geosyst., 8, Q02008, doi:10.1029/2006GC001359.

1. Introduction

[2] This study focuses on the structure of mantleflow in the North Atlantic region, in particular on thelarge-scale region around the Iceland plume whereplume-driven flow and spreading-driven flow fromthe Mid-Atlantic Ridge System are superimposed.On the basis of tomography and gravity data, amodel for mantle dynamics in the North Atlantichas been derived in a previous study [Marquart andSchmeling, 2004]. Complemented by another glob-ally based flow model we study some consequenceson geophysical and geochemical observables. Sincemantle flow pattern is expected to give rise toseismic anisotropy, the flow fields are used todetermine the related elastic deformation tensor ofolivine (based on the formalism of Kaminski andRibe [2001, 2002]) and compare the direction of thefast axis to recent results on fast seismic velocitydirections [e.g., Bjarnason et al., 2002; Li andDetrick, 2003]. Another constraint on mantle flowat least along the ridges can be found in the geo-chemical signature of mid-oceanic ridge basaltsdrilled and dredged along the Reykjanes and Kol-beinsey Ridges. Therefore the mantle flow fieldderived in this study will also be discussed in thelight of geochemical observations.

[3] Iceland is one of the few locations on Earthwhere a mantle plume and a spreading ridge coin-cide. This is not a permanent setting, since litho-spheric plates and their boundaries move withrespect to the mantle, and plumes are believed tobe either fixed in the mantle or move with differentspeeds relative to plate boundaries [Steinberger andO’Connell, 1998]. In the North Atlantic the plumearrived under central or southeastern Greenlandabout 58 Ma ago [Torsvik et al., 2001], presumablycaused rifting and plume-related volcanism in west-ern, northern and eastern Greenland [Skogseid et al.,1992;White, 1997; Scarrow et al., 2000] and finallyinitiated massive volcanism and continental breakupbetween the North American and Eurasian Plate.Thus, in the early phase the plume was located to thewest of the ridge, which may not only have causedthe ridge jump from the extinct Aegir Ridge to thepresent Kolbeinsey Ridge north of Iceland (seeFigure 1), but may also have contributed to a higherplate velocity of the North American Plate comparedto the Eurasian Plate. Today the plate velocity of

Iceland on theNorthAmerican Plate at 65�N is about2.6 cm/a in roughly westward direction compared to1.4 cm/a in SW direction on the Eurasian side;arrows of motion are shown in Figure 1 (values referto the hot spot reference frame based on the HS3-Nuvel1a plate motions [Gripp and Gordon, 2002]).These plate motion vectors indicate a westwardmigration of the spreading ridge with approximately1 cm/a. This migration of the ridge system in respectto the mantle very likely explains the observationthat after the strong initial volcanic phase in theNorth Atlantic Basalt Province volcanic activityceased but was renewed about 20 Ma ago. By thistime the ridge axis has migrated close enough to thedecaying plume that hot upwelling plume materialcould rise to a shallower level leading to increasedmelt production and thus to a renewal of volcanicactivity forming the Iceland plateau [Mihalffy et al.,2006]. Continuous westward movement of the ridgesystem since then has led to the present situationwiththe Reykjanes and Kolbeinsey Ridge system alreadywestward of Iceland, while the on-land spreadingridge, the neovolcanic zones of Iceland, is anchoredto the mantle fixed plume and forms an indentationto the east (see Figure 1). The link to the NorthAtlantic ridge is provided by the Tjoernes FractureZone in the north, and the ‘‘bookshelf’’-type faults ofthe South Iceland Shear Zone. To anchor the rift axison Iceland to the hot mantle anomaly beneath, whilethe North Atlantic Ridge System moves westward,repeated eastward ridge jumps on Iceland during theentire time of the island’s existence have occurred.

[4] The repeated migration of the rift axis is alreadya clear indication of the importance of plume flux,interacting with ridge perpendicular spreading fluxand large-scale plate movement. The complexflow can be separated into four contributions(see Figure 2): (1) Plume-related flow can beexpected to be predominately vertical below�200 km and radial outward at shallower depth(Figure 2a). (2) A ridge parallel flow componenthas been proposed on the basis of geochemicalobservations (Figure 2b). Variations of rare Earthelements and trace element concentrations along theReykjanes Ridge [Schilling, 1973] indicate thatmaterial from the Iceland plume has been mixedwith more normal MORB type basalts several ofhundreds of kilometers along the Mid-AtlanticRidge, but with decreasing degree. This finding

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has stimulated studies of along ridge channel likeflow transport [e.g., Albers and Christensen, 2001].(3) Spreading-related flow should follow the diver-gence of the plates at least in the uppermost mantlein an extended area on both sides of the ridge(Figure 2c). (4) Global plate motion related flowshould coincide with the plate motion vectors inFigure 1 and be characterized by its long wave-lengths nature (Figure 2d).

[5] In the following sections, dynamic flow modelsbased on the P-wave tomography model byBijwaard and Spakman [1999] and S-wavetomography model smean [Becker and Boschi,2002] will be presented and compared to the fouridealized flow models and to results from seismicanisotropy and geochemical studies.

2. Dynamic Flow Models for the NorthAtlantic Region Based on SeismicTomography

[6] If one is interested in mantle flow within aparticular region, either global tomography based

flow models may be used, and one may zoom intothe region under consideration, or both tomographyand flow modeling may be restricted to the partic-ular region. In the first approach far field (i.e.,global) flow and density structures are consistentlyincluded in the regional flow field. However, dueto computational limitations the flow model isusually restricted to long-wavelength structures.Such structures are appropriately represented byglobal S-wave tomography models. A furtherrestriction of such flow models is that they usuallyonly allow for 1-D radially variable viscosity. Inthe second approach, smaller-scale structures maybe included, which are better represented byshorter-wavelength P-wave tomography. Theregional formulation allows for higher resolutionof the flow modeling and for 3-D variableviscosity. However, as the far field is not includedcare has to be taken when choosing the regionalboundaries and boundary conditions. Boundariesmust not cut through significant tomographicanomalies. The flow region should be considerablylarger than the region with tomographic anomaliesto allow for large-scale return flows.

Figure 1. Topography/bathymetry of the North Atlantic shows major tectonic structures (data are based onETOPO5 [National Oceanic and Atmospheric Administration, 1988]). The red arrows indicate the plate motion of theNorth American and Eurasian Plates in a hot spot reference frame (HS3-Nuvel1a [Gripp and Gordon, 2002]).


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[7] In this paper we present one flow model basedon each of the above approaches, namely a regionalflowmodel based on the P-wave model by Bijwaardand Spakman [1999] regionalized for the NorthAtlantic, and a global flow model, based on theS-wave model smean [Becker and Boschi, 2002].We then compare the similarities and differences.

2.1. Tomography Models for the MantleBeneath the North Atlantic

[8] The North Atlantic has been studied by anumber of regional and global tomography studies.A global P-wave tomography model with highresolution within the North Atlantic region hasbeen developed by Bijwaard and Spakman[1999]. In this model they have identified a clearsignature of a plume conduit related to Iceland,rising through the entire mantle. This anomalystarts at a location at the CMB beneath the southerntip of Greenland and stretches as a strongly north-eastward inclined structure through the lowermantle and seems to branch off in several anoma-lies at the mantle transition zone. Bijwaard andSpakman also observed reduced seismic wavevelocities in the upper mantle below Iceland andthe east Greenland margin, at a depth of �300 kmin a rather narrow zone approximately below theKolbeinsey Ridge, and even deeper below theGreenland shield.

[9] When comparing this tomography model toothers from the same region we have to distinguishbetween global and regional models which alsoaddress the controversial topic of whether a plume-like anomaly exists in the lower mantle belowIceland.

[10] Regional tomography models using dense datacoverage retrieved on Iceland [e.g., Bjarnason etal., 1996; Wolfe et al., 1997; Allen et al., 1999;Foulger et al., 2001] show a strong approximatelycylindrical low-velocity anomaly beneath Iceland.However, due to the limited width of the seismicstation arrays, the outermost parts of these regionalmodels may not be suitable as a reference for anundisturbed mantle, but might be part of a largeranomaly.

[11] Low seismic velocities in the upper mantlebeneath the North Atlantic region are also found inall global tomography models [e.g., Grand et al.,1997; Grand, 2002; Masters et al., 2000; Megninand Romanowicz, 2000; Zhao, 2001], but the areais considerably larger than Iceland and even themaximum of the low-velocity anomaly is notcentered on Iceland in all cases. Looking to theupper mantle in more detail, a southward tiltedplume in the upper mantle roughly below Icelandcan be inferred from an along-ridge section of thesomewhat older global tomography model ofZhang and Tanimoto [1993]. They also see a

Figure 2. Possible idealized mantle flow geometry in the North Atlantic region [after Li and Detrick, 2003].



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low-velocity anomaly located in deeper parts of theupper mantle beneath the Kolbeinsey Ridge. Atilted plume is also found in a N-S section of theglobal tomography model by Zhao [2001]. Thismodel also shows a weak indication of a branchingof the plume head at shallow depth in the directionof the Reykjanes Ridge. A new global tomographymodel with higher resolution by Montelli et al.[2004] also indicates a seismic slow region belowIceland, slightly tilted to the south, but their modelcan trace the plume at most down to the mid lowermantle. Concluding, a low-velocity anomaly in theupper mantle in a large area around Iceland is arobust feature and therefore also clearly shows upin the composite S-tomography model smean[Becker and Boschi, 2002], which we used forour global flow model.

[12] Apart from tomography, there are a few otherseismic observations indicating that the plumeconduit reaches down into the lower mantle. Shenet al. [1998, 2002] studied P-S conversions fromthe 410 km and 660 km discontinuities anddiscovered a maximum thinning of the transitionzone of about 20 km, slightly shifted to the southin relation to the surface volcanic center. Shen etal. [2003] also reported an anomalous region oflimited lateral extent at 1050 km depth (whichthey also observed below Hawaii and which theycalled a discontinuity), which might be regardedas additional evidence for a deep origin of theplume.

[13] For the lower mantle, global models showonly a weak low velocity beneath the NorthAtlantic, and details of the structure are highlyvariable. A low-velocity anomaly in the lowermantle beneath the North Atlantic similar to the‘‘plume conduit’’ seen by Bijwaard and Spakmanwas only found in the P-wave tomography modelby Zhao [2001]. One reason for the rather strongdifferences among tomography models of the lowermantle is due to the fact that seismic travel timeresiduals are often stronger in the upper mantlecompared to the lower mantle. For the Bijwaard-Spakman data set used here the maximum rootmean square velocity variations for the upper400 km is 7.0%, for the depth interval 400 km to1400 km it is �1.5%, and for the lower mantledown to 2500 km it is only 0.74%. Similarvariations have been observed in other data setsas well and have been explained by the increase ofthe elastic moduli with depth. Since the variationsin the mid lower mantle are so small, the resolvedanomalies are strongly biased by assumptions used

for the inversion routine (e.g., damping). Further-more, one has to keep in mind that the resolutionof most of the global models is relatively coarse(e.g., smean is given for spherical harmonics 1–31;the model byMegnin and Romanowicz [2000] has ahorizontal resolution between 450 and 850 km).Bijwaard and Spakman [1999] performed syntheticresolution tests for their model and claimed anoverall resolution of the order of 300–500 km. Asthey used a non-equidistant inversion grid, themodel has a somewhat higher resolution close toseismic stations where the grid elements aresmall. In general, pure body wave models mayhave less resolution in the upper mantle awayfrom seismic stations compared to the lowermantle.

[14] In summary, at least down to a depth of themid lower mantle most tomography models includ-ing smean [Grand, 2002; Ritsema et al., 1999;Masters et al., 2000; Montelli et al., 2004; Beckerand Boschi, 2002] show a clear low-velocityanomaly beneath the North Atlantic. Whether thisanomaly is related to a plume conduit and can betraced to greater depth cannot be conclusivelyanswered on the basis of tomography models, yet.

[15] In addition, a global view of most tomographymodels below 1400 km down to the core mantleboundary, is dominated by two stronger low-velocity anomalies further south, beneath westernand southern Africa, respectively. These anomaliesare important for global flow models and have tobe kept in mind when interpreting flow structuresin the North Atlantic (see below).

2.2. Mantle Flow Model for the NorthAtlantic Based on Regional Modeling

[16] The mantle flow model for the North Atlanticwhich is presented in this section is based on asection of the global P-wave tomography model byBijwaard and Spakman [1999]. The section liesbetween 49.8�N, 50�W and 85.2�N, 15.4�E with aresolution of 0.6 degrees and is given in 26 non-equidistant depth layers. This section of the mantleincludes the above mentioned plume conduitrelated to Iceland [Bijwaard and Spakman, 1999].From seismic velocity anomalies within this sec-tion internal mantle densities and temperatureshave been inferred, assuming a constant conver-sion factor of 0.3 between density to seismicvelocity anomalies [Karato, 1993] and a relationbetween density and temperature with a depth-dependent thermal expansivity as described bySchmeling et al. [2003]. This leads to a relation



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between seismic velocity and mantle temperatureof the form

@ ln T

@ ln vp¼ �0:3

1

a T¼ �0:3

KT Pð Þr0 cv g

� 1T

ð1Þ

The notations are defined in Table 1 together withthe numerical values of the parameters which are inaccordance with a PREM mantle [Stacey, 1992].All parameters in equation (1) vary with pressure,but the effect is strongest for the bulk modulus KT;therefore all other parameters in equation (1) are setconstant, and for KT a relation of the form KT(P) =KT0

+ dKdPP(z) is used. This approach leads to

buoyancy forces in the deep mantle which areabout a factor 4 smaller compared to the uppermantle for the same excess temperature.

[17] The temperature anomaly, as derived from theBijwaard and Spakman [1999] tomographic modelis shown in Figure 3. The excess temperature of thestructure, defined as the plume conduit by Bij-waard and Spakman, is between 200 and 250Kthroughout most of the mantle. Such an excesstemperature is well in agreement with temperatureestimates for the Iceland plume from differentobservations and modeling approaches (for exam-ple, see the review by Ruedas et al. [2006]). Fromgeochemical studies on Icelandic lavas, Schilling[1991] and Nicholson and Latin [1992] estimated aplume excess temperature around 250K. Similar orslightly lower excess temperature had been de-duced from seismic velocity variations in theuppermost mantle below Iceland [Allen et al.,1999; Wolfe et al., 1997; Foulger et al., 2001].

Fully dynamical models of plume rise includingmelting in the plume center [Ruedas et al., 2004;Kreutzmann et al., 2004; Ito et al., 1999; Keen andBoutilier, 2000] obtained somewhat lower excesstemperatures of 100 to 180K. Excess temperaturesof 200K for the lower mantle are low if comparedto fully dynamical global mantle convection mod-els. Such models suggest that lower mantle excesstemperatures are higher than upper mantle excesstemperatures by a factor of 2–3 [Zhong, 2006].This suggests that the plume like structures in thelower mantle as inferred from the Bijwaard andSpakman tomography model might not be stronglycoupled with the well resolved upper mantleplume.

[18] Once the temperature field was estimated, itwas used as the driving force for a dynamic modelof mantle flow. As we only have a Cartesian 3-Dvariable viscosity convection code available, wetransformed the temperature field to a rectangularbox. Using Mercator projection this box wasdefined as a 4000 � 4000 � 2880 km box,representing a region within a sphere bordered bythe latitudes and longitudes as given above. Thisbox was embedded in the central part of a largercomputational box of 16 000 � 16 000 � 2880 kmin order to minimize boundary effects and toallow for larger-scale return flows. For this regionwe solved the Navier-Stokes equation forincompressible, variable viscosity flow

0 ¼ �r pþr � m ru þ ruð ÞTh in o

þ g �r0 aT þ DrO�S fO�S þ DrS�P fS�Pð Þezð2Þ

Table 1. Definition of Parameters Used in This Study

Symbol Parameter Value and Unit

a thermal expansivity K�1

g Grueneisenparameter 1.2m mantle shear viscosity Pa sr0 mean mantle density 4250 kg/m3

DrO –S density jump at the olivine spinel transition 196 kg/m3

DrS–P density jump at the spinel perovskite transition 253 kg/m3

cv specific heat at constant volume 1.3 103 J/kg KfO/S fraction of material transformed from olivine to spinel %fS/P fraction of material transformed from spinel to perovskite %g gravity acceleration 9.87 m/s2

KT bulk modulus at constant temperature above 410 km 135 109 PaKT bulk modulus at constant temperature below 410 km 200 109 PadKdP

above 410 km 6

dKdP

below 410 km 3.5

P hydrostatic pressure Pap dynamical pressure PaT temperature �C, Ku flow velocity cm/avP seismic P-wave velocity km/s



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Notations are found in Table 1. This equationcontains the effect of olivine phase transitions inthe mantle; for the phase parameters we usedvalues according to Akaogi et al. [1989] and treatedthe phase transitions numerically as described byMarquart et al. [2000]. The thermal expansivity awas assumed to be depth-dependent in the sameway as explained in equation (1). The model set upalso included a free slip lower and upper boundarywith a highly viscous lithosphere according to30 Ma of cooling. An alternative mechanicalboundary condition would be a prescribedkinematic condition with observed plate velocities.However, this would imply exerting external forcesto the system in contrast to the stress free surface ofthe earth. A draw back of the chosen boundarycondition is the non-plate like behavior of themodel near the surface. A flow model withprescribed kinematic surface velocities will bepresented in the next section.

[19] Equation (1) was solved by a hybrid spectral/FD method, described by Marquart et al. [2000]and Marquart [2001]. For the temperature- andpressure-dependent mantle viscosity we used an

equation of the type m(T, z) = m0w(z) � ecT. Thedepth-dependent function w(z) and the constant chave been determined by fitting the geoid andgravity data of the EGM96 potential field model[Lemoine et al., 1998] for the North Atlantic fora wavelengths range between 400 and 4000 km.This fitting procedure is the topic of the study byMarquart and Schmeling [2004] where also thenumerical modeling is explained. In this studywe tested various viscosity functions with gradualor stepwise increases with depth and calculatedthe flow model and the related gravity anomaliesfor each of these viscosity distributions. Wefound the best fit to the observed gravity anoma-lies for a model with a stepwise viscosity increaseby a factor 30 from the upper to the lower mantleand with a moderate dependence on temperatureof 1 order of magnitude for 500�C temperaturevariation.

[20] Most dynamic models for the Iceland plume(as for plumes in general) are fully dynamic in thesense that the Navier-Stokes equation and the heattransport equation are solved simultaneously andan idealized plume rise is studied during its tem-

Figure 3. Three-dimensional view of tomographic data [Bijwaard and Spakman, 1999] converted to temperatureassuming a pressure-dependent thermal expansivity and projected on a Cartesian grid. Notice the uprising lowseismic velocity anomaly, originating at the CMB at a position beneath the southern tip of Greenland. This anomalyrises, strongly northeastward inclined, through the lower mantle and intersects with the mantle transition zone beneaththe western European margin at the latitude of Great Britain. Through the transition zone itself, no clear continuationcould be identified, but a strong upper mantle anomaly is present beneath Iceland.



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poral evolution [e.g., Ribe et al., 1995; Ito et al.,1999; Ruedas et al., 2004]. In the approach byMarquart and Schmeling [2004] a buoyancy fieldwas determined from a tomography model and thedynamic response was calculated in a limited area.Such approaches have been widely used for globalstudies [e.g., Richards and Hager, 1984] but hardlyfor regional studies and temperature-dependentviscosity. One exception is the study of Mihalffyet al. [2006], who dynamically combined globalflow models based on seismic tomography with aregional convection model of the Iceland plume.

[21] In the following we discuss the mantle flowfield model by Marquart and Schmeling [2004](with the best fit to the observed gravity anoma-lies). In Figure 4, vertical flow is shown in colorcoding and horizontal flow by arrows with lengthscales different for the upper and lower mantle asindicated in the two lowermost panels in the lowerright figure (note that the numerical grid wasdenser and only every second vector is drawn forclearer visualization). As the numerical grid wasfour times larger as the area under investigationmost of the return flow occurred outside this area.

Figure 4. Mantle flow field at various depths in the North Atlantic based on the temperature field of Figure 3 and aviscosity profile with a stepwise increase by a factor 30 from the upper to the lower mantle and a dependence ontemperature of 1 order of magnitude for 500�C. Horizontal flow is shown by arrows (scale in lower figures for upper(left) and lower (right) mantle) and vertical flow is color coded.



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In the lower mantle the flow is very slow, less than1 cm/a, and of long-wavelength nature, mainlycharacterized by a broad upwelling. The flow inthe mantle transition zone shows considerablymore detailed small-scale characteristics than inthe mantle below and above, with upwellings in abroad region around Iceland and beneath the north-ern ridge system. Above 300 km, the flow field isdominated by horizontal flow. While Iceland issituated approximately above the center of maxi-mum vertical flow at the transition zone, it is not inthe center of radially divergent horizontal flux atshallow depth. Below Iceland, horizontal flow ismainly directed southward, changing from a N-Sdirection in east Iceland to a more NE-SW directedflow in the western part of the island. Horizontalflow is mainly along-ridge for the ReykjanesRidge, and vertical flow from greater depth occursonly in the part of this ridge which is adjacent toIceland. The Kolbeinsey Ridge, in contrast, isdominated by divergent flow with strong verticalflow throughout the upper mantle. In the light ofthe idealized flow models shown in Figure 2 onemay conclude that the flow pattern is explained bya combination of large-scale plume flux andspreading flux to the north.

[22] The mantle flow field around Iceland has in thepast often been predicted on the basis of numericalor analogue modeling of idealized plume ridgeinteraction. Ribe et al. [1995] and Feighner et al.[1995] studied the case of a plume, initiated under aspreading ridge, and found that radial plume fluxnormally predominates for slow spreading veloci-ties as in the North Atlantic. Only in case of a verystrong temperature-dependent viscosity along ridgechannel flow superimposing plume flow could beobtained [Albers and Christensen, 1996].

2.3. Mantle Flow Model for the NorthAtlantic Based on Global Modeling

[23] In the dynamic model, described so far, all flowcomponents of global or very large scale nature havebeen neglected, in order to allow more sophisticatedmodeling of mantle parameters and to enhanceregional resolution. Furthermore, the model param-eters, especially the pressure- and temperature-dependent viscosity law was scaled to fit regionalgravity observations. On the other hand, effects ofthe far flow field have not been included. Therefore,in the following test we modified the approach andconsider a global flow field in spherical harmonicsl = 1 to 30, deduced from a robust tomographymodel (smean [Becker and Boschi, 2002]).

[24] We varied the free parameters of the model(upper to lower mantle viscosity contrast, seismicvelocity to density scaling factor in the upper andlower mantle, and a layering coefficient character-izing the permeability of upper/lower mantle inter-face) in order to maximize the fit between theobserved and modeled geoid at low degreeharmonics (l = 2–12). The optimum values ofthe model parameters were determined by system-atic exploration of the model space. The best-fitting model demands somewhat higher viscosityincrease from the upper to lower mantle of a factor100 compared to a factor 30 found for the regionalmodel, and a scaling factor of 0.1 in the uppermantle and 0.25 in the lower mantle. We note thatin case of the global model, we consider differentboundary conditions at the top boundary and theupper/lower mantle interface than in the regionalmodel. The observed plate motion is imposed as akinematic boundary condition at the surface andthe permeability at the 660-km depth is modulatedby a free model parameter, called the layeringcoefficient, the value of which is determined fromthe geoid inversion. The layering coefficient allowsto study the whole range of partially layered flowmodels, with a whole mantle flow model (layeringcoefficient equal to 0) and a perfectly layered flowmodel (1) as end members (for more details, seeCadek and Fleitout [1999, 2003]). The inversion ofthe geoid gives the value of the layering coefficientclose to 0.5 which means that the flow across theupper/lower mantle boundary is reduced by 50% incomparison with the whole mantle flow model.

[25] Zooming in to the North Atlantic region theresulting flow field is shown in Figure 5a with thenet rotation toroidal component of degree 1 beingremoved. Also the global model shows a pro-nounced upwelling flow in the upper as well aslower mantle. The upwelling region within theupper mantle has a maximum vertical velocity ofmore than 6 cm/a and is elongated in SW-NEdirection, roughly following the North Atlanticridge system. The upwelling in the lower mantleis less elongated in shape and reaches almost 4 cm/a in the upper part. The horizontal flow in theupper mantle is characterized by a superposition ofa ridge-perpendicular and a radial divergence flowcomponent (compare Figures 2a and 2c), on whicha large-scale northerly flow is superimposed. Incontrast, the lower mantle shows a large-scalesoutherly flow, which is strongest in the lower partof the lower mantle. These large-scale upper andlower mantle flow components seem to correspond



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to the large-scale upwelling flow induced by theAfrican superplume.

[26] It is interesting to note that the seismic low-velocity anomalies identified by the tomographymodel smean show a lateral shift between theupper and lower mantle: As can be seen inFigure C1 of Becker and Boschi [2002], the uppermantle anomaly is centered beneath Iceland, whilethe (weaker) anomaly in the upper part of the lower

mantle is shifted toward the SE, similarly to theBijwaard and Spakman [1999] model. In contrast,the upwelling flow in the upper and lower mantledoes not show this lateral shift (Figure 5a).

[27] For a better comparison with the regional flowmodel, Figure 5b shows a high pass filtered versionof the flow field of the global model (harmonicdegrees 1 to 8 are subtracted). The upwelling flowstructures within the lower and upper mantle are

Figure 5. Mantle flow field at various depths in the North Atlantic from the globally derived flow field based on thetomography model smean and a global fit of the geoid. The optimal model has a viscosity profile with a stepwiseincrease by a factor 100 from the upper to the lower mantle and a layering coefficient of about 0.5. Horizontal flow isshown by arrows (scale below the figures, different for upper and lower mantle), and vertical flow is color coded.(a) Flow model in spherical harmonics 1 to 30 with the toroidal mode l = 1 removed. (b) High pass filtered flowmodel with spherical harmonics between 9 and 30.



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even more isolated compared to the unfiltered caseof Figure 5a, but the flow velocities are damped.The upwelling flows in the upper part of the lowermantle and in the upper mantle are clearly interre-lated and may be associated with the Icelandplume. The horizontal component of the high passfiltered flow (Figure 5b) is considerably differentfrom the unfiltered field of Figure 5a: the large-scale northerly flow within the upper mantle andsoutherly flow within the lower mantle vanished.Without this mantle wind, the horizontal flowclearly converges and feeds the elongated upwell-ing region at mid lower mantle depth, and divergesfrom a point near western Iceland in the sublitho-

spheric upper mantle. It is interesting to note thatthe horizontal feeding flow takes place at 1500 kmdepth and not near the core mantle boundary.

2.4. Comparison of Flow Fields Based onRegional and Global Modeling

[28] We now compare the characteristic features ofthe flow fields based on regional and globalmodeling (Figure 4 and Figures 5a and 5b).

[29] Starting with the lower mantle, the verticalupwelling flow structures are remarkably similar,both strongest in the upper part of the lowermantle, and situated at almost identical locations.

Figure 5. (continued)



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However, lateral feeding of the upwelling flow(i.e., the ‘‘source region of the plume’’) reachesdown to greater depth in the regional model(compare 1500 and 2540 km depths panels ofFigure 4 and Figure 5b). As expected, the southerlymantle wind in the global model (Figure 5a, depths1500 km and 2540 km) is not present in theregional model. It is also remarkable that despitesignificant differences in model assumptions (lat-erally variable versus stratified viscosity, 30 versus100 times viscosity jump between upper and lowermantle, an olivine-spinel-perovskite phase transi-tion versus the assumption of a layering coeffi-cient), both models show an increase of upwellingvelocity between lower and upper mantle by asimilar factor of about 1.5.

[30] It is interesting to note that the regional modelshows a lateral shift between the upper and lowermantle upwellings, associated with a horizontalflow component near 660 km depth (Figure 4),while in the global model this shift is not observed(Figure 5b). This is surprising, as both tomographymodels show such a shift. We attribute this differ-ent behavior at least partly to the lateral variation ofthe viscosity in the regional model, which moreeffectively focuses the upwellings into the hot, lowviscosity regions.

[31] Comparing the upper mantle flow structuresclearly show the different resolution of structures:in the regional model different upwelling regionsbeneath Iceland and beneath the Kolbeinsey ridgecan be distinguished, which seems to be beyondthe resolution of the global model. The differentsurface boundary conditions (kinematic platevelocities versus free slip) are also clearly visiblewhen comparing the 200km and 100 km depthpanels of Figure 4 and Figure 5b: In the model withkinematic boundary conditions the high astheno-spheric velocities drop to the slower plate veloci-ties on top. This drop is associated with a strongvertical shear flow, which has a considerable effecton the LPO (see below). In contrast, in the free slipmodel surface velocities are higher, and no verticalshear flow is visible. Below 100 to 200 km depththe effect of the difference in surface boundaryconditions becomes small.

[32] It is also interesting to compare our flow fieldmodels with the flow field for the North Atlantic ofMihalffy et al. [2006]. As this is also a globalflow model it closely resembles the flow field ofFigure 5a, and shows all features discussed abovesuch as the strong upwelling through the lower andupper mantle centered beneath Iceland and the

northerly mantle wind induced by the large-scaleupwelling beneath South Africa.

3. Anisotropy in theNorth Atlantic Region

[33] In this section seismic anisotropy modelsbased on seismic observation will be compared toseismic anisotropy as predicted by the differentflow models.

3.1. Seismic Anisotropy Models

[34] Early studies on seismic anisotropy gave in-consistent results for the North Atlantic regions.The map by Montagner and Tanimoto [1991]indicates a weak frequency-(depth)-dependent an-isotropy with a predominantly NW-SE fast seismicdirection for the entire region. However, anotherglobal study on azimuthal seismic anisotropy usinglong-period SS - S times [Woodward and Masters,1991] did not give indication for upper mantleanisotropy in the North Atlantic. Yang and Fischer[1994] analyzed SS arrivals for shear-wave split-ting in and found some indication of upper mantleanisotropy beneath the Atlantic, with the inferredorientation for the central North Atlantic inapproximate agreement with the global anisotropymodel of Montagner and Tanimoto [1991]. Anewer global study by Ekstrom [2001] shows for50s to 150s periods (maximum sensitivity between80 and 250 km) also a constant WNW-ESE direc-tion for the region around Iceland, but their reso-lution does neither allow to reveal any details sinceazimuthal anisotropy studies on a global scale onlyprovide the averaged effect over large areas.

[35] More recently an inversion based on Rayleighwaves was done by Debayle et al. [2005] and byPilidou et al. [2005] to infer seismic wave speedanomalies including anisotropy for the upper500 km of themantle. They found also an anisotropyfield in NW-SE direction over most of the NorthAtlantic but rotation to nearly E-W slightly north ofthe Charlie-Gibbs Fracture Zone (southward of alatitude of �56�N) and north of 82�N, and apredominately N-S direction across the Greenlandshield.

[36] Another observation based anisotropy fieldhas been provided by Jeannot Trampert [Trampertand Woodhouse, 2003] on the basis of Rayleighwaves of 40s period with resolution of sphericalharmonic degree of 8. This model generally indi-



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cates more WNW-ESE directions at latitudes southof Iceland even across the Greenland Shield.

[37] Better lateral resolution than the global modelscan be expected from a surface wave dispersionstudy of a large cap around the arctic by Levshin etal. [2001]. For a period of 50s their map indicates ageneral W-E to WNW-ESE fast seismic directionand widely agrees with the global studies, with theexception that they report a clear N-S fast directionin the northern part of the Greenland shield. For a100s period, which is better to compare with ouranisotropy model, their map indicates considerablymore spatial variations, with a NW-SE directionnorth of Iceland up to the Knipovich Ridge.

[38] In conclusion still considerable differences arepresent between the anisotropy directions in theNorth Atlantic published by various authors. Onereason for this might be the sensitivity of theinversion to the damping factor used, as discussedby Levshin et al. [2001].

[39] Focussing on Iceland, a number of seismicanisotropy measurements have been done. Anisot-ropyobservationson Icelandat veryshallow(crustal)depth [Menke et al., 1994] show ridge parallelanisotropy in the SW. However, one should keep inmind that these shallow layers cannot be compared toour model and might be related to crustal accretionprocesses. New detailed S-wave anisotropy studiesprobing the upper mantle [Bjarnason et al., 2002; Liand Detrick, 2003; Xue and Allen, 2005] neitherreflect a spreading-parallel nor a radial patternacross Iceland. Bjarnason et al. [2002] observed afast direction in NNW-SSE direction in eastern Ice-land and of NNE-SSW direction in western Iceland.

[40] Li and Detrick [2003] studied Rayleigh-waveanisotropy and observed a few flips in the orienta-tion of the fast axis beneath W-Iceland above adepth of about 60 km. These different anisotropydirections mainly in the thick crust are explained asbeing due to the combined effect of fossil spread-ing ridge jumps and a frozen-in crystal orientationand cannot be linked to the numerical flow model.At a depth greater than 80 km (better at periodsgreater than 60s), Li and Detrick [2003] also founda NE-SW fast direction in western Iceland, and anindication for NW-SE direction in eastern Icelandthough the signal is more unclear. They alsoreported only weak seismic anisotropy beneathcentral Iceland at a depth greater 50 km, whichthey interpreted as an indication that buoyantupwelling may control the orientation of crystals.Though weak, Li and Detrick [2003] found the

anisotropy direction in this central part to beconsistently rotated by �90� to the directionsbelow the other parts of Iceland, being nearlyperpendicular to the rift axis. While this strongvariation in azimuthal anisotropy on small scale isdifficult to explain by mantle flow, it might reflectprocesses of different kind, such as the presence ofa melting zone at a depth of 80 to 100 km withwater release, which would lead to an alignment ofthe b-axis of olivine with the flow direction and thea-axis perpendicular [Mizukami et al., 2004]. Analternative model has been proposed by Ruedasand Schmeling [2006] in which the change inanisotropy is explained by a dynamically con-trolled change in deep dyke or channel orientation.

[41] On the basis of the above discussion wecompare the Levshin et al. [2001] anisotropy modelwith the predictions of our regional flow model.Although the resolutions are quite different, it isstill worthwhile to compare our predictions withthe findings for Iceland by Li and Detrick [2003].On the other hand, it is consistent to compare ourglobal flow model with a global rather than aregional anisotropy model. Therefore we choosethe recent model of Debayle et al. [2005] as themost appropriate mode for comparison.

3.2. Mantle Flow and Seismic Anisotropy

[42] From laboratory and theoretical studies it iswell known that plastic deformation of mantlerocks may lead to strain induced lattice preferredorientations (LPO) of olivine crystals (alignment ofthe a-axes) which results in seismic anisotropy[e.g., Nicolas and Christensen, 1987; Mainpriceet al., 2000]. As LPO is a function of the finitedeformation associated with mantle convection,and the long axis of the finite strain ellipsoid(fse) often points into the flow direction, seismicanisotropy has in the past often been interpreted asa direct image of flow directions [Tanimoto andAnderson, 1984] and a measure for mantle con-vection [e.g., Ribe, 1989; Russo and Silver, 1994;Tommasi, 1998; Blackman and Kendall, 2002;Becker et al., 2003]. However, this is only a roughapproximation, since the fse may locally stronglydepart from the flow directions, e.g., near stagna-tion points [McKenzie, 1979], and LPO and thelong axis of the fse only converge after sufficientlyhigh strain [Kaminski and Ribe, 2002]. In themodels presented below, the differences and sim-ilarities between flow direction, the long axis of thefse and fast axis of LPO will be discussed for aparticular flow case.



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[43] We determined the long direction of the fse aswell as the LPO for an assemblage of olivine grainswith three slip-planes undergoing straining. Wefollowed the formalism by Kaminski and Ribe[2001], who determined this direction in a statisti-cal sense, following the assemblage of initiallyarbitrarily oriented olivine grains along variousstreamlines. For each grain three perpendicularslip-planes were assumed and each can deformby intra-crystalline slip [Ribe and Yu, 1991] anddynamic recrystallization. Dynamic recrystalliza-tion depends on the dislocation density on eachslip plane which is a function on the applied stressand the orientation of the slip plane. Dislocationcreep may lead to grain boundary migration andformation of stress free sub grains and by thatchanging the overall elasticity tensor of the variousaggregate assemblages. The detailed formalism aswell as the various parameters are described in anumber of papers [Kaminski and Ribe, 2001;Kaminski, 2002; Kaminski and Ribe, 2002] andwill not be repeated here.

[44] The flow field as shown in Figure 4 wasassumed to be steady state (for a time period of�10 Ma). Further assumption for the anisotropycalculations are: 100% olivine grains, and initialpositions of the olivine assemblages not deeper than420 km, and to result in a finite strain of at most 10at the positions where the LPO is determined. Themagnitude of anisotropy may be overestimatebecause the mantle also contains other minerals.

3.3. Comparison of Modeled and ObservedSeismic Anisotropy for Regional Models

[45] From the flow model shown in Figure 4 it isobvious that horizontal flow dominates above300 km depth, except for the area around Icelandand along the Kolbeinsey Ridge, where stronguprising flow is present with about the samemagnitude as horizontal flow. This is consistentwith the observation by Montagner and Tanimoto[1991], who studied anisotropy on a global scaleand showed that azimuthal anisotropy is importantto depths of �300 km. Therefore we determinedthe anisotropy at a depth of 75 to 175 km, which isjust below the lithosphere. The longest axis of thefse and the locally averaged LPO directions aregiven in Figure 6 for a depth of 175 km. The lengthof these axes represents the natural strain (ln(a/c)) incase of fse and the percentage of anisotropy (nonisotropic part of the elasticity tensor) for LPO. Thevertical components of these axes are coded bycolor, the horizontal components are shown as bars.

[46] The preferred directions for the fse and theLPO in Figure 6 are quite different. The long axisof the fse as well as the fast axis of LPO aredetermined by the accumulated strain along thestreamlines above 420 km and, in case of the LPO,by the ability of olivine slip planes to adjust to thestrain field changing along the streamline. In thecentral upwelling region the flow is characterizedby vertical uni-axial compression, leading to hor-izontal long strain axes with arbitrary directions,although a tendency of radially diverging fsedirections is visible close to Iceland. In the periph-eral area the flow is still directed radially outward,but it slows down with increasing distance fromthe plume. As expected this flow type producesflow-perpendicular stretching with the long fseaxes tangential to the center of upwelling. This isclearly visible in the western part of the model. Afurther flow component of our model is associatedwith a radially outward directed, horizontal shearflow (at shallow depth the radial flow velocities arefaster than at greater depth). Because dynamicrecrystallization leads to faster adjustment of thepreferred directions in respect of the macroscopicflow field [Kaminski and Ribe, 2002], the LPOadjusts faster to this radial shear flow than the fse.As a result the horizontal flow vectors and the fastazimuthal directions of the fast axis of olivine pointin similar directions, however, in areas of strongvertical flow large deviations are possible and evenfor some areas outside strong upwelling (e.g.,south of Iceland) deviations up to 90 degrees arepossible.

[47] The general appearance of the anisotropy fieldis characteristic for a large-scale upwelling, with itscenter north of Iceland. The area to the north ofIceland and along the northern ridge system, wherethe flow is essential vertical, is characterized byweak horizontal anisotropy of varying direction. Inthe Atlantic south of Iceland, azimuthal anisotropychanges from NW-SE close to the European con-tinent to NE-SW across the Reykjanes Ridge. Theanisotropy model, presented here, would furtherpredict a mainly NW-SE direction of fast seismicvelocity in the mantle below the eastern Greenlandshield, and weaker, varying directions in the west-ern and southern part. Variations around an E-Wanisotropy direction are also found in the southerlyadjacent oceanic region.

[48] We now compare our modeled fast LPOdirections to those obtained by Levshin et al.[2001] for moderate damping. We relate Rayleighwave periods of 50s to a depth for the LPO



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direction of 75 km and 100s to a depth of 125 km(Figures 7a and 7b). Levshin et al. [2001] foundthat a high damping factor leads to a preference ofthe long wavelengths parts of the field. Since in ourregional flow model, the maximum wavelength isrestricted to 1600 km, and no global wide large-scale flow is present, it is more consistent tocompare our modeled anisotropy directions to

regional anisotropy studies with a moderate damp-ing factor.

[49] For shallow depth (Figure 7a) both, modeledLPO directions and seismic anisotropy directionsare more irregular and show stronger small-scalevariations. In Figures 7a and 7b we have alsomarked in light gray the regions where the modeledazimuthal directions of the fast LPO and the

Figure 6. (bottom) Regional mantle flow model at a depth of 150 km, (middle) long axis of the finite strainellipsoid, and (top) LPO of olivine at a depth of 175 km based on the 3-D flow model. vertical components areindicated by color.



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observed seismic anisotropy directions fit betterthan 45�.

[50] For both periods a robust feature is a goodagreement of the azimuthal directions in large areasnorth, east and SE of Iceland. No coincidencebetween the azimuthal directions for both periodsis found south of Iceland in a narrow zone alongthe northern end of the Reykjanes Ridge, in the

eastern part of the North Sea, in the oceanic areasouth of Greenland, and to the east of the Charlie-Gibbs Fracture Zone.

[51] Along the Reykjanes Ridge our modeled LPOdirections are mainly ridge parallel, in agreementwith the predicted along ridge shallow flow. Theseismic anisotropy directions show at most in thesouthern part of the Reykjanes Ridge a ridge

Figure 7. Horizontal component of LPO directions of the regional flow model (black bars) and azimuthalanisotropy (red bars) in the North Atlantic region based on Rayleigh waves after Levshin et al. [2001] for periods of(a) 50 s and (b) 100 s. Regions where the fit is better than 45� are indicated in light gray.



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parallel direction, but closer to Iceland clear ridgeperpendicular directions are found. Roughly ridgeperpendicular directions are found for the ridgesnorth of Iceland, both, in our modeled LPO direc-tions and the seismic anisotropy.

[52] Even though the regional numerical flowmodel does not resolve any lateral variations onlength scales much smaller than 100 km, a com-parison to anisotropy observations on Icelandmight be of interest.

[53] Figures 8a and 8b show a close-up of themodeled horizontal components of the LPO vector

around Iceland at 75 and 125 km depth togetherwith the azimuthal seismic anisotropy directionsobtained by Li and Detrick [2003] for Rayleighwave periods of 50s and 67s. Since Iceland is notdirectly in the center of the large-scale divergingflow, which is about 150 km north of Iceland, theLPO fast directions change only moderately acrossIceland from NNW-SSE direction on the easternside to more NE-SW direction on the western part.By comparing our fast LPO directions to theseismic anisotropy directions of Li and Detrickfor 50s (Figure 8a), we found a good agreementbetween the two data sets. For 67s of Rayleigh

Figure 8. Horizontal component of LPO directions of the regional flow model (black bars) and azimuthalanisotropy (red bars) in the area of Iceland based on Rayleigh waves after Li and Detrick [2003] for periods of (a) 50 sand (b) 67 s.



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wave periods however, the eastern part of Icelandbehaves quite differently. However, by performinga sensitivity study, Li and Detrick stated that theirresults in the eastern part of Iceland are not robustwhile the NNW-SSE directions in the western partare reliable.

[54] The travel time differences between the fastand the slow S-wave which they observed is largerin the eastern part of Iceland and had been inter-preted as due to a 100–200 km thick anisotropiclayer under E-Iceland. Though the fast directionsthey observed are very similar to the fast olivinedirections modeled here and may very well beexplained by a slight rotation, they defined twodifferent anisotropic domains with a divide to belocated about 100 km west of the neo-volcaniczones and interpreted this finding as an indicationof a former spreading center still conserved due touncompleted crystal reorientation.

3.4. Comparison of Modeled and ObservedSeismic Anisotropy for the Models

[55] On the basis of the globally derived flow fieldwe also determined seismic anisotropy directionsand compared them to global observation basedanisotropy directions [Debayle et al. [2005]. Theirinversion is based on surface wave observationsand is done for seismic anomalies including an-isotropy. While it would be consistent to use theirdata on seismic anomalies also for the tomographymodel, this can hardly be done, since their data set

is rather shallow (down to 500 km) and not suitablefor estimation of deep mantle density anomalies.While the model parameters are chosen (or betterfound) in a way to give the best fit to the long-wavelength geoid, the fit to the anisotropy isestimated for the North Atlantic region only. Ourbest model is shown in Figure 9, where the blackbars indicate the modeled anisotropy and the redbars indicate the observations. The overall fit is33.7�, which is not too bad, since inspectingFigure 9 in more detail, it becomes obvious thatthe misfit is mainly attributed to the GreenlandShield Area and the northern part of Fennoscandiaand the adjacent Barents Sea. For the Greenlandshield the azimuthal anisotropic in the upper200 km might not be related to present mantleflow, nor are the density and viscosity conditionsfor this thick shield properly modeled in ournumerical approach.

[56] As for the regional model a good fit is found ina broad region of the North Atlantic north, east andSE of Iceland, as well as for Iceland itself. In thearea NE of Iceland directions are roughly perpen-dicular to the ridge system. As in the regional flowmodel (Figures 7a and 7b), a significant regionassociated with the Reykjanes Ridge area southwest of Iceland shows a pronounced misfit.

3.5. Discussion of the Anisotropy Results

[57] Although the regional and global flow modelsshow satisfactory agreement in their flow fields

Figure 9. Horizontal component of LPO directions of the global flow model (black bars) and azimuthal anisotropy(red bars) in the North Atlantic region based on Rayleigh waves after Debayle et al. [2005].



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(see section 2.4), their predictions for LPO aremore different. While both approaches are charac-terized by circular or elongated regions of upwell-ing and diverging flow beneath Iceland, only theregional model shows divergent LPO directions,while the global model is more characterized by aroughly ridge-perpendicular LPO direction. Weattribute this difference to (1) the higher sensitivityof the LPO to strain rates rather than flow direc-tions, in particular to the vertical shear strain rate atshallow depth associated with the imposed kine-matic boundary condition which is absent inthe regional free slip model (compare flow fieldsshown in Figures 5 and 7), and (2) with the NE-SWelongation of the upwelling region present in theglobal model. In the Atlantic region northeast ofIceland most seismic anisotropy models roughlyagree on a ridge perpendicular fast direction, whichwe found for both, our global and our regionalmodel.

[58] The comparisons shown in sections 3.3 and 3.4reveal a few robust features: Fast directions of theseismic anisotropy models and both flow modelsagree well in large sections to the north, east, and SEof Iceland. We therefore conclude that in theseregions the observed seismic anisotropy is causedby LPO with fast a-axes oriented in NW-SE direc-tions, induced by the North Atlantic upwelling flowassociated with the Iceland plume and the thermalanomalies beneath the Kolbeinsey Ridge.

[59] Another robust feature seems to be the pro-nounced misfit for the Greenland shield and SW ofIceland including the Reykjanes Ridge (compareFigures 7a, 7b, and 9, as well as in other compar-isons not shown here). One explanation might bethat in these regions the mechanisms producing theobserved azimuthal anisotropy may be differentfrom the one used in our model. For the Greenlandshield seismic anisotropy down to the base of thethick lithosphere at about 200 km depth may notexclusively be generated to the ongoing mantleflow but may be inherit from older dynamicprocesses. For the Reykjanes Ridge and closelyadjacent regions the presence of water may beimportant. In the presence of water olivine aggre-gates have been found to exhibit B-type anisotropy,in which the fast a-axes are oriented perpendicularto the shear flow within the shear plane [Kneller etal., 2005]. Thus, if applied to our regional flowmodel (Figures 7a and 7b), fast axes in such waterbearing mantle regions would have to be flipped by90�. Because for the Kolbeinsey Ridge the agree-ment between observed and modeled anisotropy

does not require such a flip, we might speculatethat we have B-type anisotropy south of Icelandand A-type anisotropy north of it. This wouldsuggest that high degree melting beneath the Kol-beinsey ridge would have extracted all the water,while it might be still present in the mantle beneaththe Reykjanes ridge. We will address the issue ofdifferent temperatures within the mantle regionsnorth and south of Iceland in the following section.

4. Comparison to GeochemicalObservations

[60] It might be of some interest to relate ourfindings on temperature distribution and mantleflow to some geochemical data obtained by drillingor dragging along the North Atlantic Mid-OceanRidge north of the Charlie-Gibbs-Fracture Zone[e.g., Hart et al., 1973; Schilling, 1973; Mertz etal., 1991; Devey et al., 1994; Hanan et al., 2000].

[61] First we summarize our main results for theregional temperature and flow model (Figure 4 andFigure 10) and then relate them to geochemicalobservations: We found a strong temperatureanomaly below Iceland and the northwesterly ad-jacent oceanic region (Figure 10, 100–630 km)which drives an upwelling flow and which seemsto be weakly connected to a deeply rooted lowermantle anomaly southeast of Iceland (light upperpart of the lower mantle anomaly visible inFigure 3). In our regional flow model the upwellingregion of the lower mantle feeds material to theupper mantle by an obliquely northwest orientedflow (see the horizontal flow component at 680 kmdepth in Figure 4). A second pronounced temper-ature anomaly is located in the upper mantle at adepth of about 200–300 km below the Kolbeinseyand southern Mohns Ridge (Figure 10). Thisanomaly has no direct deep seated root, althoughit might be weakly connected to the first anomalyand its lower mantle feeding system (Figure 4).(It should be noted that the flow field representsonly a snap shot of a time-dependent flow, andstatements about feeding and ‘‘flow connections’’must be handled with care). A super adiabatictemperature of �200 K was estimated using equa-tion (1) for the Iceland centered anomaly and a valueof �120 K for the Kolbeinsey-Mohns Ridge anom-aly. Temperature anomalies along the ReykjanesRidge exist too, but are shallower than 100 km andthe super adiabatic temperature increase is less than100 K. These temperature anomalies should lead toexcess melting, thick crust and shallow bathymetry.A thick crust in excess of 24 km has not only been



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Figure 10. Temperature anomaly in the upper mantle beneath the North Atlantic derived from the tomographymodel of Bijwaard and Spakman [1999] and the seismic velocity temperature relation of equation 1.



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repeatedly reported for Iceland [e.g., Darbyshire etal., 2000] but is also well established for theadjacent ridges. Both the Kolbeinsey and theReykjanes Ridge show some morphological fea-tures which are unusual for slow spreading ridges:overlapping spreading centers, absence of a medianvalley and a ridge topography untypical for the bulkpart of the Mid-Atlantic Ridge System [Sinha et al.,1998; Kodaira et al., 1997]. All these featuresindicate a high production rate of magma [Kodairaet al., 1997] and a crustal thickness in excess ofabout 9 km [Detrick et al., 1995]. For the ReykjanesRidge at 61�4400N, Searle et al. [1998] found athickness of the crust of 11.2 km and Kodaira etal. [1997] observed a crustal thickness at theKolbeinsey ridge of 10 km at 70�2000N, whichcorresponds with a water depth of not more than500 m [Moeller, 2002].

[62] On the basis of Hf and Nd isotopes ratios andtrace element data from various mid-ocean ridgebasalts, Salters [1996] found indications that theonset of melting for the Kolbeinsey Ridge is deeperthan for normal oceanic ridges. Low Na2O andhigh FeO contents have been observed for Kol-beinsey Ridge basalts [Moeller, 2002; Humler andBesse, 2002], being a clear hint for a high degree ofpartial melting over a large depth range of adepleted source. Corresponding temperature esti-mates of 1460�C for Kolbeinsey Ridge basalts atmelting [Humler and Besse, 2002] are in fairlygood agreement with the temperature of 1440�Cbelow the Kolbeinsey Ridge at 250 km depth,which we estimated for our flow model assumingan adiabatic background temperature of 1320�C atthat depth. Extensive melting and an anomalouslyhigh temperature along the Kolbeinsey Ridge isalso supported by the observation of a high deple-tion in rare earth elements (REEs) [Schilling et al.,1983; Haase and Devey, 1994; Devey et al., 1994].Since REEs are incompatible with mantle rocks,they become extracted with the first melts pro-duced, thus further melting only ‘‘sees’’ an increas-ingly depleted source. Kolbeinsey Ridge basaltsare even more depleted in the light REEs thanReykjanes Ridge basalts. This extreme depletionhas even been interpreted as being produced by thehighest degrees of partial melting found anywherein the world’s oceans [Klein and Langmuir, 1987].

[63] Another important question addresses thematerial supply from the lower to the upper mantlebeneath Iceland and the amount of deep mantlematerial injected into the ridge system. Our flowmodel suggests that at least in the vicinity of

Iceland the deep mantle upwelling would lead todeep mantle tracers exposed in extruded basalts.On Iceland, a plateau-shaped maximum of 3He/4Heis well established [e.g., Breddam et al., 2000], andcommonly regarded as an indication for at leastsome deep mantle material in the melting source.The detailed compositions of basalts on Iceland,however, have been found to be very heteroge-neous indicating that beside a common MORBsource a number of non-MORB sources areinvolved [e.g., Hanan and Schilling, 1997; Hananet al., 2000; Stracke et al., 2003; Thirlwall, 1995;Fitton et al., 1997, 2003; Kempton et al., 2000].Our flow model supports the common geochemicalview that the latter sources might at least partlyoriginate from the lower mantle.

[64] Since in the shallow upper mantle an essen-tially radial flow is predicted around Iceland and tothe south of it, ridge parallel flow is a consequencealong the Reykjanes Ridge. Thus we expect adecreasing influence of deep mantle material trac-ers away from Iceland along Reykjanes Ridgerather than a sharp transition. This is in agreementwith the steady decrease of several geochemicalobservables [see Murton et al., 2002] along theridge away from Iceland, being characteristic forocean island basalts or deep mantle influence, suchas the La/Sm ratio [Schilling, 1975], or the ratio of3He/4He and 22Ne/21Ne [Dixon et al., 2000].

[65] For the Kolbeinsey Ridge our model predictsdivergent flow driven by a separate upper mantleheat source with at most only a weak connection tothe deep mantle source to the south. Thus weexpect a geochemical signature for Kolbeinseybasalts typical for extensive MORB melting, butdeep mantle tracers should be essentially absent.Very much in line with our model Mertz et al.[1991] could not find MORB-plume mixing indi-cations north of the Tjoernes Fracture Zone (TFZ)along the Kolbeinsey Ridge. In contrast to basaltsfrom the Reykjanes Ridge south of Iceland, closeto the TFZ a sharp decrease in the La/Sm ratio isobserved.

[66] A number of studies [Macpherson et al., 1997;Botz et al., 1999; Breddam et al., 2000] investigatednoble gas concentrations in samples from theKolbeinsey Ridge and generally observed remark-ably constant isotope ratios for He, Ne, and Aralong the ridge with the exception of a maximumin 3He/4He ratio at the Tjoernes Fracture Zone.However, while Ne and Ar isotope ratios do notindicate a deep mantle source, 3He/4He valuesare significantly greater than typically found in



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N-MORB. The obviously high degree of meltingand deep melt source along Kolbeinsey Ridge hasbeen interpreted in the past as due to the proximityof the thermal anomaly associated with the Icelandmantle plume. However, the absence of latitudinalvariations for most geochemical trace elementsdo not indicate a gradual plume-MORB mixing.Schilling et al. [1999] presented a review about Pb,Nd, Sr, and He systematics along the KolbeinseyRidge and claimed to see some plume mixingacross the TFZ and defined the boundary of theplume influence about 300 km north of Iceland,which would even fit better to our flow model (seeFigure 4).

[67] The superadiabatic temperature of about 120Kat 400 km and above which we estimated from theBijwaard and Spakman [1999] tomography modelfor the Kolbeinsey and the southern Mohns Ridge isin agreement with a deep melt source with a highdegree of melting. The related flow model showsnearly uniform upwelling along the KolbeinseyRidge at depth and ridge perpendicular flow atshallow depth which fits to the observation of hardlyany along ridge variations in basaltic compositionand noble gas isotope ratios. However, whether therelativelyhigh3He/4Heratio foundalongKolbeinseyis indeed a tracer of deepmantle material andmay beexplained by a lateral connection to the Icelandicanomaly remains an open question.

5. Discussion and Conclusion

[68] Dynamic models of temperature anomalies andmantle flow in the North Atlantic region betweenthe Charlie-Gibbs and Jan-Mayen Fracture Zonesbased on the regionalized P-wave tomographymodel of Bijwaard and Spakman [1999] and onthe global, long-wavelength S-wave tomographymodel smean [Becker and Boschi, 2002] werepresented and compared. The regional temperaturemodel predicts an anomaly throughout the uppermantle below Iceland and beneath the westerlyadjacent section of the mantle and vanishes at theGreenland margin and a second anomaly between400 and 100 km depth below the Kolbeinsey andpartly Mohns Ridge. The long-wavelength modelsmean does not resolve these features, but shows astrong and broad low-velocity anomaly in the uppermantle of the North Atlantic centered at Iceland.From the regional flow model we deduce that theresulting mantle flow has a strong upwelling com-ponent beneath Iceland of more than 4 cm/a,however, beneath Iceland the flow is oblique inS or SW direction, turning horizontally at shallow

depth into ridge parallel direction along theReykjanes Ridge. In contrast, the Kolbeinsey Ridgeis subjected to strong vertical flow diverging ridgeperpendicular above 200 km. The global flowmodel is dominated by a NE-SWelongated upwell-ing region associated with a ridge perpendicularcomponent all over the North Atlantic. If theidealized flow models on Figure 2 are recalled,we conclude that the flow in the asthenospherebeneath the North Atlantic can be characterizedby a large-scale radial plume flux around Iceland,superimposed with a ridge parallel component inthe SW-parts of Iceland and the Reykjanes Ridge,and a larger-scale ridge perpendicular componentbeing a consequence of plate divergence and thelarge-scale NE-SWelongated upwelling flow in theupper mantle (see Figure 5).

[69] Pipe-like flow of plume material along theReykjanes Ridge rift axis south of Iceland mayalso occur in addition to the flow model presented.Such a flow type has been proposed on the basisof the observation of geochemical signature andV-shaped pattern in bathymetry and gravity anoma-lies south of Iceland first by Vogt [1971] and beenmodeled by Albers and Christensen [2001] and Ito[2001], but the model approach used here cannotresolve such small-scale features. Along ridgechannel flow demands a localized strong viscosityreduction and will thus be largely decoupled fromthe (upper) mantle wide circulation. Thereforealong ridge channel flow might simply add to theflow field shown in Figure 4.

[70] A strong seismic low-velocity anomaly in theupper mantle directly below Iceland is seen in thelarge-scale smean [Becker and Boschi, 2002] and inthe Bijwaard and Spakman [1999] model and evenclearer in regional tomography models. Wolfe et al.[1997] used the data obtained in the ICEMELTexperiment [Bjarnason et al., 1996] and detectedan approximately circular vertical structure between100 and 400 km centered beneath Vatnajokull, andestimated its radius to about 150 km and the excesstemperature to be �200K which is in very goodagreement with the estimates obtained in our study(see Figure 3 and Figure 10). However, this anom-aly of limited size, though hot, is not large enough todrive the bulk of the upper mantle flow. In the viewof our flow models, the plume below the NorthAtlantic rooting down (see Figure 4) into the lowermantle is a large feature of the upper mantle possi-bly centered to the west of Iceland, whereby theanomaly below Iceland is a hot, but smaller sidebranch. Rapid vertical flow within the sub Iceland



22 of 26

anomaly is likely, but beyond the resolution of ourmodel. The upper mantle anomaly is likely to be fedby a lower mantle upwelling centered east of Ice-land, implying a significant horizontal flow compo-nents near 660 km depth as seen in the regional flowmodel.

[71] In summary the following conclusions can bedrawn from our dynamic flow models:

[72] 1. Low seismic velocity anomalies in theupper mantle and upper part of the lower mantlebeneath Iceland as seen by the regionalized P-wavemodel of Bijwaard and Spakman [1999] and theglobal S-wave model smean [Becker and Boschi,2002] are associated with regional-scale upwellingflows at least down to mid lower mantle depth.

[73] 2. The upwellings within the upper andlower mantle are probably laterally shifted, result-ing in a pronounced horizontal flow componentnear 660 km depth.

[74] 3. At least two distinct upwelling flow regionsare identified by the regional flow model, onesituated beneath Iceland extending to NW of Ice-land and one beneath the Kolbeinsey ridge.

[75] 4. Modeled seismic anisotropy based on flowinduced LPO is sensitive to dynamic flow charac-teristics such as kinematic versus free slip bound-ary conditions: While the free slip flow model isdominated by seismically fast directions divergingfrom Iceland and the Kolbeinsey Ridge, the kine-matic boundary condition model shows predomi-nantly ridge-perpendicular fast directions for mostof the area.

[76] 5. A satisfying fit to observed seismic anisot-ropy directions is achieved in several but not allregions. Robust features include the following:(1) a good fit in a broad region north, east, andSE of Iceland indicates that the NW-SE anisotropyobserved in this area is induced by the Iceland plume(i.e., the large-scale North Atlantic upwelling flow),(2) a good fit beneath Iceland indicates the possibleimportance of a SW oriented flow beneath westernIceland, and (3) a robust misfit between the regionalmodels and observations in the Reykjanes ridge arealeads to the speculation that in this region B-typeanisotropy with the fast axis perpendicular tothe flow direction may be dominating, possiblyindicating the presence of water.

Acknowledgments

[77] Thanks are extended to Wim Spakman for giving us

access to his tomography model data and to Edouard Kaminski

for making his code DRex available to us. Our colleagues in

the ‘‘Frankfurt-Mainz Iceland Plume Working Group,’’espe-

cially Wolfgang Jacoby and Peter Mihalffy, made essential

contributions to this work through their interest and discus-

sions. Thomas Ruedas’s superb Iceland review facilitated our

literature search considerably. Numerous comments of two

anonymous reviewers helped to considerably improve the

manuscript. The original study was supported by a research

grant from the Deutsche Forschungsgemeinschaft; the Space

Research Organization of the Netherlands (SRON) and the

MAGMA grant supported by the European Commission made

it possible to finish this manuscript.

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