On subducting slab entrainment of buoyant asthenosphere · mantle between 60 and 250 km depths (see also Gung et al., 2003). The LVZ is an observed fact. Most researchers believe

On subducting slab entrainment of buoyant asthenosphere

J. Phipps Morgan,1 Jorg Hasenclever,2,3 M. Hort,3 L. Rupke2,4 and E. M. Parmentier5

1Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, NY, USA; 2IfM-GEOMAR, Kiel University, Kiel, Germany;3Institute of Geophysics, Hamburg University, Hamburg, Germany; 4Department of Physics, PGP, Oslo University, Oslo, Norway;5Department of Geological Sciences, Brown University, Providence, RI, USA

Introduction

Typical cartoons of mantle convectionshow oceanic lithosphere moving inconcert with the underlying mantleflow (Fig. 1a). If, however, theasthenosphere layer – presumed tounderlie at least much of the oceaniclithosphere – is itself less dense andviscous than the underlying mantle,then the resulting asthenosphere cir-culation can be strongly altered inways that may have significant geolo-gical consequences (Fig. 1b,c). Here,we study the question of how effi-ciently a subducting slab will entrainbuoyant asthenosphere near a subduc-tion zone, a question that leads to abetter understanding of the behaviourof the asthenosphere–lithosphere sys-tem in mantle convection.There exist several different views

on what exactly is asthenosphere.Seismic observations of the radialEarth structure show a low-seismic-velocity (Dziewonski and Anderson,1981) and high attenuation (Widmeret al., 1991) layer between �100 and300 km depths. This layer with lowseismic wavespeeds is now known tobe well developed beneath the �60–100 km-thick oceanic lithosphere(Gaherty et al., 1999) and activeyoung continental margins such as

the Basin and Range (Goes and vander Lee, 2002) in the western U.S.However, beneath stable continentalregions such as the Archaean shieldsin W. Australia (Gaherty et al., 1999)and N. America (Goes and van derLee, 2002) seismic wavespeeds aresignificantly faster, implying the pres-ence of much stronger (lithospheric)mantle between 60 and 250 km depths(see also Gung et al., 2003). The LVZis an observed fact. Most researchersbelieve the LVZ also corresponds to alower viscosity region within Earth�smantle. Rock-mechanics arguments,for example, imply the existence of alow-viscosity zone at the shallowesthot mantle where competing tempera-ture and pressure effects lead to aviscosity minimum (Weertman andWeertman, 1975; Buck and Parmen-tier, 1986; Karato and Wu, 1993).Studies of the distribution of stresses inthe oceanic plates require a low vis-cosity asthenosphere of 1018–1019 Pa-sunderneath the ocean lithosphere(Richter and McKenzie, 1978; Wiensand Stein, 1985), consistent with the�1018–1019 Pa s viscosities inferredfrom post-glacial rebound at Iceland(Sigmundsson and Einarsson, 1992).Several potential mechanisms have

been suggested to cause the formationof a shallow asthenosphere layer withlow wavespeeds, high attenuation andreduced viscosity. Silicate mantle closeto its melting temperature will haveslower wavespeeds, higher attenuation(Faul and Jackson, 2005), and lowerviscosity (Weertman and Weertman,

1975; Karato and Wu, 1993) than itdoes as cooler temperatures. As theuppermost suboceanic mantle is clo-sest to its solidus temperature, theasthenosphere could simply be a con-sequence of this effect, and the pres-ence of any partial melt wouldenhance the reduction in viscosity(Hirth and Kohlstedt, 1995) and seis-mic wavespeeds (Hammond andHumphreys, 2000). A higher watercontent has also been suggested to bethe origin for the LVZ (Karato andJung, 1998), and to lead to a viscosityreduction in this region of the mantle(cf. Hirth and Kohlstedt, 1996).We favour the hypothesis that

Earth�s asthenosphere forms as a sim-ple consequence of mantle plume up-welling – as hot buoyant plumematerial naturally tends to rise, up-welling plumes bring hot buoyantmaterial to the �ceiling� of the con-vecting mantle that is formed by thebase of the mechanical lithosphere(Phipps Morgan et al., 1995a). In thiscase, asthenosphere material will actu-ally be hotter than the underlyingmantle (at least in terms of its poten-tial temperature; e.g. temperature cor-rected for adiabatic effects). If theasthenosphere�s average temperatureis that of hot upwelling plumes, thenan observation of the deflection of the410- and 660-km discontinuities be-neath Iceland suggests that it may be�10–20% (i.e. �150–300�C) warmerthan the underlying ambient mantle(Shen et al., 1996). This would resultin a �0.5–1% reduction in density of

ABSTRACT

Laboratory and numerical experiments and boundary layeranalysis of the entrainment of buoyant asthenosphere bysubducting oceanic lithosphere implies that slab entrainment islikely to be relatively inefficient at removing a buoyant andlower viscosity asthenosphere layer. Asthenosphere wouldinstead be mostly removed by accretion into and eventualsubduction of the overlying oceanic lithosphere. The lower (hot)side of a subducting slab entrains by the formation of a �10–30 km-thick downdragged layer, whose thickness depends

upon the subduction rate and the density contrast and viscosityof the asthenosphere, while the upper (cold) side of the slabmay entrain as much by thermal �freezing� onto the slab as bymechanical downdragging. This analysis also implies thatproper treatment of slab entrainment in future numericalmantle flow experiments will require the resolution of �10–30 km-thick entrainment boundary layers.

Terra Nova, 19, 167–173, 2007

Correspondence: J. Phipps Morgan,

Department of Earth and Atmospheric

Sciences, Cornell University, Ithaca, NY

USA. Tel.: +1 607-821-1741; fax: +1 607-

254-4780; e-mail: [email protected]

� 2007 Blackwell Publishing Ltd 167

doi: 10.1111/j.1365-3121.2007.00737.x

the asthenosphere in comparison withthe underlying cooler mantle (Dq ⁄q =aDT, with a � 3.3 · 10)5 �C)1). Anypartial melt extraction from upwellingplumes will actually increase the buoy-ancy of plume material while trans-forming some of the plume�s thermalbuoyancy into compositional deple-tion buoyancy. This happens becausethe increase in depletion buoyancyfrom melt-extraction in the garnetstability field (Boyd and McCallister,1976; Oxburgh and Parmentier, 1977;Jordan, 1979) is roughly three times

larger than the decrease in thermalbuoyancy due to cooling associatedwith the latent heat of partial melting(cf. PhippsMorgan et al., 1995b).Dep-letion buoyancy would also arise fromthe preferential melting of denser andlower-solidus eclogite veins within aneclogite–peridotite �marblecake�mantle.If both hotter and shallower than

the underlying mantle, the astheno-sphere would also be much weaker.Karato and Wu�s (1993) summary ofthe rheology of the uppermost mantlesuggests that, for the most probable

viscous deformation mechanism in theuppermost mantle, dislocation creepof olivine at 1600 K, a 200 K increasein temperature leads to a >100-foldviscosity reduction.

Numerical and laboratoryexperiments of asthenosphereslab-entrainment

We first discuss several numericalexperiments that explore the entrain-ment surrounding a subducting slab.The key question we are interested in ishow much asthenospheric material isdowndragged by subducting slabs.Focusing on the effects of subductionprocesses, we also exclude mantleplumes as part of the numerical mod-elling domain. In order to highlight theimplications of a hot and depleted,hence weak and buoyant astheno-sphere layer we have simplified oursetup for the numerical calculations inthat phase transitions and melting pro-cesses are not considered. Instead, webegin with a preexisting 200 km-thickasthenosphere layer with a higher tem-perature and more depletion than theunderlyingmantle and ask howquicklyit can be removed by plate subduction.Consider the buoyant astheno-

sphere entrainment scenario sketchedin Fig. 1(b) and Box 1. Intuition sug-gests a simple flow pattern may form,and this pattern is in fact seen in thenumerical and laboratory experi-ments. The moving plate tends todrag shallower asthenosphere towardsthe subduction zone, which creates arelative pressure high that, in turn,locally tilts the asthenosphere baseand drives deeper lateral astheno-sphere flow away from the subductionzone as discussed in previous studies(Schubert and Turcotte, 1972; PhippsMorgan and Smith, 1992). In addi-tion, the subducting slab tends to dragthin sheets of asthenosphere into thedeeper mantle at its top and bottomboundaries (Fig. 1b,c). All of theseeffects are clearly visible in our numer-ical (Figure 1c) and laboratory (bot-tom-side only) experiments (Figure 3)as well as in the supplementary videos.The numerical experiments were car-

ried out in two dimensions with a codethat uses a penalty finite-element for-mulation to determine viscous flow,a Smolarkiewicz-upwind-finite differ-ence technique to solve for the ther-mal advection–diffusion problem, and

(a)

(b)

(c)

Fig. 1 (a) Cartoon illustrating the �typical� connection of lithospheric plate motionswith deeper mantle flow. (b) Cartoon illustrating the possible connection between platemotions and deeper flow when a buoyant low-viscosity asthenosphere decouplingzone is present. (c) A snapshot after 7 Ma from a numerical experiment discussed inthe text that shows the behaviour illustrated in (b). Here the white line marks theboundary of asthenosphere material – a gradational asthenosphere–mesospheretransition (centred around 300 km depth) was made by having the asthensosphere–mesosphere interface conductively cool for 50 Ma in a static configuration beforesurface plate motions were �turned on�. The oceanic lithosphere and slab are assumedto move at 63 km Ma)1. White streamlines in the mantle wedge show the recirculationthat develops in this region, colours show asthenosphere and mesosphere speeds, andarrows show instantaneous flow directions. (The experiments with a gradationalasthenosphere–mantle interface evolve quite similarly to previous experiments inwhich this transition was assumed to be discontinuous, as seen by comparing theevolution of supplementary videos oceanside.mpg and oceanside_sharp.mpg.) Theasthenosphere return flow beneath the base of the oceanic lithosphere is in goodagreement with Schubert and Turcotte�s (1972) simple boundary layer model, aboundary layer theory for the entrainment around the slab is discussed in Box 1.

On subducting slab entrainment of buoyant asthenosphere • J. Phipps Morgan et al. Terra Nova, Vol 19, No. 3, 167–173

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168 � 2007 Blackwell Publishing Ltd

tracer particles to track asthenosphereentrainment on both sides of the sub-ducting slab – for code details, seeRupke et al.�s (2004) paper. In eachexperiment, the solution region is a2250 km-wide by 1050 km-deep (ocea-nic side) or 2500 km-wide by 1300 km-deep (overriding plate side) box,respectively, allowing for a minimumgrid-spacing of 4 km in the slab�s sur-roundings. The top boundary is insu-lating representing the base of the

overlying lithosphere, while heat con-ducts into the slabmoving at a fixed 45�angle of subduction. Entrained low-viscosity asthenosphere is assumed toregain the ambient mesosphere visco-sity (1021 Pa s) after passing deeperthan 660 km. The initial asthenospherelayer is taken to be 0.3% composition-ally buoyant (due to prior melt extrac-tion) and up to 200�C hotter thanunderlying mesosphere, resulting in amaximum density reduction of 1%.

The asthenosphere viscosity and sub-duction rate are then varied in a suite ofexperiments. Typical asthenosphereflow exhibited in these experiments isshown in Fig. 1(c), which represents aninset near the slab. The experimentsfrom which Fig. 1(c) is taken areshown as two of the videos in theSupplementary Material, which alsocontains more details of the initial andboundary conditions.Low viscosity asthenospheric

material is entrained on the (hot)bottom side by downdrag only, withthe thickness of the entrained sheetranging from 10 to 55 km, dependingupon the asthenosphere�s viscosityand buoyancy, and the slab�s subduc-tion rate (Fig. 2a). A boundary layertheory was developed to estimate theentrainment beneath the base of thesubducting slab (Box 1). This simpletheory that neglects conductive heatloss into the slab is in good (>90%)agreement with the numerical resultsfor the moderate to large astheno-sphere viscosities (1019 up to 1020 Pa s)which are numerically best resolved(Fig. 2a). Possibly the growing relat-ive importance of conductive heat losswith decreasing subduction speed isthe main reason for the more stronglydeviating numerical results at theslowest subduction rates. For the low-est asthenosphere viscosity consid-ered, the boundary layer theoryconsistently predicts smaller entrain-ment thicknesses than observed in thenumerical calculations, which wethink is predominantly an effect ofour inability to resolve the predictedvery thin entrainment boundary layerwith even a 4-km grid, an observationthat will be further discussed below.On the cold top of the slab, entrain-ment is much larger for the samemodel parameters (Fig. 2b), withroughly twice the entrainment thick-ness subducting, roughly half by cool-ing and �freezing� to the top of the slaband half by downdragging like at theslab�s base. If other effects such as therelease of fluids from the dehydratingslab significantly reduce the viscosityof the wedge region, then top-sideentrainment may be even more com-pletely a �freezing� instead of �down-dragging� phenomenon.To verify our numerical techniques

and further explore entrainment at thebase of a downgoing slab, we alsoperformed several laboratory experi-

MOR

ASTHENOSPHERE

MESOSPHEREm ~ 1021Pa-s

µasth

µ

Δρasth

~ 1019 Pa-s~ 1%

ARC

LITHOSPHERESLA

BMESOSPHERE

ENTRAINEDASTHENOSPHERE

SLAB

VVw(x)

he

he

QE = ∫w(x)dx

V

V

PLUME

(a)

(b)

Box 1 (a) Cartoon of Earth�s lithosphere, asthenosphere and mesosphere flow. Toestimate the rate of asthenosphere entrainment by a subducting slab, we treat theentrainment process as the Poiseuille flow problem sketched in (b). The slab andadjacent mesosphere drag down a sheet of buoyant asthenosphere at a flux Vh, whereV is the vertical speed of the downgoing slab and h is the thickness of the entrainedasthenosphere sheet. Within the sheet itself, local buoyancy resists subduction andasthenosphere flows upward at a flux Qb ¼ ðh3=12lasthÞDqasthg. The net verticalentrainment fluxQE is the difference between these two opposing flux components, i.e.QE ¼ Vh� ðh3=12lasthÞDqasthg. Maximum entrainment as a function of h occurs atdQE=dh ¼ 0 ¼ V � ðh2

e=4lasthÞDqasthg or he ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi4lasthV =Dqasthg

p. The resulting flux is

QE = 2Vhe ⁄3, or twice this value if the subducting slab entrains the asthenosphere onboth sides of the slab. The width he of the entrainment finger is 20 km forlasth = 1019 Pa s, V = 100 mm year)1, and Dqasth = 1%, with a corresponding two-sided entrainment flux Ee = 4Vhe ⁄3 which is equivalent to 26 km of asthenospheresubducting as part of the downgoing slab. While this solution geometry assumedvertical subduction and entrainment flow, it also applies to slabs dipping at any anglefrom the vertical direction (as the buoyancy force and vertical velocity component ofthe subducting slab depend the same way upon the angle of subduction).

Terra Nova, Vol 19, No. 3, 167–173 J. Phipps Morgan et al. • On subducting slab entrainment of buoyant asthenosphere

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ments (Fig. 3). In each experiment, acorn-syrup + water mixture overlay adenser and more viscous pure corn-

syrup layer, with the lithosphereboundary simulated by a sheet ofplastic moving along a fixed litho-

sphere-slab geometry (angle of subduc-tion 45�). The development of theentrainment boundary layer and tiltingof the base of the buoyant astheno-sphere towards the subduction zone isshown in Fig. 3 (see also the two videosof laboratory experiments in the Sup-plementary Material). Numerical cal-culations using the measured densitiesand viscosities of the laboratory fluidsand the dimensions of the experimentalsetup can reproduce the laboratorymeasurements of the entrainmentthickness to within our measurementerror (<10%).

Discussion

These magnitudes and modes of pre-dicted slab entrainment are of geologi-cal interest. For reasonableasthenosphere density contrasts andviscosities (�0.5–1% less dense and1018–1019 Pa s), the entrained down-ward flux of asthenosphere is roughly�20–40% of the downward flux withinthe lithospheric slab which itself isbuild up of former asthenosphere,implying that most asthenosphere re-turns to the deeper mantle by subduct-ing as cold lithosphere instead of beingdirectly downdragged by subductingslabs. This happens because �60–100 km of former asthenosphere hasaccreted to form the lithosphere of thesubducting slab; both as part of themid-ocean spreading process (PhippsMorgan, 1994, 1997; Hirth and Kohl-stedt, 1996) and as additional conduc-tive thickening of the lithosphere as itcools to a �100-km-thick thermalboundary layer beneath 100 Ma sea-floor (Parsons and Sclater, 1977). Theimplication is that the asthenosphere isprimarily consumed by forming andsubducting oceanic lithosphere insteadof by direct downward entrainment atsubduction zones. In this case, if up-welling mantle plumes supply newasthenosphere at the rate that plategrowth and subduction consumeasthenosphere, then there should exista persistent plume-fed asthenospherelayer. The entrainment rates foundrepresent upper rather than lower esti-mates since our two-dimensionalexperiments are idealizing an infinitelyextended slab in the third dimension,thus not allowing for trench parallelasthenosphere flow or the chancefor buoyant asthenosphere to escapearound slab edges or through slab

0

10

20

30

40

50

60

Sla

b-bo

ttom

thic

knes

s of

ent

rain

ed la

yer

(km

)

20 40 60 80 100

Subduction rate (km Ma–1)10 30 50 70 90

1018 Pa s

1019 Pa s

3.1019 Pa s

μasth = 1020 Pa s

No. experiments

Theory

0

10

20

30

40

50

60

20 40 60 80 100

Subduction rate (km Ma–1)10 30 50 70 90

1018 Pa s

1019 Pa s

3.1018 Pa s

μasth = 3.1019 Pa s

Sla

b-to

pth

ickn

ess

of e

ntra

ined

laye

r (k

m) No. experiments

0

20

40

60

80

100

120

140

4 16 64Numerical grid spacing (km)

theory8 32

2 km grid spacing

160

95 km/Ma

Theory

μasth = 1020 Pa s

No. experiments

19 km/Ma63 km/Ma

95 km/Ma

19 km/Ma63 km/Ma

Sla

b-bo

ttom

thic

knes

s of

ent

rain

ed la

yer

(km

)

μasth = 1019 Pa s

(a)

(b)

(c)

Fig. 2 (a) Thickness of the entrained asthenosphere sheet beneath the base of thesubducting slab as a function of slab subduction speed and asthenosphere viscosity. Ineach experiment, the asthenosphere is assumed to be up to 1% less dense thanunderlying mesosphere of density 3300 kg m)3, with a gradational base as discussed inFig. 1(c). Experiments are marked as solid circles, dashed lines are theoretical maximagiven by a boundary layer theory (see Box 1). (b) Thickness of the entrainedasthenosphere sheet for the slab top-side – same asthenosphere initialization as in (a).Consistently higher entrainment rates are due to the effects of asthenosphere cooling tothe top of the slab, which can be as large or even larger than the purely dynamicalentrainment effect seen at the base of the slab. (c) Thickness of the entrainedasthenosphere sheet at the slab�s bottom side as observed in numerical experimentsusing different numerical grid-spacing. The theoretical maximum thickness (see Box 1)is marked at the �zero� grid-size edge of the plot. Artificially high asthenosphereentrainment is observed until a finemesh of�4 km is used tomodel flow in the region ofthe entrained sheet.


.............................................................................................................................................................


windows. In addition a non-New-tonian (stress-dependent) astheno-sphere rheology would further weakenthe entrained sheet of asthenosphere,making it more difficult to drag downasthenosphere.The convection style observed – a

thin sheet of asthenosphere entrain-ment by a subducting slab coexistingwith large-scale asthenosphere returnflowaway from the trench–occurs onlywhen the asthenosphere layer is bothless dense and less viscous than theunderlying mantle. Changing eitherasthenosphere density or viscosityalone results in an asthenosphere layerthat is easily dragged down and re-moved by a subducting slab as seen inFig. 4(b,c), respectively. Figure 4(a)shows the asthenosphere flow thatevolves after 20 Ma of plate subduc-tion through a buoyant, low viscosityasthenosphere – the same structure asthat seen and analysed previously inFigs 1-3. Figure 4(b) shows the evolu-tion of a system inwhich a shallow low-viscosity asthenosphere has the samedensity as its underlying mantle, ascould happen if the asthenosphere were

not plume-fed and had the same poten-tial temperature and potential densityas the underlying mantle. In this case,the whole weak layer is downdraggedand removed by plate subduction. Fig-ure 4(c) shows the evolution of a sys-tem in which a shallow buoyant layerhas the same viscosity (1021 Pa s) as itsunderlyingmantle. Here too, the wholebuoyant layer is rapidly downdraggedand removed by plate subduction.Note that the evolution seen in panels4(b) and 4(c) is similar to the numericalevolution of panel 4(a) for numericalexperiments with a too-coarse(>8 km) mesh on which slab entrain-ment cannot be properly resolved – inthis case poor numerical resolutionleads to runs (cf. panel 4d) that evolvelike simulations with entrainment ofmuch higher viscosity asthenosphere.In our numerical experiments, we ob-served spuriously high entrainmentrates until the numerical grid-spacingin the region of the downdragged sheetis less than about 4–8 km (Fig. 2c).Implications for mantle convection

are evident in the differences betweenthe convection scenarios sketched in

Fig. 1(a,b). For geologically reason-able asthenosphere parameters, aplume-fed asthenosphere can form adynamic low-viscosity boundary layerdecoupling the oceanic lithospherefrom deeper mantle flow. Indeed theentrained asthenosphere layer mayalso form a �lubricating� layer aroundthe slab itself to at least the depth ofthe 410-km phase transition, an effectthat may greatly shape the regionalupper mantle flow around a subduct-ing slab, as seen in the detailed flowvectors in Fig. 1(c) or the animationof Fig. 3. As the asthenosphere is then�consumed� mainly at mid-ocean rid-ges and by incorporation into thecooling plate, it can form a persistentand geochemically distinct depletedMORB source as proposed by PhippsMorgan and Morgan (1999). Theevolution of the asthenosphere wedgebeneath an island arc may also besignificantly affected by this process.Instead of the �corner flow� typicallyassumed beneath an island arc, the arcwould be underlain by a buoyantwedge of material that recirculates(seeFig. 1c) andbecomes progressivelydepleted with time until back-arcspreading, a changing subduction geo-metry, or apulseof �fresh�asthenosphereis brought into the back-arc region.It is important to recognize that

none of these effects will be resolv-able in global 3-D mantle convectioncodes until they can incorporate a�5–10 km numerical resolution oflow viscosity regions adjacent tohigher viscosity subducting slabs.We think that poor numerical reso-lution is likely to be the main reasonwhy the asthenosphere entrainmentseen in Figs 1, 3 and 4(a) and in theonline supplementary videos was notnoticed in previous numerical studiesexploring asthenosphere–lithospheredynamics on a global scale. If anumerical experiment�s resolution isworse than �5–10 km, it will tend toimproperly entrain too muchasthenosphere as a numerical arte-fact (Figs 2c and 4d), thus generatingthe scenario sketched in Fig. 1(a) asa consequence of poor numericalresolution. Because of the continuingrapid progress in computationalmantle flow, we believe that thecomputational challenge to success-fully include slab entrainment ofbuoyant asthenosphere will be rap-idly overcome.

0 5 10 15 20cm

0

5

10

Plate speed 31 mm min–1 upper layerviscosity 14 Pa-s1% lower density

lower layerviscosity 66 Pa-s

density 1420 kg m–3

t = 360 sec

cm

Fig. 3 An example laboratory experiment discussed in the text, shown for the casewhere the asthenosphere viscosity is 14 Pa s, the density contrast between astheno-sphere and deeper layer (66 Pa s, 1420 kg m)3) is �1%, and the plate �subduction�speed is 31 mm min)1. The snapshot of the glass-bead rich �asthenosphere layer� andthe deeper glass-bead poor �mesosphere layer� is shown 360 s after the initiation of theexperiment. The predicted pressure-induced tilting of the base of the asthenosphereand the entrainment of a thin asthenosphere sheet between the subducting slab andthe higher-viscosity layer are evident. (The full time-history of this run is shown as theLabMovie31mmpermin.mpg video in the Supplementary material, where there is also avideo of an additional laboratory experiment for a slower plate speed).


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References

Boyd, F.R. and McCallister, R.H., 1976.Densities of fertile and sterile garnet per-idotites. Geophys. Res. Lett., 3, 509–512.

Buck, W.R. and Parmentier, E.M., 1986.Convection beneath young oceaniclithosphere: Implications for thermalstructure and gravity. J. Geophys. Res.,91, 1961–1974.

Dziewonski, A. and Anderson, D.L., 1981.Preliminary reference Earth model.Phys. Earth Planet. In., 25, 297–356.

Faul, U.H. and Jackson, I, 2005. The seis-mological signature of temperature andgrain size variations in the upper mantle.Earth Planet. Sci. Lett., 234, 119–134.

Gaherty, J.B., Kato, M. and Jordan, T.H.,1999. Seismological structure of the up-per mantle: a regional comparison ofseismic layering. Phys. Earth Planet. In.,110, 21–41.

Goes, S. and van der Lee, S., 2002. Ther-mal structure of the North American

uppermost mantle inferred from seismictomography. J. Geophys. Res., 107,ETG2-1–ETG2-13.

Gung,Y.,Panning,M.andRomanowicz,B.,2003.Global anisotropy and the thicknessof the continents.Nature, 422, 407–411.

Hammond, W.C. and Humphreys, E.D.,2000. Upper mantle seismic wave velo-city: effects of realistic partial meltgeometries. J. Geophys. Res., 105,10975–10986.

Hirth, G. and Kohlstedt, D.L., 1995.Experimental constraints on thedynamics of the partially molten uppermantle: deformation in the dislocationcreep regime. J. Geophys. Res., 100,15441–15449.

Hirth, G. and Kohlstedt, D.L., 1996. Waterin the oceanic upper mantle: implicationsfor rheology, melt extraction, and theevolution of the lithosphere. EarthPlanet. Sci. Lett., 144, 93–108.

Jordan, T.H., 1979. In: Proceedings of theSecond International Kimberlite Confer-

ence, Vol. 2 (F.R. Boyd and H.O.A.Meyer, eds). American GeophysicalUnion, Washington D.C.

Jordan, T.H., 1979. mineralogies, densities,and seismic velocities of garnet Iherzolitesand their geophysical implications.In: Themantle Sample: Inclusions in Kimberlitesand Other Volganics (F.R. Boyd andH.D.A. Meyer, eds), pp. 1–14. AmericanGeophysical Union, Washington D.C.

Karato, S.-I. and Jung, H., 1998. Water,partial melting and the origin of theseismic low velocity and high attenuationzone in the upper mantle. Earth Planet.Sci. Lett., 157, 193–207.

Karato, S.-I. and Wu, P., 1993. Rheologyof the upper mantle: a synthesis. Science,260, 771–778.

Oxburgh, E.R. and Parmentier, E.M.,1977. Compositional and density strati-fication in oceanic lithosphere – causesand consequences. J. Geol. Soc. London,133, 343–355.

(a) (b)

(d)(c)

Fig. 4 Comparison of: (a) the dynamics of a buoyant low-viscosity asthenosphere layer; (b) the dynamics of a low-viscosityasthenosphere that has the same density as its underlying mantle; (c) the dynamics of a shallow layer with the same buoyancycontrast as in (a) but for a uniform viscosity mantle. All runs show the evolution after 20 Ma of plate subduction, starting with thesame initial conditions apart from differences in viscosity- or density-contrast between the asthenosphere and its underlyingmantle. Only the situation in (a) of an asthenosphere that is simultaneously less dense and less viscous (e.g. hotter) than underlyingmantle exhibits limited asthenosphere entrainment by slab subduction and the development of classic Schubert&Turcotte-like(Schubert and Turcotte, 1972) asthenosphere return flow away from the trench. Calculations using a too coarse numerical grid(d) cannot resolve a thin entrainment layer and thus tend to drag down too much asthenosphere, resulting in a flow pattern closerto (b) or (c) than to (a), even though calculation (d) used the same physical parameters as calculation (a).


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Parsons, B. and Sclater, J., 1977. Ananalysis of the variation of ocean floorbathymetry and heat flow with age.J. Geophys. Res., 82, 803–827.

Phipps Morgan, J., 1994. The effect of mid-ocean ridge melting on subsequent off-axis hotspot upwelling and melting. EOSTrans. AGU, 75, 336.

Phipps Morgan, J., 1997. The generation ofa compositional lithosphere bymid-oceanridge melting and its effect on subsequentoff-axis hotspot upwelling and melting.Earth Planet. Sci. Lett., 146, 213–232.

Phipps Morgan, J. and Morgan, W.J.,1999. Two-stage melting and the geo-chemical evolution of the mantle: Arecipe for mantle plum-pudding. EarthPlanet. Sci. Lett., 170, 215–239.

Phipps Morgan, J. and Smith, W.H.F.,1992. Flattening of the seafloor depth-age curve as a response to astheno-spheric flow. Nature, 359, 524–527.

Phipps Morgan, J., Morgan, W.J., Zhang,Y.-S. and Smith, W.H.F., 1995a.Observational hints for a plume-fed sub-oceanic asthenosphere and its role inmantle convection. J. Geophys. Res., 100,12753–12768.

PhippsMorgan, J.,Morgan,W.J. andPrice,E., 1995b. Hotspot melting generatesboth hotspot volcanism and a hotspotswell? J. Geophys. Res., 100, 8045–8062.

Richter, F.M. and McKenzie, D., 1978.Simple plate models of mantle convec-tion. J. Geophys., 44, 441–471.

Rupke, L.H., Phipps Morgan, J., Hort, M.and Connolly, J.A.D., 2004. Serpentineand the subduction zone water cycle.Earth Planet. Sci. Lett., 223, 17–34.

Schubert, G. and Turcotte, D.L., 1972.One-dimensional model of shallow-mantle convection. J. Geophys. Res., 77,945–951.

Shen, Y., Solomon, S.C., Bjarnason, I.T.and Purdy, G.M., 1996. Hot mantletransition zone beneath Iceland and theadjacent Mid-Atlantic Ridge inferredfrom P-to-S conversions at the 410- and660-km discontinuities. Geophys. Res.Lett., 23, 3527–3530.

Sigmundsson, F. and Einarsson, P., 1992.Glacio-isostatic crustal movementscaused by historical volume change ofthe Vatnajokull ice cap. Geophys. Res.Lett., 19, 2123–2126.

Weertman, J. and Weertman, J.R., 1975.High temperature creep of rock andmantle viscosity. Annu. Rev. Earth Pla-net. Sci., 3, 293–315.

Widmer, R., Masters, G. and Gilbert, F.,1991. Spherically symmetric attenuationwithin the Earth from normal modedata. Geophys. J. Int., 104, 541–553.

Wiens, D.A. and Stein, S., 1985. Implica-tions of oceanic intraplate seismicity forplate stresses, driving forces, and rheol-ogy. Tectonophysics, 116, 143–162.

Supplementary Material

The following material is available athttp://www.blackwell-synergy.com:These .mpg videos show eight exam-ples of the time evolution of numericalexperiments (one each for entrainmenton the bottom and top of a subductingslab, two to compare the effects of asharp and diffuse asthenosphere–mes-osphere transition, two in which eitherthe asthenosphere density or viscosity,respectively, is equal to that of themesosphere, one showing the effects ofan inadequate numerical resolution,and one numerical simulation of alaboratory experiment) and two exam-ples of laboratory experiments.The video oceanside.mpg shows

25 Ma of entrainment beneath a plateand slab moving at 63 km Ma)1. Theasthenosphere viscosity is 1019 Pa sand the underlying mesosphere visco-sity is 1021 Pa s, with a gradationaltransition between asthenosphere andmesosphere like that after 50 Ma ofheat conduction. Asthenosphere andmesosphere densities are 3270 and3300 kg m)3, respectively. Below thedepth of 660 km, the entrained mater-ial is assumed to regain the ambientmesosphere viscosity. (The aboveparameters are referred to as the�reference case� in the following.)Video oceanside_blow-up.mpg shows15 Ma of evolution in a zoom of theentrainment region. The oceanward(left) part of Fig. 1(c) is taken after7 Ma from this numerical calculation.The video oceanside_sharp.mpg

shows 25 Ma of entrainment beneatha slab for the same scenario as videooceanside.mpg, but with a sharp (dis-continuous) asthenosphere–meso-sphere transition. There are only smalldifferences between the evolution ofexperiments with sharp and diffuseasthenosphere-mesosphere transitions.We think this is because asthenosphereentrainment occurswhere the topof theasthenosphere meets the subductingslab, for which the �sharp-transition�and �diffuse-transition� experimentshave the same physical properties.The videos oceanside_drho0.mpg and

oceanside_dmu0.mpg show the evolu-tion of systems in which the astheno-sphere has the same density(3300 kg m)3) or viscosity (1021 Pa s)respectively, as the underlying mantle.Figure 4b and 4c are snapshots of these

movies after 20 Ma, when the wholeasthenosphere is being rapidly down-dragged and removed by plate subduc-tion. This evolution is similar tonumerical experiments incorporatingan insufficient grid resolution in theregion where a sheet of asthenosphereis dragged-down: the video ocean-side_32 km-grid.mpg shows 25 Ma oftime evolution of the reference-case,but using a 32 kmnumerical grid in thisregion instead of the 4 km grid-spacingused in all other calculations. The flowfield in the asthenosphere is completelydifferent with a much weaker returnflow evolving and much more astheno-sphere being dragged down by platesubduction. Video overridingside.mpgshows 20 Ma of time evolution ofasthenosphere and mesophere flow onthe top or overriding plate side of thesubducting slab for the parameters ofour reference-case. In this example theoverriding �landward� plate does notmove. The landward (right) part ofFig. 1(c) is taken after 7 Ma from thisnumerical experiment.The two videos for the laboratory

experiments show examples with iden-tical �mesosphere� densities and viscos-ities (1420 kg m)3 and 66 Pa s,respectively), and nearly identicalasthenosphere densities and viscosities(1% lower density than the meso-sphere, viscosities of 12 and 14 Pa-s,respectively). Examples for two differ-ent subduction rates are shown,7 mm min)1 (LabMovie7mmper-min.mpg) and 31 mm min)1 (LabMov-ie31 mmpermin.mpg). Figure 3 shows astatic image of the LabMovie31mmper-min.mpg video. Video Num-Sim_of_Lab3 1mmpermin.mpg showsthe time evolution of a numericalexperiment with an initial geometry,plate speed, and viscosity structurechosen to try to match the conditionsof the 31mm min)1 laboratory experi-ment. The numerical simulation mat-ches the detailed entrainment thicknessand asthenosphere counterflow struc-tures of the laboratory experimentquite well, even matching the time-evolution in deep flow that occurs withthe progressive growth of a low-visco-sity asthenosphere decoupling zonealong the subducting plate interface.

Received 6 April 2006; revised versionaccepted 16 October 2006


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Documents

On subducting slab entrainment of buoyant asthenosphere · mantle between 60 and 250 km depths (see also Gung et al., 2003). The LVZ is an observed fact. Most researchers believe