On Subducting Slab Entrainment of Buoyant Asthenosphere

doi: 10.1111/j.1365-3121.2007.00737.x On subducting slab entrainment of buoyant asthenosphere

J. Phipps Morgan,1 Jo¨ rg Hasenclever,2,3 M. Hort,3 L. Ru¨ pke2,4 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

ABSTRACT Laboratory and numerical experiments and boundary layer upon the subduction rate and the density contrast and viscosity analysis of the entrainment of buoyant asthenosphere by of the asthenosphere, while the upper (cold) side of the slab subducting oceanic lithosphere implies that slab entrainment is may entrain as much by thermal ÔfreezingÕ onto the slab as by likely to be relatively inefﬁcient at removing a buoyant and mechanical downdragging. This analysis also implies that lower viscosity asthenosphere layer. Asthenosphere would proper treatment of slab entrainment in future numerical instead be mostly removed by accretion into and eventual mantle ﬂow experiments will require the resolution of 10– subduction of the overlying oceanic lithosphere. The lower (hot) 30 km-thick entrainment boundary layers. side of a subducting slab entrains by the formation of a 10– 30 km-thick downdragged layer, whose thickness depends Terra Nova, 19, 167–173, 2007

the Basin and Range (Goes and van 1975; Karato and Wu, 1993) than it Introduction der Lee, 2002) in the western U.S. does as cooler temperatures. As the Typical cartoons of mantle convection However, beneath stable continental uppermost suboceanic mantle is clo- show oceanic lithosphere moving in regions such as the Archaean shields sest to its solidus temperature, the concert with the underlying mantle in W. Australia (Gaherty et al., 1999) asthenosphere could simply be a con- flow (Fig. 1a). If, however, the and N. America (Goes and van der sequence of this effect, and the pres- asthenosphere layer – presumed to Lee, 2002) seismic wavespeeds are ence of any partial melt would underlie at least much of the oceanic significantly faster, implying the pres- enhance the reduction in viscosity lithosphere – is itself less dense and ence of much stronger (lithospheric) (Hirth and Kohlstedt, 1995) and seis- viscous than the underlying mantle, mantle between 60 and 250 km depths mic wavespeeds (Hammond and then the resulting asthenosphere cir- (see also Gung et al., 2003). The LVZ Humphreys, 2000). A higher water culation can be strongly altered in is an observed fact. Most researchers content has also been suggested to be ways that may have significant geolo- believe the LVZ also corresponds to a the origin for the LVZ (Karato and gical consequences (Fig. 1b,c). Here, lower viscosity region within EarthÕs Jung, 1998), and to lead to a viscosity we study the question of how effi- mantle. Rock-mechanics arguments, reduction in this region of the mantle ciently a subducting slab will entrain for example, imply the existence of a (cf. Hirth and Kohlstedt, 1996). buoyant asthenosphere near a subduc- low-viscosity zone at the shallowest We favour the hypothesis that tion zone, a question that leads to a hot mantle where competing tempera- EarthÕs asthenosphere forms as a sim- better understanding of the behaviour ture and pressure effects lead to a ple consequence of mantle plume up- of the asthenosphere–lithosphere sys- viscosity minimum (Weertman and welling – as hot buoyant plume tem in mantle convection. Weertman, 1975; Buck and Parmen- material naturally tends to rise, up- There exist several different views tier, 1986; Karato and Wu, 1993). welling plumes bring hot buoyant on what exactly is asthenosphere. Studies of the distribution of stresses in material to the ÔceilingÕ of the con- Seismic observations of the radial the oceanic plates require a low vis- vecting mantle that is formed by the Earth structure show a low-seismic- cosity asthenosphere of 1018–1019 Pa-s base of the mechanical lithosphere velocity (Dziewonski and Anderson, underneath the ocean lithosphere (Phipps Morgan et al., 1995a). In this 1981) and high attenuation (Widmer (Richter and McKenzie, 1978; Wiens case, asthenosphere material will actu- et al., 1991) layer between 100 and and Stein, 1985), consistent with the ally be hotter than the underlying 300 km depths. This layer with low 1018–1019 Pa s viscosities inferred mantle (at least in terms of its poten- seismic wavespeeds is now known to from post-glacial rebound at Iceland tial temperature; e.g. temperature cor- be well developed beneath the 60– (Sigmundsson and Einarsson, 1992). rected for adiabatic effects). If the 100 km-thick oceanic lithosphere Several potential mechanisms have asthenosphereÕs average temperature (Gaherty et al., 1999) and active been suggested to cause the formation is that of hot upwelling plumes, then young continental margins such as of a shallow asthenosphere layer with an observation of the deflection of the low wavespeeds, high attenuation and 410- and 660-km discontinuities be- Correspondence: J. Phipps Morgan, reduced viscosity. Silicate mantle close neath Iceland suggests that it may be Department of Earth and Atmospheric to its melting temperature will have 10–20% (i.e. 150–300C) warmer Sciences, Cornell University, Ithaca, NY slower wavespeeds, higher attenuation than the underlying ambient mantle USA. Tel.: +1 607-821-1741; fax: +1 607- (Faul and Jackson, 2005), and lower (Shen et al., 1996). This would result 254-4780; e-mail: [email protected] viscosity (Weertman and Weertman, in a 0.5–1% reduction in density of

2007 Blackwell Publishing Ltd 167 On subducting slab entrainment of buoyant asthenosphere • J. Phipps Morgan et al. Terra Nova, Vol 19, No. 3, 167–173 ......

(a) viscous deformation mechanism in the uppermost mantle, dislocation creep of olivine at 1600 K, a 200 K increase in temperature leads to a >100-fold viscosity reduction.

Numerical and laboratory experiments of asthenosphere (b) slab-entrainment We first discuss several numerical experiments that explore the entrainment surrounding a subducting slab. The key question we are interested in is how much asthenospheric material is downdragged by subducting slabs. (c) Focusing on the effects of subduction processes, we also exclude mantle plumes as part of the numerical mod- elling domain. In order to highlight the implications of a hot and depleted, hence weak and buoyant asthenosphere layer we have simplified our setup for the numerical calculations in that phase transitions and melting processes are not considered. Instead, we begin with a preexisting 200 km-thick asthenosphere layer with a higher temperature and more depletion than the underlying mantle and ask how quickly Fig. 1 (a) Cartoon illustrating the ÔtypicalÕ connection of lithospheric plate motions with deeper mantle flow. (b) Cartoon illustrating the possible connection between plate it can be removed by plate subduction. motions and deeper flow when a buoyant low-viscosity asthenosphere decoupling Consider the buoyant astheno- zone is present. (c) A snapshot after 7 Ma from a numerical experiment discussed in sphere entrainment scenario sketched the text that shows the behaviour illustrated in (b). Here the white line marks the in Fig. 1(b) and Box 1. Intuition sug- boundary of asthenosphere material – a gradational asthenosphere–mesosphere gests a simple flow pattern may form, transition (centred around 300 km depth) was made by having the asthensosphere– and this pattern is in fact seen in the mesosphere interface conductively cool for 50 Ma in a static configuration before numerical and laboratory experi- surface plate motions were Ôturned onÕ. The oceanic lithosphere and slab are assumed ments. The moving plate tends to to move at 63 km Ma)1. White streamlines in the mantle wedge show the recirculation drag shallower asthenosphere towards that develops in this region, colours show asthenosphere and mesosphere speeds, and the subduction zone, which creates a arrows show instantaneous flow directions. (The experiments with a gradational relative pressure high that, in turn, asthenosphere–mantle interface evolve quite similarly to previous experiments in locally tilts the asthenosphere base which this transition was assumed to be discontinuous, as seen by comparing the and drives deeper lateral astheno- evolution of supplementary videos oceanside.mpg and oceanside_sharp.mpg.) The sphere flow away from the subduction asthenosphere return flow beneath the base of the oceanic lithosphere is in good zone as discussed in previous studies agreement with Schubert and TurcotteÕs (1972) simple boundary layer model, a (Schubert and Turcotte, 1972; Phipps boundary layer theory for the entrainment around the slab is discussed in Box 1. Morgan and Smith, 1992). In addition, the subducting slab tends to drag the asthenosphere in comparison with larger than the decrease in thermal thin sheets of asthenosphere into the the underlying cooler mantle (Dq ⁄q = buoyancy due to cooling associated deeper mantle at its top and bottom ) ) aDT, with a 3.3 · 10 5 C 1). Any with the latent heat of partial melting boundaries (Fig. 1b,c). All of these partial melt extraction from upwelling (cf. Phipps Morgan et al., 1995b). Dep- effects are clearly visible in our numer- plumes will actually increase the buoy- letion buoyancy would also arise from ical (Figure 1c) and laboratory (bot- ancy of plume material while trans- the preferential melting of denser and tom-side only) experiments (Figure 3) forming some of the plumeÕs thermal lower-solidus eclogite veins within an as well as in the supplementary videos. buoyancy into compositional deple- eclogite–peridotite ÔmarblecakeÕ mantle. The numerical experiments were car- tion buoyancy. This happens because If both hotter and shallower than ried out in two dimensions with a code the increase in depletion buoyancy the underlying mantle, the astheno- that uses a penalty finite-element for- from melt-extraction in the garnet sphere would also be much weaker. mulation to determine viscous flow, stability field (Boyd and McCallister, Karato and WuÕs (1993) summary of a Smolarkiewicz-upwind-finite differ- 1976; Oxburgh and Parmentier, 1977; the rheology of the uppermost mantle ence technique to solve for the ther- Jordan, 1979) is roughly three times suggests that, for the most probable mal advection–diffusion problem, and

168 2007 Blackwell Publishing Ltd Terra Nova, Vol 19, No. 3, 167–173 J. Phipps Morgan et al. • On subducting slab entrainment of buoyant asthenosphere ......

The asthenosphere viscosity and sub- (a) MOR ARC duction rate are then varied in a suite of experiments. Typical asthenosphere flow exhibited in these experiments is V 19 LITH µasth~ 10 Pa-s OS shown in Fig. 1(c), which represents an ASTHENOSPHERE PH Δρasth~ 1% E R inset near the slab. The experiments E S L from which Fig. 1(c) is taken are A B shown as two of the videos in the V Supplementary Material, which also MESOSPHERE 21 µ m ~ 10 Pa-s contains more details of the initial and boundary conditions. PLUME Low viscosity asthenospheric material is entrained on the (hot) bottom side by downdrag only, with the thickness of the entrained sheet ranging from 10 to 55 km, depending upon the asthenosphereÕs viscosity and buoyancy, and the slabÕs subduction rate (Fig. 2a). A boundary layer (b) theory was developed to estimate the entrainment beneath the base of the he MESOSPHERE subducting slab (Box 1). This simple theory that neglects conductive heat w(x) V V loss into the slab is in good (>90%) ENTRAINED ASTHENOSPHERE agreement with the numerical results for the moderate to large astheno- Q = ∫w(x)dx E sphere viscosities (1019 up to 1020 Pa s) h SLAB e which are numerically best resolved (Fig. 2a). Possibly the growing relative importance of conductive heat loss with decreasing subduction speed is the main reason for the more strongly Box 1 (a) Cartoon of EarthÕs lithosphere, asthenosphere and mesosphere flow. To deviating numerical results at the estimate the rate of asthenosphere entrainment by a subducting slab, we treat the slowest subduction rates. For the low- entrainment process as the Poiseuille flow problem sketched in (b). The slab and est asthenosphere viscosity consid- adjacent mesosphere drag down a sheet of buoyant asthenosphere at a flux Vh, where ered, the boundary layer theory V is the vertical speed of the downgoing slab and h is the thickness of the entrained consistently predicts smaller entrain- asthenosphere sheet. Within the sheet itself, local buoyancy resists subduction and ment thicknesses than observed in the asthenosphere flows upward at a flux Q h3=12l Dq g. The net vertical b ¼ð asthÞ asth numerical calculations, which we entrainment flux QE is the difference between these two opposing flux components, i.e. 3 think is predominantly an effect of QE ¼ Vh ðh =12lasthÞDqasthg. Maximum entrainmentpffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi as a function of h occurs at 2 our inability to resolve the predicted dQE=dh ¼ 0 ¼ V ðhe =4lasthÞDqasthg or he ¼ 4lasthV =Dqasthg. The resulting flux is Q =2Vh ⁄ 3, or twice this value if the subducting slab entrains the asthenosphere on very thin entrainment boundary layer E e with even a 4-km grid, an observation both sides of the slab. The width he of the entrainment finger is 20 km for 19 )1 that will be further discussed below. lasth =10 Pa s, V = 100 mm year , and Dqasth = 1%, with a corresponding two- On the cold top of the slab, entrain- sided entrainment flux Ee =4Vhe ⁄ 3 which is equivalent to 26 km of asthenosphere subducting as part of the downgoing slab. While this solution geometry assumed ment is much larger for the same vertical subduction and entrainment flow, it also applies to slabs dipping at any angle model parameters (Fig. 2b), with from the vertical direction (as the buoyancy force and vertical velocity component of roughly twice the entrainment thick- the subducting slab depend the same way upon the angle of subduction). ness subducting, roughly half by cooling and ÔfreezingÕ to the top of the slab tracer particles to track asthenosphere overlying lithosphere, while heat con- and half by downdragging like at the entrainment on both sides of the sub- ducts into the slab moving at a fixed 45 slabÕs base. If other effects such as the ducting slab – for code details, see angle of subduction. Entrained low- release of fluids from the dehydrating Ru¨pke et al.Õs (2004) paper. In each viscosity asthenosphere is assumed to slab significantly reduce the viscosity experiment, the solution region is a regain the ambient mesosphere visco- of the wedge region, then top-side 2250 km-wide by 1050 km-deep (ocea- sity (1021 Pa s) after passing deeper entrainment may be even more com- nic side) or 2500 km-wide by 1300 km- than 660 km. The initial asthenosphere pletely a ÔfreezingÕ instead of Ôdown- deep (overriding plate side) box, layer is taken to be 0.3% composition- draggingÕ phenomenon. respectively, allowing for a minimum ally buoyant (due to prior melt extrac- To verify our numerical techniques grid-spacing of 4 km in the slabÕs sur- tion) and up to 200C hotter than and further explore entrainment at the roundings. The top boundary is insu- underlying mesosphere, resulting in a base of a downgoing slab, we also lating representing the base of the maximum density reduction of 1%. performed several laboratory experi-

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(a) sphere-slab geometry (angle of subduc- 60 Theory tion 45). The development of the No. experiments entrainment boundary layer and tilting 50 of the base of the buoyant astheno- μ = 1020 Pa s asth sphere towards the subduction zone is 40 shown in Fig. 3 (see also the two videos

30 of laboratory experiments in the Sup- 3.1019 Pa s plementary Material). Numerical cal- Slab-bottom 20 culations using the measured densities 1019 Pa s and viscosities of the laboratory ﬂuids 10 and the dimensions of the experimental

thickness of entrained layer (km) layer of entrained thickness 1018 Pa s setup can reproduce the laboratory 0 10 20 3040 50 60 70 80 90 100 measurements of the entrainment Subduction rate (km Ma–1) thickness to within our measurement error (<10%). (b) 60 No. experiments μ = 3.1019 Pa s asth Discussion 50 1019 Pa s These magnitudes and modes of pre- 40 3.1018 Pa s dicted slab entrainment are of geologi- 1018 Pa s cal interest. For reasonable 30 asthenosphere density contrasts and Slab-top 20 viscosities (0.5–1% less dense and 1018–1019 Pa s), the entrained down- 10 ward ﬂux of asthenosphere is roughly

thickness of entrained layer (km) layer of entrained thickness 20–40% of the downward ﬂux within 0 the lithospheric slab which itself is 10 20 3040 50 60 70 80 90 100 Subduction rate (km Ma–1) build up of former asthenosphere, implying that most asthenosphere re- (c) 160 turns to the deeper mantle by subduct- Theory ing as cold lithosphere instead of being 140 No. experiments directly downdragged by subducting 120 slabs. This happens because 60– 100 km of former asthenosphere has 100 μ 20 asth = 10 Pa s 95 km/Ma accreted to form the lithosphere of the 80 63 km/Ma subducting slab; both as part of the 19 km/Ma 60 mid-ocean spreading process (Phipps Slab-bottom Morgan, 1994, 1997; Hirth and Kohl- 40 μ = 1019 Pa s asth stedt, 1996) and as additional conduc- 95 km/Ma 20 63 km/Ma tive thickening of the lithosphere as it

thickness of entrained layer (km) layer of entrained thickness 19 km/Ma 2 km grid spacing cools to a 100-km-thick thermal 0 theory 4 83216 64 boundary layer beneath 100 Ma sea- Numerical grid spacing (km) floor (Parsons and Sclater, 1977). The implication is that the asthenosphere is Fig. 2 (a) Thickness of the entrained asthenosphere sheet beneath the base of the primarily consumed by forming and subducting slab as a function of slab subduction speed and asthenosphere viscosity. In subducting oceanic lithosphere instead each experiment, the asthenosphere is assumed to be up to 1% less dense than ) of by direct downward entrainment at underlying mesosphere of density 3300 kg m 3, with a gradational base as discussed in subduction zones. In this case, if up- Fig. 1(c). Experiments are marked as solid circles, dashed lines are theoretical maxima given by a boundary layer theory (see Box 1). (b) Thickness of the entrained welling mantle plumes supply new asthenosphere sheet for the slab top-side – same asthenosphere initialization as in (a). asthenosphere at the rate that plate Consistently higher entrainment rates are due to the effects of asthenosphere cooling to growth and subduction consume the top of the slab, which can be as large or even larger than the purely dynamical asthenosphere, then there should exist entrainment effect seen at the base of the slab. (c) Thickness of the entrained a persistent plume-fed asthenosphere asthenosphere sheet at the slabÕs bottom side as observed in numerical experiments layer. The entrainment rates found using different numerical grid-spacing. The theoretical maximum thickness (see Box 1) represent upper rather than lower esti- is marked at the ÔzeroÕ grid-size edge of the plot. Artificially high asthenosphere mates since our two-dimensional entrainment is observed until a fine mesh of 4 km is used to model flow in the region of experiments are idealizing an infinitely the entrained sheet. extended slab in the third dimension, thus not allowing for trench parallel ments (Fig. 3). In each experiment, a syrup layer, with the lithosphere asthenosphere flow or the chance corn-syrup + water mixture overlay a boundary simulated by a sheet of for buoyant asthenosphere to escape denser and more viscous pure corn- plastic moving along a fixed litho- around slab edges or through slab

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–1 0 Fig. 1(a,b). For geologically reason- Plate speed 31 mm min t = 360 sec upper layer able asthenosphere parameters, a viscosity 14 Pa-s plume-fed asthenosphere can form a 1% lower density dynamic low-viscosity boundary layer lower layer decoupling the oceanic lithosphere 5 viscosity 66 Pa-s from deeper mantle ﬂow. Indeed the density 1420 kg m–3 entrained asthenosphere layer may also form a ÔlubricatingÕ layer around the slab itself to at least the depth of

10 the 410-km phase transition, an effect cm that may greatly shape the regional upper mantle flow around a subducting slab, as seen in the detailed flow vectors in Fig. 1(c) or the animation of Fig. 3. As the asthenosphere is then ÔconsumedÕ mainly at mid-ocean rid- ges and by incorporation into the cooling plate, it can form a persistent and geochemically distinct depleted 0 5101520cm MORB source as proposed by Phipps Morgan and Morgan (1999). The Fig. 3 An example laboratory experiment discussed in the text, shown for the case evolution of the asthenosphere wedge where the asthenosphere viscosity is 14 Pa s, the density contrast between astheno- beneath an island arc may also be ) sphere and deeper layer (66 Pa s, 1420 kg m 3)is1%, and the plate ÔsubductionÕ significantly affected by this process. speed is 31 mm min)1. The snapshot of the glass-bead rich Ôasthenosphere layerÕ and Instead of the Ôcorner flowÕ typically the deeper glass-bead poor Ômesosphere layerÕ is shown 360 s after the initiation of the assumed beneath an island arc, the arc experiment. The predicted pressure-induced tilting of the base of the asthenosphere would be underlain by a buoyant and the entrainment of a thin asthenosphere sheet between the subducting slab and wedge of material that recirculates the higher-viscosity layer are evident. (The full time-history of this run is shown as the (see Fig. 1c) and becomes progressively LabMovie31mmpermin.mpg video in the Supplementary material, where there is also a depleted with time until back-arc video of an additional laboratory experiment for a slower plate speed). spreading, a changing subduction geometry, or a pulse of ÔfreshÕ asthenosphere is brought into the back-arc region. windows. In addition a non-New- not plume-fed and had the same poten- It is important to recognize that tonian (stress-dependent) astheno- tial temperature and potential density none of these effects will be resolv- sphere rheology would further weaken as the underlying mantle. In this case, able in global 3-D mantle convection the entrained sheet of asthenosphere, the whole weak layer is downdragged codes until they can incorporate a making it more difficult to drag down and removed by plate subduction. Fig- 5–10 km numerical resolution of asthenosphere. ure 4(c) shows the evolution of a sys- low viscosity regions adjacent to The convection style observed – a tem in which a shallow buoyant layer higher viscosity subducting slabs. thin sheet of asthenosphere entrain- has the same viscosity (1021 Pa s) as its We think that poor numerical reso- ment by a subducting slab coexisting underlying mantle. Here too, the whole lution is likely to be the main reason with large-scale asthenosphere return buoyant layer is rapidly downdragged why the asthenosphere entrainment flow away from the trench – occurs only and removed by plate subduction. seen in Figs 1, 3 and 4(a) and in the when the asthenosphere layer is both Note that the evolution seen in panels online supplementary videos was not less dense and less viscous than the 4(b) and 4(c) is similar to the numerical noticed in previous numerical studies underlying mantle. Changing either evolution of panel 4(a) for numerical exploring asthenosphere–lithosphere asthenosphere density or viscosity experiments with a too-coarse dynamics on a global scale. If a alone results in an asthenosphere layer (>8 km) mesh on which slab entrain- numerical experimentÕs resolution is that is easily dragged down and re- ment cannot be properly resolved – in worse than 5–10 km, it will tend to moved by a subducting slab as seen in this case poor numerical resolution improperly entrain too much Fig. 4(b,c), respectively. Figure 4(a) leads to runs (cf. panel 4d) that evolve asthenosphere as a numerical arte- shows the asthenosphere flow that like simulations with entrainment of fact (Figs 2c and 4d), thus generating evolves after 20 Ma of plate subduc- much higher viscosity asthenosphere. the scenario sketched in Fig. 1(a) as tion through a buoyant, low viscosity In our numerical experiments, we ob- a consequence of poor numerical asthenosphere – the same structure as served spuriously high entrainment resolution. Because of the continuing that seen and analysed previously in rates until the numerical grid-spacing rapid progress in computational Figs 1-3. Figure 4(b) shows the evolu- in the region of the downdragged sheet mantle flow, we believe that the tion of a system in which a shallow low- is less than about 4–8 km (Fig. 2c). computational challenge to success- viscosity asthenosphere has the same Implications for mantle convection fully include slab entrainment of density as its underlying mantle, as are evident in the differences between buoyant asthenosphere will be rap- could happen if the asthenosphere were the convection scenarios sketched in idly overcome.

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(a) (b)

Fig. 4 Comparison of: (a) the dynamics of a buoyant low-viscosity asthenosphere layer; (b) the dynamics of a low-viscosity asthenosphere that has the same density as its underlying mantle; (c) the dynamics of a shallow layer with the same buoyancy contrast as in (a) but for a uniform viscosity mantle. All runs show the evolution after 20 Ma of plate subduction, starting with the same initial conditions apart from differences in viscosity- or density-contrast between the asthenosphere and its underlying mantle. Only the situation in (a) of an asthenosphere that is simultaneously less dense and less viscous (e.g. hotter) than underlying mantle 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 closer to (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. An Supplementary Material movies after 20 Ma, when the whole analysis of the variation of ocean floor asthenosphere is being rapidly down- The following material is available at bathymetry and heat flow with age. dragged and removed by plate subduc- http://www.blackwell-synergy.com: J. Geophys. Res., 82, 803–827. tion. This evolution is similar to These .mpg videos show eight exam- Phipps Morgan, J., 1994. The effect of mid- numerical experiments incorporating ocean ridge melting on subsequent off- ples of the time evolution of numerical an insufficient grid resolution in the axis hotspot upwelling and melting. EOS experiments (one each for entrainment region where a sheet of asthenosphere Trans. AGU, 75, 336. on the bottom and top of a subducting is dragged-down: the video ocean- Phipps Morgan, J., 1997. The generation of slab, two to compare the effects of a side_32 km-grid.mpg shows 25 Ma of a compositional lithosphere by mid-ocean sharp and diffuse asthenosphere–mes- ridge melting and its effect on subsequent time evolution of the reference-case, osphere transition, two in which either off-axis hotspot upwelling and melting. but using a 32 km numerical grid in this the asthenosphere density or viscosity, Earth Planet. Sci. Lett., 146, 213–232. region instead of the 4 km grid-spacing respectively, is equal to that of the Phipps Morgan, J. and Morgan, W.J., used in all other calculations. The flow mesosphere, one showing the effects of 1999. Two-stage melting and the geo- field in the asthenosphere is completely chemical evolution of the mantle: A an inadequate numerical resolution, different with a much weaker return recipe for mantle plum-pudding. Earth and one numerical simulation of a flow evolving and much more astheno- Planet. Sci. Lett., 170, 215–239. laboratory experiment) and two exam- sphere being dragged down by plate Phipps Morgan, J. and Smith, W.H.F., ples of laboratory experiments. subduction. Video overridingside.mpg 1992. Flattening of the seafloor depth- The video oceanside.mpg shows age curve as a response to astheno- shows 20 Ma of time evolution of 25 Ma of entrainment beneath a plate spheric flow. Nature, 359, 524–527. ) asthenosphere and mesophere flow on and slab moving at 63 km Ma 1. The Phipps Morgan, J., Morgan, W.J., Zhang, the top or overriding plate side of the asthenosphere viscosity is 1019 Pa s Y.-S. and Smith, W.H.F., 1995a. subducting slab for the parameters of and the underlying mesosphere visco- Observational hints for a plume-fed sub- our reference-case. In this example the sity is 1021 Pa s, with a gradational oceanic asthenosphere and its role in overriding ÔlandwardÕ plate does not mantle convection. J. Geophys. Res., 100, transition between asthenosphere and move. The landward (right) part of 12753–12768. mesosphere like that after 50 Ma of Fig. 1(c) is taken after 7 Ma from this Phipps Morgan, J., Morgan, W.J. and Price, heat conduction. Asthenosphere and numerical experiment. E., 1995b. Hotspot melting generates mesosphere densities are 3270 and ) The two videos for the laboratory both hotspot volcanism and a hotspot 3300 kg m 3, respectively. Below the swell? J. Geophys. Res., 100, 8045–8062. experiments show examples with iden- depth of 660 km, the entrained mater- Richter, F.M. and McKenzie, D., 1978. tical ÔmesosphereÕ densities and viscos- ial is assumed to regain the ambient ) Simple plate models of mantle convec- ities (1420 kg m 3 and 66 Pa s, mesosphere viscosity. (The above tion. J. Geophys., 44, 441–471. respectively), and nearly identical parameters are referred to as the Ru¨pke, L.H., Phipps Morgan, J., Hort, M. asthenosphere densities and viscosities and Connolly, J.A.D., 2004. Serpentine Ôreference caseÕ in the following.) (1% lower density than the meso- and the subduction zone water cycle. Video oceanside_blow-up.mpg shows sphere, viscosities of 12 and 14 Pa-s, Earth Planet. Sci. Lett., 223, 17–34. 15 Ma of evolution in a zoom of the respectively). Examples for two differ- Schubert, G. and Turcotte, D.L., 1972. entrainment region. The oceanward One-dimensional model of shallow- ent subduction rates are shown, (left) part of Fig. 1(c) is taken after ) mantle convection. J. Geophys. Res., 77, 7 mm min 1 (LabMovie7mmper- 7 Ma from this numerical calculation. ) 945–951. min.mpg) and 31 mm min 1 (LabMov- The video oceanside_sharp.mpg Shen, Y., Solomon, S.C., Bjarnason, I.T. ie31 mmpermin.mpg). Figure 3 shows a shows 25 Ma of entrainment beneath and Purdy, G.M., 1996. Hot mantle static image of the LabMovie31mmper- a slab for the same scenario as video transition zone beneath Iceland and the min.mpg video. Video Num- oceanside.mpg, but with a sharp (dis- adjacent Mid-Atlantic Ridge inferred Sim_of_Lab3 1mmpermin.mpg shows from P-to-S conversions at the 410- and continuous) asthenosphere–meso- the time evolution of a numerical 660-km discontinuities. Geophys. Res. sphere transition. There are only small experiment with an initial geometry, Lett., 23, 3527–3530. differences between the evolution of plate speed, and viscosity structure Sigmundsson, F. and Einarsson, P., 1992. experiments with sharp and diffuse Glacio-isostatic crustal movements chosen to try to match the conditions asthenosphere-mesosphere transitions. ) caused by historical volume change of of the 31mm min 1 laboratory experi- We think this is because asthenosphere the Vatnajo¨kull ice cap. Geophys. Res. ment. The numerical simulation mat- entrainment occurs where the top of the Lett., 19, 2123–2126. ches the detailed entrainment thickness asthenosphere meets the subducting Weertman, J. and Weertman, J.R., 1975. and asthenosphere counterflow struc- slab, for which the Ôsharp-transitionÕ High temperature creep of rock and tures of the laboratory experiment mantle viscosity. Annu. Rev. Earth Pla- and Ôdiffuse-transitionÕ experiments quite well, even matching the time- net. Sci., 3, 293–315. have the same physical properties. evolution in deep flow that occurs with Widmer, R., Masters, G. and Gilbert, F., The videos oceanside_drho0.mpg and the progressive growth of a low-visco- 1991. Spherically symmetric attenuation oceanside_dmu0.mpg show the evolu- sity asthenosphere decoupling zone within the Earth from normal mode tion of systems in which the astheno- data. Geophys. J. Int., 104, 541–553. along the subducting plate interface. sphere has the same density Wiens, D.A. and Stein, S., 1985. Implica- )3 21 tions of oceanic intraplate seismicity for (3300 kg m ) or viscosity (10 Pa s) Received 6 April 2006; revised version plate stresses, driving forces, and rheol- respectively, as the underlying mantle. accepted 16 October 2006 ogy. Tectonophysics, 116, 143–162. Figure 4b and 4c are snapshots of these

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