Lecture 10

Seismic Tomography and

Mantle Convection

Tomography: Deviations of real Earth structure from a reference Earth model

Dziewonski & Anderson, 1981

Zhao et al., Science, 1997

Masters et al. (2000)

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spots—such as Iceland, the Azores, and Tris-tan in the mid-Atlantic—are located on orclose to a mid-ocean ridge; the plume be-neath Hawaii now lies in the middle of thePacific Plate but may have interacted strong-ly with a ridge early in its evolution (12).

The earliest studies of ridge-plume in-teractions demonstrated that a plume and aridge can interact over distances of morethan 1000 km (13, 14). In the classicalsource-sink model (14), plume and ridgeinteract via a sublithospheric channel; asthe plume (or ridge) migrates, the ridge isfed through this channel. Since the firstnumerical modeling of plume-ridge inter-action dynamics (15), the latest simula-tions consider a radially expanding andpulsating plume model and allow forplume flux estimates (16). They are able toexplain, for instance, the V-shaped gravityanomalies over the Reykjanes Ridge.

To understand whether plumes take anactive or passive role and how they interactwith mid-ocean ridges and lithosphere faraway from plate boundaries, we need toknow the depth extent of mantle upwellingbeneath hot spots. Seismological analyses

help to provide the answer because melt pro-duction, crustal thickness, and sublithospher-ic flow all leave a seismic signature. Modelsof seismic anisotropy can be used to infer theflow geometry in Earth’s interior because thefast axis of olivine (a key mantle mineral)tends to align in the flow direction (17).Mapping the three-dimensional isotropicseismic velocity structure of plumes at depthremains a challenging task, however (18).Plume conduits may be very narrow (100km) and cannot easily be detected. In addi-tion, temperature and vertical alignment ofolivine crystals have opposite effects on ver-tical seismic waves. To date, most seismicmodels are isotropic, but a first attempt (1)to relate seismic anisotropy to plume-ridgeinteractions looks very promising.

A recent global-scale tomographic mod-el (see the figure) (19) suggests that lowseismic velocity anomalies beneath theMid-Atlantic Ridge and hot spots in the At-lantic Ocean are confined to the upper 150km of the mantle, with the exception of Ice-land. Low velocities beneath Iceland extendwell into the upper mantle transition zone.A comparable anomaly is seen in high-res-

olution regional tomographic models (20).Global models of seismic anisotropy (21)still lack the resolution required for precisemapping of flow patterns around plumes,but Gaherty (1) was able to derive an aver-age model of radial anisotropy for the Mid-Atlantic Ridge south of Iceland. Assumingthat deformation does not occur under wa-ter-rich conditions (22), his model suggestsvertical flow below the shallowest (<100km) part of the ridge and horizontal flow toa depth of 200 km. This indicates that theIceland plume induces buoyancy-driven up-welling beneath the ridge.

Precise estimates of the lateral extent ofthe upwelling beneath Iceland requiremore detailed anisotropic tomographicmodels. The new anisotropy data and the50 K temperature anomaly along theReykjanes Ridge (which is much smallerthan that proposed by Schilling (14)] pro-vide strong constraints for numerical mod-eling of plume-ridge interaction. This isimportant because seismic models can on-ly provide an instantaneous image of flow.

Numerical modeling with realistic pa-rameters and boundary conditions is re-quired to reconcile geophysical and geo-chemical data with seismic tomographicimages and to make accurate estimates ofthe geodynamic importance of the interac-tion between a plume and a ridge. Seismicexperiments, possibly with ocean-bottomseismometers to extend seismic arrays be-yond islands, are crucial for constrainingthe mantle transition zone structure be-neath ridges and intraplate hot spots andaddressing whether plumes play an inde-pendent role in mantle convection.

References1. J. B. Gaherty, Science 293, 1645 (2001).2. W. J. Morgan, Nature 230, 42 (1971).3. J. T. Wilson, Can. J. Phys. 41, 863 (1963).4. W. J. Morgan, J. Geophys. Res. 83, 5355 (1978).5. V. C. Courtillot et al., Earth Planet. Sci. Lett. 166, 177

(1999).6. M. A. Richards, R. A. Duncan, V. Courtillot, Science

246, 103 (1989).7. R. S. White, D. McKenzie, J. Geophys. Res. 94, 7685,

(1989).8. M. Ishida et al., Earth, Planets and Space 51, 1 (1999).9. D. L. Anderson, Science 293, 2016 (2001).

10. S. D. King, D. L. Anderson, Earth Planet. Sci. Lett. 136,269 (1995).

11. S. D. King, J. Ritsema, Science 290, 1137 (2000).12. R. A. Keller, M. R. Fisk, W. M. White, Nature 405, 673

(2000).13. J. G. Schilling, Nature 242, 565 (1973).14. ———, Nature 314, 62 (1985).15. N. M. Ribe, U. Christensen, J. Theissing, Earth Planet.

Sci. Lett. 134, 155 (1995).16. G. Ito et al., Nature 411, 681 (2001).17. A. Nicolas, N. I. Christensen, Earth Planet. Sci. Lett.

134, 155 (1995).18. H.-C. Nataf, Annu. Rev. Earth Planet. Sci. 28, 391

(2000).19. J. Ritsema, H. J. van Heijst, J. H. Woodhouse, Science

286, 1925 (1999).20. R. M. Allen et al., J. Geophys. Res., in press.21. J. P. Montagner, L. Guillot, in Problems in Geophysics

for the Next Millenium, E. Boschi et al., Eds. (EditriceCompositori, Bologna, Italy, 2000), pp. 218–253.

22. H. Y. Jung, S.-I. Karato, Science 293, 1460 (2001).

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Beneath the Mid-Atlantic Ridge. Modeled (18) shear velocities (bottom) along a cross sectionalong the Mid-Atlantic Ridge from the South Atlantic to the Arctic Ocean (top). Circles on the ridge(green line) mark 10° intervals. Triangles mark hot spots. Low shear velocities relative to the globalaverage are represented by shades of red, with an intensity proportional to the magnitude of theanomaly. These anomalies mainly correspond to elevated temperatures with respect to the ambi-ent mantle. Low seismic velocity anomalies beneath the ridge and hot spots in the Atlantic Oceanare confined to the upper 150 km of the mantle (dashed line), with the exception of Iceland.

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model Crust5.1 (17). We assumed an isostati-cally compensated crust in the calculation ofthe gravity field. Our “dynamic topography,”therefore, is the nonisostatic topography ofthe free surface that is produced by densityanomalies within the entire mantle, includingthe lithosphere.

In the inversion, we allowed for lateral vari-ations in S velocity, P velocity, density, dynam-

ic free-surface topography, and topography onthe 660 and the CMB (18). Initially, we invert-ed for topography on the 410-km discontinuity(410) as well but found that undulations on thisboundary are poorly constrained by the data(19). Therefore, topographic variations on 410are those determined in a recent travel-timestudy (20). The starting model for the inversionincluded S velocity model SKS12WM13 (1)and P velocity model P16B30 (3). The densitycomponent of the starting model was obtainedby scaling velocity model SKS12WM13 by afactor of 0.2. There was no starting dynamictopography on the free surface or the CMB,except for its excess ellipticity, which was de-termined by very long baseline interferometry(21). Starting topography on 660 was takenfrom Gu et al. (20). This starting model ex-plains 74% of the variance in the mode data buthas a relatively high !2/N of 6.6, where N "2850 is the number of normal-mode data (22).Model SPRD6, obtained by an inversion of

normal-mode and gravity data, explains 92% ofthe variance in the mode data and 96% of thevariance in the gravity data, which correspondto !2/N " 2.0 and !2/Nf " 0.07, where Nf " 27is the number of free-air gravity coefficients.The fit to the free-air gravity anomaly is excel-lent; however, it is well known that the gravityanomaly can be relatively easily fit because ofits highly nonunique dependence on densityand boundary topography. Therefore, we didnot allow free-air gravity to change models ofdensity and boundary topography substantiallyfrom models determined with only normal-mode data.

We compared the S model obtained fromthe normal-mode inversion with a recent modelbased on body-wave data, SKS12WM13 (1)(Fig. 1). In the shallow mantle, continents arecharacterized by fast velocity anomalies, andmid-ocean ridges correspond with slow ve-locity anomalies. There are strong heteroge-neities in the lowermost mantle, with a dis-tinct ring of high-velocity anomalies aroundthe Pacific Ocean. The strong negative anom-alies in the lowermost mantle underneath thePacific Ocean and Africa are interpreted asmantle upwellings. The correlation betweenthe two models is high in the upper mantleand near the CMB, but a lower correlation isfound in the mid-mantle, which is relativelypoorly constrained by travel-time and wave-form data.

We compared the P part of model SPRD6with P velocity model P16B30 (3) (Fig. 2). Theelongated velocity anomalies at mid-mantledepths underneath the Americas and Eurasiaare characteristic of several recent travel-timemodels (23). The two P velocity models arewell correlated at the top of the mantle andaround 2500-km depth but show lower correla-tion in the mid-mantle and near the CMB. The

Fig. 3. Relative perturbations in even-degree den-sity at six discrete depths as in Figs. 1 and 2. Thescale for each map (from the top) is #1.0, #0.5,#0.5, #0.5, #0.8, and #1.0%, respectively. Blueregions denote higher than average density, andred regions denote lower than average density.The maximum and minimum values of the den-sity model are summarized in Table 1.

A BFig. 4. Shear velocity,bulk sound velocity, anddensity models at 2800-km depth. (A) Map viewsof shear velocity (top),bulk sound velocity (mid-dle), and density (bot-tom) from an inversionwith these model pa-rameters (model SBRD6).The color scheme is thesame as in the previousfigures. The scale for themaps is #2.5, #2.0,and#1.5% for S veloc-ity, bulk sound velocity,and density, respec-tively. Note the stronganticorrelation betweenshear and bulk soundvelocity (correlation co-efficient of $0.63). (B)Same as in (A), butshowing only the de-gree 2 component. Thescales for the maps are #0.7, #0.4, and #0.5%, respectively.

Table 1. The maximum and minimum values ofthe density model with the associated error at thegiven depth. The average value of the model isalways zero because of the absence of a degree 0coefficient. The errors are uniformly distributedover the sphere, which reflects the fact thatmodes are sensitive to structure everywhere onthe globe. The error values are much smaller thanthe extrema of the model.

Depth(km) Maximum Minimum Maximum

error

100 0.86 $0.83 0.12600 0.62 $0.51 0.081300 0.60 $0.60 0.101800 0.53 $0.53 0.102300 0.99 $0.79 0.122850 1.7 $1.5 0.16

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