© 1999 Macmillan Magazines Ltd

Seismic tomography, named in analogyto medical imaging, is a child ofthe early 1980s now grown to vigorous

adulthood. It is the technique involvinganalysis of seismic waves that travel throughthe Earth and are affected in various ways bywhat they pass through en route from source

to sensor. Among other things, seismictomography has shown that flow in theEarth’s mantle is organized into concentrat-ed downwelling and upwelling structures.High-velocity anomalies, where seismicwaves speed up, are comparatively commonand correspond to regions where cold litho-

spheric plates have sunk into the mantle atthe convergent margins of tectonic plates1

(subduction zones) (Fig. 1a). Concentratedlow-velocity (hot) structures are less preva-lent, the largest lying beneath southernAfrica and causing considerable uplift of theAfrican continent2.

On page 246 of this issue3, Van der Voo etal. advance the geological interpretation ofseismic tomography by recognizing the like-ly connection between a major high-velocityanomaly lying 1,500–2,800 km beneathSiberia and the subduction of an oceanicplate during Jurassic time, about 150–200million years ago. This discovery is signifi-cant in several respects — it is the oldest slabof subducted lithosphere individually recog-nized in the deep mantle; it supports the ideathat some slabs sink essentially to the bottomof the mantle1; and it calibrates the time-scale for sinking. Furthermore, this workshows that seismic tomography is a powerfultool for palaeogeography, providing a freshsource of information about the history ofglobal plate motions and the processes ofcontinental growth.

This level of maturity in understandingmantle dynamics stems from continuousimprovements in both seismic imaging andits geodynamic interpretation. In the mid-1980s it was discovered that large-scale(10,000–20,000-km) horizontal variationsin seismic velocity represent mantle densityanomalies that give rise to the shape of theEarth’s gravity field, or geoid4. This large-scale structure is a consequence of wherecold lithospheric plates have sunk into themantle during the past 200 million years ofEarth history5 (Fig. 1b) — that is, most ofCenozoic and Mesozoic time. But subductedslabs in the deep mantle are only a few hun-dred kilometres thick and could not be wellresolved in early tomographic studies. Nowthey can be, allowing detailed geologicalinterpretation of the history of convergentplate margins in particular.

Seismic tomographers employ manytypes of seismic vibration to image the deepEarth. But the most compelling images havebeen produced using higher-frequency‘body’ waves travelling from earthquakesources to the worldwide network of seis-mometers. The speeding-up or slowing-down of a seismic ray as it passes through ahigh- or low-velocity region perturbs itsarrival time. If the locations of the earth-quakes are well known, thousands of suchmeasured arrival times (and waveforms) canbe mathematically ‘inverted’ to provide athree-dimensional model of seismic velocitystructure in the mantle.

This kind of analysis is highly complicat-ed, requiring powerful computers and hugeamounts of seismic data. In 1994, however, abreakthrough came in a study, published byGrand6, in which body shear-waves wereused to image the extinct Farallon plate,

NATURE | VOL 397 | 21 JANUARY 1999 | www.nature.com 203

news and views

Prospecting forJurassic slabsMark A. Richards

Increasingly precise seismic imaging of the Earth’s deep interiorcontinues to sharpen our understanding of mantle convection and platetectonics. One such study has traced events down to 2,800 km beneathEarth’s surface, and back to 200 million years ago.

Figure 1 Comparison of models of mantle structure at 1,500 km depth. a, Shear-wave tomographymodel1 (velocity variations in percent; blue is high velocity, red is low velocity). b, Geodynamic, orsubduction, model14 (temperature variations in kelvin; blue is low temperature, red is hightemperature). The geodynamic model accounts for thermal convection beneath surface plates whosemotions are derived from plate tectonic reconstructions for the past 120 million years. The twomodels are similar in that structure is dominated by cold, high-velocity anomalies corresponding tosubduction zones. But they differ in two important respects. First, concentrated hot (upwelling)structures are absent in the geodynamic model. Second, the positions of subducted slabs in thegeodynamic model correspond only roughly to those observed seismically: for example, the seismicsignature of the Farallon plate beneath North America lies about 1,000–1,500 km east of its locationin the geodynamic model. Also, the geodynamic model has too little convergence between Europe andAfrica, and so fails to explain the high-velocity anomaly beneath the eastern Mediterranean region.The top of the sub-Siberian high-velocity anomaly, the subject of the new study by Van der Voo et al.3,is seen faintly with shear-wave tomography, but is completely absent in the subduction model, whichdoes not include the Mongol–Okhotsk subduction.

© 1999 Macmillan Magazines Ltd

sinking from 700-km to 2,800-km depthbeneath North and South America (Fig. 1a).Subsequently, using high-resolution com-pressional-wave tomography, Van der Hilstet al.7 further demonstrated the prevalenceof sheet-like high-velocity structuresbeneath past and present subduction zones,some extending to the boundary betweenEarth’s liquid outer core and rocky mantle.The difference between shear and compres-sional waves need not concern us here.Rather, the point is that these entirely inde-pendent data sets delivered spectacularagreement between images in areas wherethe quality of seismic ray coverage wassimilar1.

Van der Voo et al.3 now take ‘seismo-slabology’ a step further in relating a hypoth-esized case of continental accretion in Juras-sic time to a hitherto unexplained high-velocity seismic anomaly beneath Siberia.Large continents such as Eurasia are formedby collisions of smaller continental frag-ments, preceded by subduction of the inter-vening oceanic plate. In Late Jurassic time, asa block of continental fragments (includingmuch of present-day Mongolia, China andSoutheast Asia) moved towards Siberia fromthe southeast, the Mongol–Okhotsk Oceanbasin closed8 (see map on page 249). TheMongol–Okhotsk oceanic plate was con-sumed, and the Verkhoyansk Mountainswere pushed up, as the continental blockscollided.

Subduction ended by the Early Creta-ceous, or about 135 million years ago, so thata large slab of oceanic lithosphere becamedetached from the overlying lithosphere as itsank into the deep mantle. Improved tomo-graphic resolution of sub-Eurasian struc-ture9 reveals a large, high-velocity (cold)anomaly sinking in the mantle just below thehypothesized site of the Mongol–Okhotsksubduction, with the top of the anomalybeing at about 1,500 km depth. Coupledwith the likely date at which subductionstopped, this implies that the slab has sunkinto the deep mantle at about 1 cm yr11 —slower than, but roughly in accord with, pre-vious estimates6,10. The anomaly extends tothe core–mantle boundary, where it joins amuch broader high-velocity structure thatunderlies much of eastern Eurasia and alsoconnects with active subduction zones in thewestern Pacific.

The Mongol–Okhotsk slab seems to haveundergone little horizontal displacementsince Jurassic time relative to the present-day‘suture zone’ of the continental collision.Even more remarkably (and perhaps coinci-dentally), it preserves the ‘Z’ shape, in mapview, of the old subduction zone along themargin of the Siberian block. Although otherpossible explanations cannot be ruled outentirely, that of Van der Voo et al.3 looksto be plausible in all respects.

The interpretation of the sub-Siberian

high-velocity anomaly in terms of Mon-gol–Okhotsk subduction supports modelsof supercontinent aggregation that invokesubducted slabs as the driving mechanismfor convergence between continentalblocks11. Straightforward modelling of theoceanic lithosphere as the upper thermalboundary layer for mantle convection showsthat slabs should survive as concentratedthermal anomalies during their 100–200-million-year journey to the bottom of themantle, a view supported by the interpreta-tion of Van der Voo et al.

The case is not yet completely closed,however. Some slabs appear to be inhibitedas they encounter the mantle transition zone(400–800-km depth), where a series of min-eral phase transitions induced by increasingpressure may cause complicated buoyancyand rheological effects7,12. Another concernis that the sub-Siberian anomaly appears tomaintain a faint high-velocity connectionall the way to the surface, even though sub-duction ceased by the Early Cretaceous.Although this upper mantle extension ispoorly resolved by the seismic ray coverageused, better imaging of the sub-Siberianregion should aid in refining interpretationof the large, deep-mantle anomaly.

Seismic tomography is providing similarinsights into the fates of other subductedslabs. For example, the old Farallon plate isfound1,6,7 about 1,000–1,500 km to the eastof where a simple subduction history modelmight place it (compare Figs 1a and b). Thiskind of information is important, because

subducted slabs are probably the largestdynamic source of mantle buoyancy —which, among other things, controls theheight of continents and the stratigraphicrecord of sea level13. As illustrated by theresults discussed here, the new generation ofseismic tomography tools provides geolo-gists and geophysicists with tantalizing foodfor thought. It is allowing them to refinemodels for past plate motions and mantleconvection, and so arrive at a more completepicture of Earth history during Cenozoicand Mesozoic time.Mark A. Richards is in the Department of Geologyand Geophysics, University of California, Berkeley,California 94720, USA.e-mail: markr@seismo.berkeley.edu1. Grand, S. P., Van der Hilst, R. D. & Widiyantoro, S. GSA Today

7, 1–7 (1994).

2. Lithgow-Bertelloni, C. & Silver, P. G. Nature 395, 269–272

(1998).

3. Van der Voo, R., Spakman, W. & Bijwaard, H. Nature 397,

246–249 (1999).

4. Hager, B. H., Clayton, R. W., Richards, M. A., Dziewonski,

A. M. & Comer, R. P. Nature 313, 541–545 (1985).

5. Richards, M. A. & Engebretson, D. C. Nature 355, 437–440

(1992).

6. Grand, S. P. J. Geophys. Res. 99, 11591–11621 (1994).

7. Van der Hilst, R. D., Widiyantoro, S. & Engdahl, R. Nature 386,

578–584 (1997).

8. Ziegler, A. M. et al. in The Tectonic Evolution of Asia (eds Yin, A.

& Harrison, M.) 371–400 (Cambridge Univ. Press, 1996).

9. Bijwaard, H., Spakman, W. & Engdahl, R. J. Geophys. Res. (in

the press).

10.Ricard, Y., Richards, M. A., Lithgow-Bertelloni, C. & Le Stunff,

Y. J. Geophys. Res. 98, 21895–21909 (1993).

11.Gurnis, M. Nature 332, 695–699 (1988).

12.Fukao, Y., Obayashi, M., Inoue, H. & Nenbai, M. J. Geophys.

Res. 97, 4809–4822 (1992).

13.Gurnis, M. Nature 364, 589–593 (1993).

14.Bunge, H.-P. et al. Science 280, 91–95 (1998).

news and views

204 NATURE | VOL 397 | 21 JANUARY 1999 | www.nature.com

The apples you buy at the grocery willusually have completed a long jour-ney, including shipment from the

farmer to your home town then delivery tothe grocer. Different modes of transport mayhave been used, such as a train for the longhaul and a van for the local dispatch, and thelegs of this voyage are coordinated by closelyinteracting business partners. Similarly, ani-mal cells may use different transport systemsto deliver membrane-bound components,such as secretory vesicles, from the site atwhich they’re packaged in the centre of thecell to their destination at its periphery.Long-range movements are carried out bymolecular motors (‘trains’), using micro-tubules as their tracks. Often, however,microtubules do not reach the plasma mem-brane, and a different set of motors (‘vans’)carry out the last leg. These motors move onactin filaments, so they can traverse theactin-rich cell cortex.

According to a report by Huang et al.1 onpage 267 of this issue, there is one big dif-

ference between transporting vesicles andapples — cells combine the two differentmeans of transport in one bifunctional ve-hicle. The authors provide convincingevidence that a microtubule motor, conven-tional kinesin, and an actin motor, myosinVa, interact directly (Fig. 1).

Conventional kinesin (the prototype ofthe growing kinesin superfamily) is believedto be involved in many forms of micro-tubule-mediated vesicle and organelle trans-port2. Myosin V is one of at least a dozenclasses of unconventional (that is, non-fila-mentous) myosins that move on actin fila-ments3. Analyses of myosin V mutants inorganisms as divergent as yeast and mouseindicate that these myosins are also organellemotors. For example, mice with defects inthe dilute locus, which encodes myosinVa, have a washed-out (‘diluted’) coatcolour. This is because the transfer ofpigment organelles (melanosomes) frommelanocytes to the skin cells involved in hairformation is compromised, so melanosome

Protein transport

Molecular motors join forcesManfred Schliwa