Mesozoic subducted slabs under Siberia
ROB VAN DER VOO*†, WIM SPAKMAN† & HARMEN BIJWAARD†
Nature 397, 246 - 249 (1999) © Macmillan Publishers Ltd.
Recent results from seismic tomography demonstrate that subducted oceanic lithosphere can be observed globally as slabs of relatively high seismic velocity in the upper as well as lower mantle1,2. The Asian mantle is no exception, with high-velocity slabs being observed downwards from the west Pacific subduction zones under the Kurile Islands, Japan and farther south3,4,5, as well as under Asia's ancient Tethyan margin. Here we present evidence for the presence of slab remnants of Jurassic age that were subducted when the Mongol-Okhotsk and Kular-Nera oceans closed between Siberia, the combined Mongolia-North China blocks and the Omolon block6,7,8. We identify these proposed slab remnants in the lower mantle west of Lake Baikal down to depths of at least 2,500 km, where they join what has been interpreted as a 'graveyard'9 of subducted lithosphere at the bottom of the mantle. Our interpretation implies that slab remnants in the mantle can still be recognized some 150 million years or more after they have been subducted and that such structures may be useful in associating geodynamic to surface-tectonic processes.
Eurasia is, without doubt, the next supercontinent in the making, as continental elements such as Australia and even the Americas are steadily approaching it and seem destined to join it in the future. The growth of Asia started, however, a long time ago, when significant additions to its Siberian core began to occur through continent-continent collisions and accretion of smaller continental blocks6. The Mongolia and North and South China blocks, having amalgamated in the Permian-Triassic periods10 (or possibly as late as the Jurassic period11) joined Siberia in Late Jurassic12 or Early Cretaceous times8 (Fig. 1). We will call the former ocean between Siberia and Mongolia the Mongol-Okhotsk Ocean, but it has also been called the Khangai-Khantey Ocean6. The northeastern-most Siberian block (Omolon) accreted to the growing Asian continent in the Late Jurassic, closing the Kular-Nera Ocean6, and causing the Verkhoyansk Mountains13 (Fig. 1).
|
Figure 1 Early Cretaceous and Late Jurassic palaeoreconstructions of Siberia (SIB) and adjacent Asian blocks8. (Redrawn from ref. 8 with permission from the authors, R. Enkin et al.) a, Early Cretaceous; b, Late Jurassic. Late Jurassic subduction of the Mongol-Okhotsk Ocean is shown as occurring under the southeastern and northeastern (Verkhoyansk) margins of Siberia. Subduction probably also occurred under the Mongolian margin6, but this is not shown here. Abbreviations of block names are as follows: EUR, Europe; KAZ, Kazakhstan; MON, Mongolia; NCB and SCB, North and South China blocks; INC, Indochina; and SH, Shan-Thai block8. |
Improvements in tomographic techniques in the past few years have allowed more detailed visualization of deep fossil slabs, such as those that appear to have resulted from subduction of the Pacific and Tethyan oceans1,14. Tomography parametrizes the mantle either by spherical harmonic functions or by using local cell slowness functions. The sizes of these cells in the latter parametrization have steadily decreased from early values15 of about 10° to recent values1,14,16 of about 2°-0.6°. Improvements in the travel-time data set17, the use of a large number of composite rays, and variably sized cells that minimize the difference in cell hit count18 have now14 provided tomographic images with great detail. Examples of the resolution, data fit, and sensitivity tests for the tomographic results we interpret here are more fully described elsewhere14.
Figures 2 and 3 show map views at various lower-mantle depths and a cross-section under Siberia. The Mongol-Okhotsk and Verkhoyansk sutures are marked by a P-wave velocity contrast in the crust and uppermost mantle (not shown), also visible in surface wave studies19,20. Clear anomalies, well resolved both vertically and horizontally, are seen at depths greater than 1,200 km (Fig. 2). In the crust, the Mongol-Okhotsk-Verkhoyansk suture runs from near the northern shore of Lake Baikal to the Sea of Okhotsk (for locations, see Fig. 2), and from there northwards under the Verkhoyansk Mountains. At depths greater than 1,500 km, the velocity anomalies are pronounced; amplitude variance tests indicate that the variation (of about +0.3% or higher) is well above amplitude noise level in the lower mantle14. At 1,500 km depth, the anomalies form a 'hook', running first west-northwestwards from Mongolia and then northwards towards the Siberian Arctic coast. At depths of 1,900-2,300 km, the anomalies are gradually more displaced to the west, and display an overall 'Z'-shaped feature at 2,300 km (see curved yellow line in Fig. 2). At 2,500 km and below, the high-velocity anomalies merge with a broad east Asian high-velocity area in the lowermost mantle (Figs 2 and 3), which has been called a 'graveyard' of slabs9, an interpretation corroborated by our results.
 |
Figure 2 Tomographic
P-wave velocity anomaly patterns in the deep mantle under Asia, for four different depths
between 1,500 and 2,700 km. The superposed straight line indicates the location of
the cross-section of |
 |
Figure 3 Cross-section
through tomographic model. Top, tomographic results displayed in a vertical section along
the line indicated in |
In the cross-section (Fig. 3, top), this feature (M) is illustrated side-by-side with the currently active subduction zone (P) under northern Japan. Below 1,400 km, we see the southern leg of the hook to the northwest of Mongolia portrayed all the way down to the core-mantle boundary. Figure 3 also illustrates, for this same cross-section, how a simulated input of a layer-cake model14 is resolved in our inversion. The figure shows that below depths of 1,500 km, the M and P anomalies are both well resolved. The overall amplitude and spatial characteristics of the high P-wave velocity anomalies in the deep mantle under Siberia are comparable to those observed around the Pacific and under the Mediterranean-Himalaya-Indonesian belt and warrant a similar interpretation: as remnants of subducted slabs. Some positive anomalies are imaged between 400 and 1,200 km depth, but these are spatially scattered, of low amplitude, and not resolved. For instance, the 'connection' between upper and mid-mantle (>1,200 km) anomalies in Fig. 3 (top) is rather local and unresolved.
Identification of the Pacific and Tethyan subduction zones in the maps and section is rather straightforward1,2,14; thus, we are confident that the high-velocity zone we have described under Siberia is not related to Cenozoic subduction. However, elimination of other, Mesozoic, subduction zones under Asia as candidates for the deep-mantle portions of the Siberian slab is more difficult: this is because the Asian lithosphere may have drifted significantly with respect to the deeper mantle formerly underlying it. Because of the increasing certainty that the hotspots have moved significantly with respect to each other, they do not provide a suitable framework to assess this issue21,22. However, the western and eastern Pacific subduction zones and their deep slabs provide an alternative approach. The western Pacific subducted slab dips westwards and then sinks down vertically in the lower mantle, without large apparent displacements from where one would expect to find it (Figs 2, 3). In contrast, the deep-mantle slabs resulting from east Pacific subduction under North America in late Mesozoic-Palaeogene times can be recognized under the Great Lakes area and east of there, as far away as New England2,14. The North American lithosphere has clearly moved far to the west with respect to these deep-mantle slabs. From (1) these relative displacements of the two slabs with respect to the present lithospheric locations where they started their downward journeys, and (2) the displacements of the continents during the opening of the Atlantic Ocean, we can deduce that the Siberian lithosphere has moved very little in an east-west direction with respect to the deep-mantle slaps; we can also deduce that the Atlantic Ocean has spread primarily by westward drift of North America, which has overridden the slabs under it to the extent of some 3,500 km or more. Moreover, other deep slab remnants under Eurasia due to Tethyan subduction are not shifted in an east-west sense. It seems justifiable, therefore, to infer that the deep-mantle slab we have described under Siberia is not related to subduction other than that of the Mongol-Okhotsk and Kular-Nera oceans.
In Fig. 4 we show the migration of this slab as a function of depth, and compare it with the palaeolatitudinal location of the lithospheric Mongol-Okhotsk-Verkhoyansk suture zone as a function of geological time. This figure shows that the orientations of the present-day suture zones at the surface parallel a higher-velocity contrast within the lithosphere down to depths of at least 95 km. At depths of 1,200 km or more, the velocity anomalies are rotated anticlockwise from the surface orientation. However, their locations correspond to the (palaeogeographically restored) position of the eastern and southern margin of Siberia in early Tertiary and older times, but are further to the west, presumably owing to the angle of generally westward subduction during Late Jurassic times (Fig. 4).
 |
Figure 4 A comparison of the locations of tomographic velocity anomalies and the expected palaeolocations of the Siberian active margins. a, Locations of the lithospheric suture and the main axis of the ocean-lithospheric slab remnants as a function of depth, as determined from our tomographic results. b, Present-day and palaeogeographically reconstructed locations of the Mongol-Okhotsk-Verkhoyansk suture zones as a function of time (longitudes are arbitrary), using the palaeomagnetic pole determinations for Siberia of Zhao et al.29 (81° N, 158.6° E for 50 Myr; 73.8° N, 202.4° E for 85-120 Myr; 70.1° N, 184.3° E for 150 Myr). |
The 'Z'-shape of the slabs below 2,000 km (Fig. 2) is interpreted as the result of a similarly 'Z'-shaped surficial subduction pattern of the Kular-Nera and Mongol-Okhotsk oceans not only under Siberia itself8 (Fig. 1), but under Siberia's Jurassic northeastern and southeastern margins, as well as under the northern margin of Mongolia (see Fig. 21.39 in ref. 6).
Given that subduction ceased about 150 Myr ago, the depth of about 1,500 km
for the top of the deep slab under Siberia implies an averaged sinking velocity of
1 cm yr-1 since the Jurassic period, which is not very different from
sinking velocities inferred in some previous studies2, although higher rates of 
3 cm yr-1 have also been proposed23. Subduction velocities are thought
to reduce by a factor of about four when slabs begin to penetrate into the
higher-viscosity deeper mantle24;
a sinking velocity of 1 cm yr-1 would then imply a surficial
convergence rate of 4 cm yr-1. Even if subduction occurred on only
one (for example, the Siberian) side of the Mongol-Okhotsk Ocean, the width of this ocean
may have been at least 2,000 km at 200 Myr, which matches plate reconstructions25.
We argue that the high-velocity structures under Siberia are logically interpreted as remnants of oceanic lithosphere that subducted before the Early Cretaceous and that, therefore, subducted lithosphere of Jurassic age can still be recognized after penetration into the lower mantle even after subduction stopped some 150 Myr ago. Whether this visibility is a result of temperature26, composition, pressure, or a combination thereof, is an open question, but what is clear is that the Mongol-Okhotsk subducted lithospheric material has been feeding the 'graveyard' of slabs under Asia. Our conclusions also imply that significant downwelling is a characteristic of growing supercontinents for hundred of millions of years27, and that most, if not all, significantly fast anomalies in the deeper mantle appear to be associated with past subduction. This renders tomography an important tool for testing palaeogeographical reconstructions.
Received 23 June;
accepted 27 October 1998.
A RELATED PAPER IS HERE.
References
engör,
A. M. C. & Natal'in, B. A. in The Tectonic Evolution of Asia (eds Yin, A. &
Harrison, M.) 486-640 (Cambridge Univ. Press, 1996). Links
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Acknowledgements. We thank M. Richards and M. Gurnis for constructive comments. H.B. was supported by the Netherlands Organization for Scientific Research (NWO).
Correspondence and requests for materials should be addressed to R.V.d.V. (e-mail: voo@umich.edu).