Seismic tomography

Much important information on the three-dimensional structure of the mantle has been supplied by seismic tomography (Section 2.1.8). Convection is driven by lateral differences in temperature and density. These variables affect seismic velocity, which typically decreases with decreasing density and increasing temperature (Dziewonski & Anderson, 1984). By mapping velocities in the mantle it is possible to infer the differences in temperature and density that are a consequence of convection. Also, by mapping seismic anisotropy both vertically and laterally it is possible to obtain estimates of the direction of mantle flow.

The first three-dimensional seismic velocity models for the mantle derived by the tomographic technique were published in the early 1980s (Wooddhouse & Dziewonski, 1984). Since then there have been great improvements in data quality, geographic coverage, and processing techniques, and the resolution of subsequent models has greatly improved. However, many of the essential features of the velocity variations were apparent in the earliest models. Plate 12.1 (between pp. 244 and 245) shows the variations in the shear wave velocity at 12 depths in the mantle according to model S16B30 of Masters et al. (1996). It is immediately apparent that the greatest variations occur near the top and bottom of the mantle, presumably within or in the vicinity of the thermal boundary layers. Within the top 200 km the perturbations are very closely related to surface tectonic features. Ocean ridges, the rifts of northeast Africa, and the active backarc basins of the western Pacific are all underlain by anomalously low velocity mantle. Continental areas in general, and shield areas in particular, are underlain by the highest velocities, and older oceanic crust by relatively high velocities. These variations essentially reflect the different thermal gradients and hence the thickness of the lithosphere in these areas (Section 11.3.1). Between 200 and 400 km most of these generalizations still apply but the velocity contrasts are lower. A notable exception is the mantle beneath the backarc basins where the slow velocities at shallower depths have been replaced by near zero anomalies. In the transition zone (e.g. 530 km) the variations are in general quite small and the correlation with surface features has largely broken down. Again an exception is the mantle beneath the backarc basins of the western Pacific, which, at this depth, is characterized by high velocities presumably associated with cold subducted lithosphere. In the lower mantle (depths greater than 660 km) the variations in shear wave velocity are generally quite small (less than ±1.5%), but a persistent feature is a ring of higher than average velocities beneath the rim of the Pacific. This becomes particularly marked in the lowest 400 km of the mantle (e.g. depths of 2500 and 2750 km). At depths greater than 2000 km, large regions of anomalously low velocity occur beneath the central Pacific, and beneath southern Africa and part of the South Atlantic.

Plate 12.2 (between pp. 244 and 245) shows four cross-sections through the shear wave velocity model of Masters et al. (1996), each on a plane passing through the center of the Earth. Three of these sections are longitudinal sections, that is, the planes also pass through the north and south poles; the fourth is an equatorial section. Note that in each cross-section the great circle showing the intersection of the plane of the section with the Earth's surface is the smallest circle on the diagram. Plate 12.2a (between pp. 244 and 245) clearly illustrates the way in which the high velocities associated with continental areas, such as North America and Eurasia, and the low velocities associated with mid-ocean ridges, such as the East Pacific Rise and the mid-Indian Ocean ridge, only extend to depths of 200-400 km within the upper mantle. The section in Plate 12.2b (between pp. 244 and 245) passes through the central Pacific and southern Africa and reveals the low velocity regions in the lowermost mantle beneath these areas, and the way in which they have their greatest extent at the core-mantle boundary. In this section it is also noteworthy that beneath Alaska higher than average velocities extend from the surface to the core, and that beneath parts of the Pacific and to the south of South Africa low velocities extend from the surface to the core-mantle boundary. Plate 12.2c (between pp. 244 and 245) shows that low velocities also exist from the surface to the core beneath the region of the Azores and the Canary Islands in the North Atlantic. As the sections shown in Plate 12.2a-c (between pp. 244 and 245) all pass through both poles they all have low velocity regions in the upper mantle associated with the Arctic ridge and high velocities in the upper mantle beneath the continent of Antarctica. The equatorial section of is particularly instructive and revealing in that it not only passes through the low velocity regions in the lowermost mantle beneath southern Africa and the central Pacific but also shows that the high velocity regions associated with subduction beneath South America and the Indonesian region extend continuously through the transition zone and the lower mantle to the core-mantle boundary. Moreover it illustrates that these two pairs of features, which may represent hot upwellings and cold downwellings respectively, are approximately diametrically opposite to each other.

As discussed in Section 2.10.6, measurements of seismic anisotropy in the mantle can yield information on the pattern of flow. Depending on the deformation mechanism and the minerals involved crystal lattices can be preferentially aligned causing seismic waves to propagate with different velocities in different directions. The preferential alignment of olivine by flow in the upper mantle for example gives rise to the highest seismic velocities in the flow direction (Karato & Wu, 1993). Studies of seismic anisotropy in the upper mantle reveal flow directions that are in general parallel to plate motions with indications of vertical flow beneath mid-ocean ridges and in the vicinity of subduction zones (Park & Levin, 2002).

Most of the lower mantle is isotropic. This is probably because under the temperature, pressure and deformation mechanism pertaining in the lower mantle the minerals present, such as perovskite and magne-towustite, are effectively isotopic (Karato et al., 1995). In the lowermost mantle, the D" layer, seismic anisotropy has been observed (Section 2.10.6). It is thought to reflect deformation due to horizontal flow in general, but at the base of the low velocity regions beneath the central Pacific and southern Africa there are indications of vertical flow suggesting the onset of upwelling (Panning & Romanowicz, 2004).

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