Page 412 - Global Tectonics

394 CHAPTER 12

Plate 12.2 (between pp. 244 and 245) shows four mechanism and the minerals involved crystal lattices
cross-sections through the shear wave velocity model can be preferentially aligned causing seismic waves to
of Masters et al. (1996), each on a plane passing through propagate with different velocities in different direc-
the center of the Earth. Three of these sections are tions. The preferential alignment of olivine by fl ow in
longitudinal sections, that is, the planes also pass the upper mantle for example gives rise to the highest

through the north and south poles; the fourth is an seismic velocities in the flow direction (Karato & Wu,
equatorial section. Note that in each cross-section the 1993). Studies of seismic anisotropy in the upper mantle

great circle showing the intersection of the plane of reveal flow directions that are in general parallel to plate

the section with the Earth’s surface is the smallest motions with indications of vertical flow beneath mid-
circle on the diagram. Plate 12.2a (between pp. 244 ocean ridges and in the vicinity of subduction zones
and 245) clearly illustrates the way in which the high (Park & Levin, 2002).
velocities associated with continental areas, such as Most of the lower mantle is isotropic. This is
North America and Eurasia, and the low velocities probably because under the temperature, pressure and
associated with mid-ocean ridges, such as the East deformation mechanism pertaining in the lower mantle

Pacific Rise and the mid-Indian Ocean ridge, only the minerals present, such as perovskite and magne-
extend to depths of 200–400 km within the upper towüstite, are effectively isotopic (Karato et al., 1995).
mantle. The section in Plate 12.2b (between pp. 244 In the lowermost mantle, the D″ layer, seismic anisot-
and 245) passes through the central Pacific and south- ropy has been observed (Section 2.10.6). It is thought to

ern Africa and reveals the low velocity regions in the reflect deformation due to horizontal flow in general,

lowermost mantle beneath these areas, and the way but at the base of the low velocity regions beneath the
in which they have their greatest extent at the core– central Pacific and southern Africa there are indications

mantle boundary. In this section it is also noteworthy of vertical flow suggesting the onset of upwelling
that beneath Alaska higher than average velocities (Panning & Romanowicz, 2004).
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– 12.8.3 Superswells
mantle boundary. Plate 12.2c (between pp. 244 and
245) shows that low velocities also exist from the The most pronounced features in the lower mantle
surface to the core beneath the region of the Azores revealed by seismic tomography are two extensive
and the Canary Islands in the North Atlantic. As the regions of low velocity beneath the south Atlantic
sections shown in Plate 12.2a–c (between pp. 244 and and southern Africa, and the central and southwest

245) all pass through both poles they all have low Pacific (Plates 12.1, 12.2 between pp. 244 and 245).
velocity regions in the upper mantle associated with These correlate with anomalously high elevation of
the Arctic ridge and high velocities in the upper mantle the Earth’s surface in these areas. Indeed the width
beneath the continent of Antarctica. The equatorial of the topographic swell in each case, several thousand
section of is particularly instructive and revealing in kilometers, is so large that they have been termed
that it not only passes through the low velocity regions superswells (McNutt & Judge, 1990; Nyblade & Rob-
in the lowermost mantle beneath southern Africa and inson, 1994). This is in contrast to the topographic
the central Pacific but also shows that the high veloc- swells associated with hot spots that are typically less

ity regions associated with subduction beneath South than 1000 kilometers across. However the elevated
America and the Indonesian region extend continu- topography and bathymetry of superswells cannot be
ously through the transition zone and the lower mantle explained by anomalously high temperatures and/or
to the core–mantle boundary. Moreover it illustrates low density rock types in the lithosphere and asthe-
that these two pairs of features, which may represent nosphere beneath these regions (Ritsema & van Heijst,
hot upwellings and cold downwellings respectively, are 2000). The only plausible explanation is that they
approximately diametrically opposite to each other. are dynamically supported by major upwellings of
As discussed in Section 2.10.6, measurements of hot material in the lower mantle (Hager et al., 1985;
seismic anisotropy in the mantle can yield information Lithgow-Bertelloni & Silver, 1998). These hot, low
on the pattern of flow. Depending on the deformation velocity regions, defined by seismic tomography, appear