Page 413 - Global Tectonics

THE MECHANISM OF PLATE TECTONICS 395

to rise from the thermal boundary layer at the core– 12.8.4 The D″ layer
mantle boundary (Plate 12.2 between pp. 244 and 245)
(Section 12.8.4).
Just as upwellings in the mantle produce regional It has long been recognized that the greatest contrasts
uplift of the Earth’s surface, downwellings produce in physical properties and chemical composition within
regional subsidence (Gurnis, 2001). The most notable the Earth occur at the core–mantle boundary and that
example of depressed crust at the present day is the this is almost certainly the location of a thermo-chemi-
Indonesian region. This is situated above anomalously cal boundary layer (Section 2.8.6). Initially, seismolo-
high seismic velocities in the transition zone and upper gists were unable to detect any layering in the lower
part of the lower mantle (Plate 12.2b between pp. 244 mantle and referred to it as Layer D (Bullen, 1949).

and 245) that probably reflect a confluence of downgo- Subsequently it was realized that a layer at the base of

ing lithospheric slabs. Seismic tomography can only the mantle, perhaps 2–300 km thick, has distinctive, if
map regions of low and high velocity, and hence pos- variable, characteristics; typically lower seismic veloci-
sible upwellings and downwellings in the mantle, at ties or a lower velocity gradient than in the lower
the present day. However, evidence from the geologic mantle above. Hence the lower mantle is now divided
record for regional scale elevation and subsidence of into two seismologic layers D′ and D″. With further
the Earth’s crust may indicate that a particular area refinements in seismologic techniques, studies of

has been underlain by a major mantle upwelling or seismic waves reflected, refracted and diffracted at the
deep subducting slabs in the past. Originally it was core–mantle boundary have revealed remarkable details
assumed that changes in sea level, causing major marine of the complexity and lateral variability of layer D″.
transgressions and regressions on continental crust, The geographic distribution of earthquakes and seis-
were synchronous worldwide, away from areas of mologic observatories is such that not all parts of the
active tectonism. However, as more data accumulated layer can be studied in the same degree of detail. Clearly
it became clear that this was not so, although an for such a remote layer, that is now thought to have
obvious explanation was lacking. It is now apparent vertical and horizontal variability analogous to that of
that elevation and subsidence of the lithosphere associ- the lithosphere, this poses quite a challenge for future
ated with convection in the mantle, could provide an seismologic studies.
explanation for what were previously some very enig- Figure 12.11 illustrates the picture that is emerging
matic observations. of the nature of layer D″ for three very different regions
Denver, Colorado in the central USA has an eleva- for which detailed studies have been possible: beneath
tion of 1.6 km but is underlain by Cretaceous sediments central America, Hawaii, and southern Africa. The
typical of shallow water deposition. At that time the upper boundary of the layer is characterized by a veloc-

Farallon plate, the eastern flank of the East Pacifi c Rise ity discontinity. Below this there may be an increase or

in the northeast Pacific, was being subducted beneath decrease in the seismic velocities, particularly the shear
western and central North America and is thought to wave velocity, or a decrease in the velocity gradient with
have caused depression of the crust above it. With the depth. A velocity increase is most marked beneath
progressive elimination of the East Pacific Rise in the regions where there are subducting slabs such as Central

northeast Pacific throughout the late Cenozoic, the Far- America (Fig. 12.11a). In a 5- to 50-km-thick layer imme-

allon plate has become detached and continues to sink diately above the core–mantle boundary there is often
eastwards, allowing the buoyancy of the crust of the a zone of ultra-low seismic velocities, with decreases in
western and central USA to reassert itself, thereby the shear wave velocity of 10–50%. This implies partial
causing the uplift of the Colorado region. Van der Hilst melting with more than 15% melt (Thybo et al., 2003).
et al. (1997), using seismic tomography, imaged the These ultra-low velocity zones (ULVZ) are most exten-
sinking Farallon plate 1600 km beneath the eastern sively developed beneath major hotspots such as Hawaii
USA. Similar anomalous vertical movements of parts of (Fig. 12.11b) and beneath the superswells, and inferred

Australia since the early Cretaceous are thought to be upwellings, of the central Pacific and southern Africa

due to the influence of downwellings created by sub- (Fig. 12.11c). Unlike the variations in seismic velocity in
duction zones, initially to the east of Australia, and the main part of the lower mantle, that are thought to
more recently to the north (Gurnis et al., 1998). be largely due to temperature differences, the marked