Page 406 - Global Tectonics

388 CHAPTER 12

Tackley et al. (1993) have numerically modeled most likely to be mantle-wide and not constrained
mantle convection in three dimensions with an endo- by the transition zone.
thermic phase change at the base of the transition zone.
They suggest that cold downwelling material accumu-

lates above 660 km and then periodically flushes into the
lower mantle. This fits well with the results of seismic 12.6 THE FORCES

tomographic imaging of subduction zones, which sug-
gests that some slabs flatten out within the transition ACTING ON PLATES

zone and others penetrate the base of the zone and
descend into the lower mantle (Section 9.4; (Plate 9.2
between pp. 244 and 245). In order to understand the structural styles and
Thus, the transition zone may not be a barrier tectonic development of plate margins and interiors,
to mantle-wide convection, and a number of workers it is necessary to consider the nature and magnitude
have presented evidence in accord with this premise. of all the forces that act on plates. Forsyth & Uyeda
Kanasewich (1976) noted an organized distribution (1975) solved the inverse problem of determining the
of plates, in which the Pacific and African plates are relative magnitude of plate forces from the observed

approximately circular with the smaller plates having motions and geometries of plates. Since the present
an approximately elliptical form and arranged sys- velocities of plates appear to be constant, each plate
tematically between these two large plates. Kanasewich must be in dynamic equilibrium, with the driving
attributed this organization to convection that is forces being balanced by inhibiting forces. Forsyth &
mantle-wide. Davies (1977) conducted model experi- Uyeda (1975) used the corollary of this, that the sum
ments and concluded that only extreme viscosity of the torques on each plate must be zero, to deter-
contrasts would restrict convection to the upper mine the relative size of the forces on the 12 plates
mantle, and maintained that such contrasts do not which they assumed make up the Earth’s surface. The
exist. Elsasser et al. (1979) employed a scaling analy- asthenosphere’s role in this scenario was considered
sis in which the depth of convection is derived as a to be essentially passive. A similar set of computations
function of known parameters, and concluded that based on a similar method, and providing similar
this depth is consistent with convection throughout results, was made by Chapple & Tullis (1977). The
the entire mantle. The topography on the base of following description of forces is based on the exten-
the mantle transition zone has an amplitude of about sions of the work of Forsyth & Uyeda (1975) made
30 km (Shearer & Masters, 1992), which is an order by Bott (1982).
of magnitude lower than predicted for a chemical, At ocean ridges the ridge push force F RP (Fig. 12.7)
rather than a phase, change at this depth. Morgan acts on the edges of the separating plates. This derives
& Shearer (1993) derived the buoyancy distribution from the buoyancy of the hot infl owing material causing
in the mantle from seismic tomographic maps and the elevation of the ridge and hence an additional
concluded that there must be signifi cant fl ow between hydrostatic head at shallow depths which acts on the
the lower and upper mantle. However, other work, thinner lithosphere at the ridge crest. It may also arise
summarized by van Keken et al. (2002), suggests that from the cooling and thickening of the oceanic litho-
the geochemical and isotopic pattern of trace ele- sphere away from the ridge (Section 6.4), which exerts
ments found in oceanic volcanic rocks supports a a pull on the ridge region. Hence, it is basically a grav-
model in which portions of the mantle have been itational force. The ridge-push force may be two or
chemically isolated for much of Earth history. This three times greater if a mantle plume (Section 5.5)
would suggest that the mixing implied by whole underlies the ridge (Bott, 1993), because of the increased
mantle convection has not occurred, and that layered pressure in the asthenosphere at the ridge crest. The
convection is more likely. However, in the light of separation of plates at ocean ridges is opposed by a
the geophysical evidence for mantle-wide convection minor ridge resistance R R that originates in the brittle
many geochemists have derived models in which dis- upper crust and whose existence is demonstrated by
tinct chemical reservoirs can be preserved within this earthquake activity at ridge crests. The resisting forces
context (e.g. Tackley, 2000; Davies et al., 2002). It are small so that the net effect is the presence of a
would seem, therefore, that convective circulation is driving force.