Page 419 - Global Tectonics

THE MECHANISM OF PLATE TECTONICS 401

12.11 THE further promotes upwelling (Lowman & Jarvis, 1999).
Sites of downwelling may be controlled by the intrinsic
MECHANISM OF THE buoyancy of continental lithosphere, which tends
to concentrate subduction zones along continental
margins. This effect was illustrated by Lowman & Jarvis
SUPERCONTINENT (1996, 1999) who showed that the collision of two con-
tinents at a site of downwelling can trigger a reorganiza-
CYCLE tion of the convection pattern, leading to downwelling
at the margins and upwelling beneath their interiors
(Fig. 12.15). These authors also showed that slab-pull
The assembly and dispersal of the supercontinents and trench suction forces (Section 12.6) probably were
reflect interactions between continental lithosphere and as important as mantle upwelling in the break-up of the

processes operating in the mantle. The first type of supercontinents.
interaction involves the broad upwellings and down- Another important process that affects the relation-

wellings that define mantle convection cells (Section ship between patterns of mantle convection and plate
12.9). The second is related to the possible impingement motions is internal heating (Section 12.5.1). Lowman et
of deep mantle plumes (Section 12.10) on the base of al. (2001, 2003) showed that, in internally heated models,
continental lithosphere. plate motion is characterized by episodic reversals
Numerical simulations have provided an important in direction as mantle circulation patterns change
means of investigating the possible relationships from clockwise to counterclockwise and vice versa.
between mantle convection patterns and plate motions. These reversals are caused by the trapping and build-up
Gurnis (1988) suggested that, during periods of disper- of heat and buoyancy forces in the interior of convec-
sal, the continents tend to aggregate over cold down- tion cells, which destabilizes the convection pattern.
wellings in the mantle, where they act as an insulating The results of modeling suggest that the downwelling
blanket. The mantle consequently heats up, altering the of cold material at one edge of a plate can entrain hot
convection pattern, and the supercontinent rifts apart material that is trapped below the plate and drag it into
in response to the resulting tension. The continental the lower mantle. The hot, buoyant material then
fragments then move toward the new cold down- begins to ascend as the drag of the cold downwelling
wellings resulting from the changed convective regime. wanes. The ascent of hot material pushes the plate
Gurnis emphasized the fact that the continents, except laterally and induces new cold downwelling on the
Africa, are currently moving to cold regions of the other side of the plate, beginning a new cycle of upwell-
mantle, which are characterized by few hotspots and ing and plate motion in the opposite direction. This
high seismic velocities. It appears that about 200 Ma type of feedback relationship between plate motion and
ago, Pangea was positioned over what is today the internally heated mantle convection may explain why
upwelling beneath southern Africa. Since Africa has some plates suddenly change direction on timescales of
moved only slowly with respect to the hotspot reference some 300 Myr.
frame, it seems that Pangea may have been situated over Many geologic investigations (e.g. Hill, 1991; Storey,
this upwelling prior to break-up, in accord with the 1995; Dalziel et al., 2000a) have demonstrated time–
model. It would thus appear that a positive feedback space relationships among LIPs, hotspots, and super-
exists between patterns of mantle convection and the continental fragmentation. Nevertheless, the role of
formation of the supercontinents. hot spots or upwelling deep mantle plumes during con-
The results of experiments also suggest that several tinental break-up is uncertain. Thermal buoyancy forces
mechanisms produce convection patterns that promote due to mantle upwellings and tractions at the base of
the growth and dispersal of supercontinents. The insu- the lithosphere caused by convecting asthenosphere
lating properties of large masses of continental litho- may contribute to a horizontal deviatoric tension that

sphere create mantle upwelling beneath their interiors is sufficient to break continental lithosphere (Section
(Gurnis, 1988; Zhong & Gurnis, 1993; Guillou & 7.5). Lowman & Jarvis (1999) showed that tensile
Jaupart, 1995). Large plates also prevent the mantle stresses in the interior of supercontinents depend on the
beneath them from being cooled by subduction, which size of the plate, the Rayleigh number of mantle