Page 409 - Global Tectonics

THE MECHANISM OF PLATE TECTONICS 391

Figure 12.9 Two concepts of plate driving mechanism: (a) cellular convection, with the cells exerting a mantle drag on
the lithosphere; (b) Orowan–Elsasser-type convection, with plates driven by edge forces (redrawn from Bott, 1982, by
permission of Edward Arnold (Publishers) Ltd).

contrast, the edge-force model recognizes the litho- quently difficult to explain how cells of simple geome-
sphere itself as the upper, cool boundary layer of the try could drive plates with irregularly shaped margins,
convection cells and proposes that the plates are driven such as the Mid-Atlantic Ridge at equatorial latitudes
by forces applied to their margins. The two models thus where it is offset along a suite of transform faults. Also,
differ in the importance placed on the various forces the constant geometry of the convection cells cannot
acting on plates (Section 12.6) described by Forsyth & explain the relative movements between plate margins,
Uyeda (1975). such as is happening between the Mid-Atlantic and
Carlsberg ridges. The large horizontal dimensions of
the cells cannot account for the movements of small
12.7.1 Mantle drag mechanism plates, such as the Caribbean and Philippine plates,
which can hardly be powered by their own individual
convective systems.
Mantle drag was the first driving mechanism to be pro-

It would therefore seem that the classical mantle
posed, and envisages plate motion in response to the drag mechanism is not the main process causing the
viscous drag exerted on the base of the lithosphere by
mobility of plates. It is possible, however, that our views
the lateral motion of the top of mantle convection cells on mantle drag are biased by the fact that the present
in the asthenosphere (Fig. 12.9a). The convection cells
continents are dispersed. Ziegler (1993) argues that
would consequently rise beneath oceanic ridges and mantle drag may have been a signifi cant mechanism
descend beneath trenches, being largely absent beneath
during supercontinent break-up and, indeed, Phanero-
continental regions. This mechanism predicts that the zoic plate motions appear to require this mechanism
oceanic lithosphere would be in a state of tension at the
(Section 12.11).
ocean ridges and compression at the trenches.
Because of their relationship to accretive and destruc-
tive plate margins, the horizontal dimensions of the
convection cells powering mantle drag would be 12.7.2 Edge-force mechanism
expected to be about half the width of an ocean, that
is, 3000 km. This great lateral extent implies that the In this mechanism the oceanic lithosphere represents
cells should have a relatively simple form. It is conse- the top of the convection system, and the plates move