Page 410 - Global Tectonics

392 CHAPTER 12

in response to forces applied to their edges (Fig. 12.9b).

The mechanism was first proposed by Orowan (1965)
and Elsasser (1969, 1971) and is sometimes referred
to as Orowan–Elsasser-type convection (Davies &
Richards, 1992).
Only a small percentage of the energy supplied from
the mantle is available to drive the plates, but this frac-
tion is adequate to power the present plate motions
(Bott, 1982). The energy is utilized by the lithosphere
to drive the plates in several ways. The ridge-push force
(Section 12.6) originates from the uplift of the ridge
crest caused by the anomalously hot asthenosphere
beneath it. This provides a lateral push to the rear of
accreting oceanic lithosphere. The slab-pull force
(Section 12.6) arises from the negative buoyancy of the
downgoing slab at trenches, and is assisted by phase
changes to denser forms that affect minerals in the slab
at increased pressure. The slab-pull force is potentially
some four times larger than the ridge-push force,
although in practice much of this force is probably uti-
lized in overcoming slab resistance (Chapple & Tullis,
1977). The trench suction force (Section 12.6) originates
from the geometry of the downgoing slab and also

provides a significant driving force.
The edge-force mechanism can account for many
phenomena more satisfactorily than the mantle drag
mechanism, in particular:
1 It is more acceptable thermodynamically and is
much more effective in transporting heat from
the mantle.
2 It is consistent with the observed pattern of
intraplate stress. As discussed in Section 12.7.1,
the mantle drag mechanism implies tension at
ocean ridges and compression at trenches. The
edge-force mechanism would give rise to the
opposite stress configuration, and this is in

accord with the stress regime indicated by focal Figure 12.10 Correlations of plate parameters with
mechanism solutions of intraplate earthquakes. plate velocity: (a) plate area; (b) plate circumference
connected to downgoing slab (open bar, total length;
3 It is reconcilable with the present plate motions,

filled bar, effective length); (c) continental area of plate
in particular with the observations of Forsyth (redrawn from Forsyth & Uyeda, 1975, with permission
& Uyeda (1975) that:
from Blackwell Publishing).
(a) plate velocity is independent of plate area
(Fig. 12.10a). If mantle drag were
operative it would be expected that the
greatest velocities would be experienced 12.10b). This is in accord with the slab-pull
by plates with the greatest area over which force being greater than other forces
the mantle drag would act; affecting the plates;
(b) plates attached to downgoing slabs move (c) plates with a large area of continental
more rapidly than other plates (Fig. crust move more slowly (Fig. 12.10c). This