Page 408 - Global Tectonics

390 CHAPTER 12

4 Tension may arise from any of several
mechanisms proposed for the formation
of backarc basins on the landward side of
subduction zones (Fig. 12.8d) as described in
Section 9.10. However, once backarc spreading
commences the landward plate becomes
decoupled from the trench system (Fig. 12.8e).
When two plates of continental lithosphere are
brought into contact after the complete consumption
of an intervening ocean at a subduction zone, the
resistance to any further motion is known as collision
resistance. The mechanism of this resistance is complex
because it takes place both at the suture between the
plates and within the overriding plate (Sections 10.4.3,
10.4.6). Finally, transform fault resistance affects conser-
vative plate margins in both continental and oceanic
areas. The resistance acts parallel to the faults and gives
rise to earthquakes with a strike-slip mechanism (Section
2.1.5) confined to a shallow depth. More complex resis-

tance is encountered where the fault trend is sinuous so
that motion is not purely strike-slip (Section 8.2).
The relative magnitude of the forces acting on plates
and their relevance to the driving mechanism of plate
tectonics will be discussed in Section 12.7.

12.7 DRIVING

MECHANISM OF

PLATE TECTONICS

The energy available to drive plate motions is the heat
generated in the core and mantle that is brought to the
surface by convection in the mantle. It now remains to
consider the manner in which this thermal energy is
employed in driving the lithospheric plates. The pro-
posal by Morgan (1971, 1972b) that plates are driven by
the horizontal flow of material brought to the base of

the lithosphere by hotspots was discounted initially
Figure 12.8 Possible sources of the trench suction force

(Chapple & Tullis, 1977), as the lateral flow would prob-
(after Forsyth & Uyeda, 1975, with permission from
ably be equal in all horizontal directions and thus would
Blackwell Publishing).
not apply a directional force to the plates. Two models
have been proposed. The classical, or mantle drag,
model considers that the upper, cool, boundary layer of
a relatively high geothermal gradient giving rise the convecting system is represented by the upper part
to a relatively low viscosity in the asthenosphere of the asthenosphere, and that plates are driven by the
(Section 12.5.2). viscous drag of the asthenosphere on their bases. By