Page 407 - Global Tectonics

THE MECHANISM OF PLATE TECTONICS 389

Figure 12.7 Some of the forces acting on plates (developed from Forsyth & Uyeda, 1975, by Bott, 1982, reproduced by
permission of Edward Arnold (Publishers) Ltd).

Beneath plate interiors a mantle drag force acts on plate resistance (R O ) is expressed in the intense earth-
the base of both the oceanic and continental litho- quake and tectonic activity observed at shallow depths
spheres if the velocity of the underlying asthenosphere at destructive plate margins. The downgoing slab
differs from that of the plate. If the asthenosphere achieves a terminal velocity when F SP is nearly balanced
velocity exceeds that of the plate, mantle drag enhances by R S + R O . If F SP exceeds R S + R O , the slab descends at
the plate motion (F DO , F DC ), but if the asthenosphere greater than the terminal velocity and throws the slab
velocity is lower, as shown in Fig. 12.7, the mantle drag into tension at shallow depths. If F SP is less than R S +
tends to resist plate movement (R DO , R DC ). Mantle drag R O the slab is thrown into compression. The balance
beneath continents is about eight times the drag beneath between driving and resistive forces may thus control
oceans; this may be due to the increased thickness of the distribution of stress types, as revealed by earth-
the subcontinental lithosphere beneath cratonic areas quake focal mechanism solutions, within downgoing
(Sleep, 2003). slabs (Section 9.4).
At subduction zones the major force acting on plates In the region on the landward side of subduction
results from the negative buoyancy (F NB ) of the zones the overriding lithosphere is thrown into tension
cold, dense slab of descending lithosphere. Part of this by the trench suction force (F SU ). There are several pos-
vertical force is transmitted to the plate as the slab-pull sible causes of this force (Fig. 12.8):
force F SP . The density contrast, and hence F NB , is greatly
1 It may arise because the angle of subduction
enhanced at depths of 300–400 km where the olivine–
becomes progressively greater with depth (Fig.
spinel transition occurs in the slab. F SP is opposed by a
12.8a). Tension would then arise as the
slab resistance (R S ), which mainly acts on the leading
overriding plate collapses toward the trench.

edge of the descending plate where it is five to eight
times greater than the viscous drag on its upper and 2 The tension could result from the “roll-back” of
lower surfaces. Underthrusting involves a downward the underthrusting plate (Fig. 12.8b). That is,
flexure of the lithosphere in response to F NB , and since the downgoing slab retreats from the overriding

it behaves in an elastic manner in the top few tens of plate.
kilometers flexure is opposed by a bending resistance 3 Tension could be generated by secondary

(R B ). A further resistance to motion at subduction zones convective fl ow in the region overlying the
is the friction between the two plates. This overriding downgoing slab (Fig. 12.8c). This would require