In this chapter, the mechanisms which are widely accepted as being important in giving rise to plate motions have been briefly considered. It is reasonable to accept that the slab-pull provided by the subducting lithosphere is potentially capable of generating tensile stresses of very large magnitude. However, the extent to which this potential is decreased by the resistance of the surrounding asthenosphere is undoubtedly also large, but difficult to quantify with precision. Indeed, this resistance is thought to be so high, that the slab-pull may only be able to engender a real tensile stress of, at most, 50 MPa (500 bar) at the top of the slab.
The widely accepted mechanism of ridge-push is also a valid one. However, much of this mechanism is directed at the base of the oceanic lithosphere, where it is very weak. Consequently, it is difficult to infer just how much of this ridge push is actually transmitted to the strong, elastic layer.
It is reasonable to conclude, therefore, that whereas the generally accepted mechanisms of slab-pull, possibly coupled with some portion of ridge-push are valid, the magnitude of the stresses associated with these mechanisms is small (almost certainly less than 1.0 kb) and completely incapable of generating significant plate movement and will certainly not be able to generate mountain chains.
When the gravity-glide mechanism is included, the situation is transformed, for this latter mechanism has the potential to provide as much as 15-20 times more stress than the combined stresses for other cited mechanisms. Moreover, it can be inferred that the gravity-glide mechanism dictates that the axis of maximum horizontal stress must usually approximate closely to the direction of absolute plate motion.
We have shown that a simple gravity-glide mechanism is able to provide potential compressive stresses in oceanic plates, which comfortably exceed the level of stress which the strong layer is able to sustain. Following failure of the weaker parts of the elastic layer, the horizontal force generated at the leading edge of the oceanic lithosphere is still capable of transmitting an average compressive stress of about 8 x10s Pa (8 kb). This potential compressive stress at a vertical section of an oceanic plate most distant from the spreading-ridge is not completely realised, because of viscous constraints at the base of the plate (as well as those provided by boundary and internal vertical faults). For a very long, relatively fast-moving oceanic plate (cf. the Pacific plate), these resistive elements are extremely important so that, at a distance of 12,500 km from the ridge, the average potential stress of 8 kb is probably reduced to about 2-4 kb.
Consequently, it is concluded, the gravity-glide mechanism, even without assistance from other possible mechanisms, is capable of generating many of the known features of an up-and-running plate.
In order that a plate can attain such a state, two features must be created. They are (1) the spreading-ridge and (2) a companion trench to accommodate a subducting slab. In this chapter we have indicated how such trenches may develop. The conditions necessary to generate subduction have been attributed to a combination of vertical loading, generated by a thick sedimentary pile, of about 10 km in thickness, which causes downward flexure of the oceanic lithosphere adjacent to a continental lithosphere, combined with a horizontal stress, generated in the oceanic lithosphere by the gravity-glide mechanism. Large horizontal compressive stress is most easily generated in a relatively short, slow-moving oceanic unit. These combined conditions are only infrequently met. Finally, it should be noted that there exist a number of relatively small, arcuate, intra-oceanic, subduction zones, several of which have developed in the last 100 Ma. These intriguing features cannot be satisfactorily explained by the models discussed in this chapter. The problem of the development of such arcuate, intra-plate subduction zones is discussed in Chapter 7.
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