In this chapter we shall discuss the main driving and resistive mechanisms which are of primary importance in defining plate motions. It is generally considered that there are two main such driving forces, namely (1) the so-called ridge-push mechanism and (2) that mechanism which gives rise to tensile forces as the result of the oceanic lithosphere becoming subducted into the asthenosphere. We use the term slab-pull to explain this tensile force, where 'slab' is an abbreviated form of 'subducting slab'. (The trench is a surface feature brought about by subduction and has, itself, no significant mechanistic role in subduction.)
We argue that the combined mechanisms of slab-pull (once the resistive effect of the subducting slab forcing its way through the asthenosphere, is taken into account) plus the ridge-push are insufficient to overcome the various elements resisting plate motion. Consequently, we resurrect and expand upon a mechanism which has been largely neglected for three decades. This is followed by an outline of the types of structures, flexures and fractures, that are known to develop in oceanic plates and we note how such features have developed and fit in with the conclusions of this and the previous chapter.
The discussion, to this point, is concerned with a plate that is up and running. To attain this state, one must explain the initiation and development of the spreading-ridge and the trench where subduction of the oceanic lithosphere is formed. Only the second of these topics is discussed in this chapter; the formation of a spreading-ridge we leave until later.
The reader will realise that the conceptual modelling that we carry out is extremely simple and approximate. Nevertheless, these models contain the essential physics of the problem and permit the stresses, strains and strain-rates to be quantified. We do not attempt a comprehensive review of all the pertinent elements of plate motion. For example, we do not consider the magnitude of the stresses involved in mountain building. However, we note that these stresses can be inferred from earthquake data and evaluation of the stresses required to produce specific geological structures (Sibson, 1975), from which one can infer that the maximum differential stress is probably in the range 4-6 kb.
Locally, when faults are generated, the rates of movement are fast; but only for brief transient periods. The dominant rates of plate movements are ponderously slow, usually in the range of 1-10 cm a-1. The models discussed in this chapter are completely unable to explain the abrupt changes of rate and direction of plate motion which are, from time to time, exhibited in the geological record.
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