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Driving and retarding mechanisms

Several attempts have been made to establish the importance of various elements involved in the main mechanisms which could reasonably be thought to give rise to plate motions. One of the more important early syntheses is illustrated in Figure 3.4. The forces acting on such a plate can be classified into two categories, namely (a) basal and (b) edge forces.

The basal forces, which may be either a driving or a retarding force, depending upon the differential motion, are represented by a series of symbols in Figure 3.4. For ease of presentation we shall represent in the text these basal force with the symbol Fb and mainly restrict the application of these forces, acting on the base of the plate to result from the viscous coupling of oceanic lithosphere with the asthenosphere.

120" W
Figure 3.2 (a) Map of the present-day East Pacific Rise (EPR), is an Atlas 3.3 representation. (b) Map showing the position of the ridge at 0, 10 and 20 Ma, indicating the migration of the ridge and 'jumps' in the ridge which represent displacement of the spreading-ridge by transform faults.

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