Figure 3.11b Curve A shows distribution of potential horizontal stress from spreading-ridge to most distant part of plate at 12,500 km. Line B indicates that the maximum potential horizontal stress is a constant for all mature ocean plates, i.e. plates that attain a depth of 100 km.

As oceanic lithosphere develops and a given point travels further away from the spreading-ridge, it is subjected to an increasing magnitude of horizontal compressive stress. This will result in an elastic shortening in the strong zone of the oceanic lithosphere of 2-4*10-3. If this shortening is attained in 150 Ma, the average strain-rate will be of the order 10-18 s-1. Hence, only the uppermost sections of the oceanic lithosphere will be able to sustain a differential stress which will, at deeper levels, be sufficiently large to give rise to brittle failure.

Based on the arguments expressed in Chapter 2 (Figure 2.30), it will be seen, in Figure 3.12, that at a depth of about 25 km in the 'strong' 0-50 km band of the oceanic lithospheric unit, the rock is sufficiently strong to support a horizontal compression somewhat greater than 16.5 kb. The failure conditions established in the previous chapter are represented by the triangular area b.x.c. Any stress condition that falls outside this triangle exceeds the failure conditions and cannot be sustained. Only stress conditions which fall within this triangle are valid. This triangular area defines the depth limits over which the average horizontal stress is 8.25 x10s Pa (8.25 kb). However, if one takes into consideration the relative weakness of the rocks in the parts of this strong zone (i.e. from 35-50 km) indicated by the dashed curve, the average sustainable stress will be probably somewhat less than 8 kb. In particular, this level of stress of about 8 kb is further reduced, as the result of basal restraint of movement of the oceanic lithosphere over the LVZ, and also of the frictional/ viscous restraint at the plate boundaries.

However, we have neglected the contribution of stress that can be attributed to the slab-pull and ridge-push mechanisms. As we have noted, the magnitudes of these mechanisms cannot be quantified with accuracy. Nevertheless, if we attribute a stress of 1 kb to these mechanisms, it follows that the average sustainable stress in the strong elastic layer of oceanic lithosphere will almost certainly be close to, or even a little in excess of, 8.0 kb. We submit, therefore, that it is this average stress in the oceanic lithosphere, of kb

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