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Stress in a continental 'inclusion'

Consider now a modified plate in which a sliver of continental lithosphere is included (Figure 3.18b). Let us assume that this sliver, from front to back, has a length (L') of 500 km. The continental sliver will be subjected to a magnitude of compressive stress which has been dependent upon its position in the plate. If it is situated near the spreading-ridge, the stress will be relatively low, while if it is situated near the trough it will be modestly high. Let us assume that the sliver is sited about 2500 km from the spreading-ridge, and it is subjected to a nett average compressive stress of 2 kb. Then, from Equation 3.5, this will result in a shortening in the mantle beneath the continental crust of 2.0 km, or 0.4 per cent strain. This strain will be transmitted to the continental crust above, where it will cause a differential stress to develop in near-surface, sedimentary rocks. If we assume that these rocks have an average value of Young's modulus of E=105 bar, a horizontal differential stress of 400 bar will result. The maximum horizontal stress will, on average, be parallel to the direction of absolute plate motion. Hence, the various models presented above are compatible with the observed relationship, that the axis of maximum horizontal stress will be compressive and approximately parallel to the direction of absolute plate motion.

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