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Figure 4.16 Plan-view, showing how deep basins may develop (a) between major strike-slip faults with the same trend and movement sense and (b) in relatively weak sediments above a major strike-slip fault in the basement.

A lateral reduction in horizontal stress can be induced, for example, where the divergent axes of greatest principal stress, generated by an arcuate spreading-ridge, will cause orthogonal extension and the possible development of a graben (Figure 4.17a).

Whether or not the extension occurs will depend upon the movements of adjacent plates. However, if this extension does give rise to failure of the continental lithosphere to form grabens, these structures will tend to die out inland from the coast. The reason for this can be inferred from Figure 4.17a, which shows that, 'inland', the continental stresses will decrease in magnitude. Near the coast, the compression will tend to displace a marker line A to A', as the result of elastic strain (the displacement is grossly exaggerated). The extension of arc A', relative to arc A, is given by dl of the hachured triangle. A similar, smaller triangle dl of arcs BB0 shows that the extension is much smaller than for arc A. Also, as arc B is greater than arc A, the elastic strains in arc B will be much smaller than in arc A. Consequently, the lateral reduction of the horizontal stress normal to the S1 direction will be commensurately smaller. Hence, even if a graben may form near the coast (Figure 4.17b), it is almost certain to die out inland. This mechanism could possibly explain the development of the graben system, shown on the Tectonic Map of Australia, that cuts the recent sediments in the Lake Ayer Basin.

In any event, this mechanism cannot be applied to explain the development of such features as the North Sea/Rhine/Bresse Graben in Europe, and the even more spectacular African Rift Valley. These major features, bounded as they are by inward dipping normal faults, present evidence that seems to demand that they be interpreted in terms of extension normal to the trend of the rift. But how can such, usually local, extension take place?

Normal faults are not the only features associated with major grabens. Such structures have outward sloping shoulders and often exhibit intrusive and eruptive activity (Figure 4.18) (see lilies, 1977, 1981; Price and Cosgrove, 1990). From these features, one may infer that during its development the graben was underlain, in places, by magma chambers, which support the rock above and also provide the upward pressure necessary to generate the outward tilting shoulders of the graben. The magmatic pressure is equal to that generated by the gravitational loading of the superincumbent rocks in the graben. Magma is a liquid of relatively low viscosity, so that its pressure is hydrostatic and pushes normal to the graben at a pressure equal to the gravitational pressure induced by the rock cover beneath the graben and its shoulders.

The horizontal magmatic stress will be equal to that of the gravitational pressure, so will be equal to S1 (i.e. Sz). Therefore, the magmatic pressure significantly exceeds the ambient horizontal stress S3, and forces the wall-rock of the magma chamber to expand in the direction normal to the trend of the graben, or protograben, by pushing back the wall-rock of the magma chamber. It is this shouldering aside of the country rock at some moderate depth from the surface that causes the rocks above to stretch and develop into a graben. Therefore, to infer from this stretching in the superficial layers of the continental crust that the a

interface

Figure 4.17 (a) Development of lateral extension in a continent driven by a curved spreading-ridge, which has also defined, in plan, the curvature of the continental lithosphere. (See text for details.) (b) Type of graben that can develop in such a continent as the result of such lateral extension.

interface

Figure 4.17 (a) Development of lateral extension in a continent driven by a curved spreading-ridge, which has also defined, in plan, the curvature of the continental lithosphere. (See text for details.) (b) Type of graben that can develop in such a continent as the result of such lateral extension.

Shoulder

Shoulder

Figure 4.18 Diagrammatic representation of rifting at the surface as a consequence of emplacement of a magma chamber below.

whole of the continental lithosphere has been subjected to stretching, may or may not, be correct. Indeed, where evidence of shoulders and igneous activity exists, such a conclusion is likely to be incorrect.

Rather, we contend that a major graben is more likely to be generated by one or more elongated magma chambers that cause superficial extension in the upper crust of the continent. How may linear or curved major magma chambers come into being, sometimes with a total extent of thousands of kilometres? This is a question that cannot readily be answered by currently held conventional concepts relating to plate tectonics. However, major grabens, especially those with shoulders at the rim of the rift valley, coupled with eruptive activity within the rift, should not be interpreted as evidence of continental tension. (The grabens that may form a 'triple point' at or near the crest of an upwelling hotspot dome do, of course, relate to tensional stresses, but these are not major features when compared with those like the African Rift system.)

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