C

Figure 4.14 (a) Vertical and horizontal axes of principal stress. (b) Relative magnitudes of principal stress required to give rise to strike-slip faults. (c) Typical orientation of such faults which may develop in the stress field indicated in (b).

The second mode of basin development occurs when the cover rock above a single major strike-slip fault acts in a ductile manner. In relatively weak cover rock, normal faults can cause the development of elliptical en echelon basins (Figure 4.16b).

A major strike-slip fault may eventually cut through and split a continent. Indeed, the Red Sea, the Gulf of Akaba and the Jordon Valley probably show the transition from a relatively simple strike-slip fault to the early stages of development of a spreading-ridge.

It is a misconception that the existence of a major graben is evidence that a continental lithosphere is, or has been, subject to a regional 'tension'. Certainly, lateral extension can generate commensurate reduction in the least horizontal stress. However, it should be noted that such extension does not give rise to a general tension in the crust.

When the least horizontal stress is sufficiently reduced by extension, the maximum differential stress (S1 -S3) will give rise to normal faults, which if they are 'opposed' can give rise to a graben. We have seen (Figure 3.21) that when a rock mass fails in shear it results in a decrease in the magnitude of the differential stress. Here we are concerned with normal faults where, at any depth, the magnitude of the maximum principal stress is determined by gravitational loading, so, at a specific depth, the vertical stress (S1) will remain reasonably constant for long periods. Once a normal fault, or a graben, has developed, the least principal horizontal stress is controlled by the criteria determining slip. Hence, when the normal fault or graben is established, the least horizontal stress near the fault plane is constrained and will remain as a compressive stress.

Away from the normal fault, or graben, extension will reduce the magnitude of the least horizontal stress. However, as soon as failure conditions are met, further normal faults will develop, so that over large areas continued extension will give rise to the development of more and more normal faults or grabens. Within a continent, very special conditions have to be met to generate internal extension. Even when these conditions

Figure 4.15 (a) Strike-slip faults from four locations around the Pacific rim (after Allen, 1969. © American Geophysical Union). (b) Details of types of superficial structures associated with deep strike-slip faults in the Jordan Valley (after Freund, 1965). (c) The San Andreas Fault (after Moody and Hill, 1956).

Figure 4.15 (a) Strike-slip faults from four locations around the Pacific rim (after Allen, 1969. © American Geophysical Union). (b) Details of types of superficial structures associated with deep strike-slip faults in the Jordan Valley (after Freund, 1965). (c) The San Andreas Fault (after Moody and Hill, 1956).

are met, the least horizontal stress, at reasonable depths of a few kilometres, or more, will remain in compression.

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