T To

Figure 2.20 (a) Effect of temperature on strength (Griggs et al., 1960). (b) Variations of strength with temperature for various rock types (Griggs et al., 1960).

the 10 kb isobar, appears within a few kilometres of the ridge, while in extension the 10 kb (1 GPa) differential stress zone comes into existence some 55 km from the ridge.

Based upon the experimental data and rheological model set out by Goertz and Brace (1972) and Goertz and Evans (1979) expressed the relationship between depth and the differential stress the lithosphere could support for a specific strain-rate in the form shown in Figure 2.24. It will be seen that the strength relationship is relatively simple, but asymmetric. In compression, the strength of the lithosphere increases linearly and rapidly down to a depth of 24 km, where it exhibits a peak value of about 17 kb (1.7 GPa). From depths of 24 to 50 km there is a rapid linear decrease in strength, so that, at 50 km, the strength is only 0.5 kb. Below 50 km, the diffusion processes of rock deformation dominate, so that at a depth of about 60 km the long-term strength is reduced almost to zero. The failure conditions in extension follow a similar pattern, but the differential stress attains a maximum value of -8.5 kb, and this peak value occurs at a depth of about 35 km. The base of the strong layer for the specified conditions is arbitrarily put at 50 km, where the differential stress which the litho-sphere can sustain, for the specified conditions, is 500 bar (50 MPa).

It will be noted that in compression, the strong layer, which we take as the 10 kb contour, and which extends to a depth of 50 km in Figure 2.25, is some 5 km deeper than the lower limit of the strong layer shown in Figure 2.24. This can be attributed to the strain-rate used. When Kirby calculated the stress contours, he chose a strain-rate two orders of magnitude faster than that assumed by Bodine et al. (1981).

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