Rock mechanics data and the strength of the lithosphere

Tests of rock-strength in uniaxial and triaxial compression have been conducted for most of the 20th century. A great deal of this work was directed to understanding the behaviour of rock types frequently found in, and for conditions likely to be encountered within, the continental crust. It was soon established that the main parameters influencing the strength of such rocks (i.e. the differential stress at which the rock failed, whether in a brittle, semi-brittle, or ductile manner (Figure 2.18)), were (1) confining pressure, (2) temperature, (3) strain-rate, and (4) fluid pressure within the rock. The strongest rock units in oceanic and

continental lithosphere are the mafic rocks which exist beneath the crust. Except in special circumstances, it is widely accepted that these rocks are probably completely dry. We will, therefore, neglect the influence of

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Figure 2.16 Bars, representing orientation of maximum horizontal stress, which have passed the Rayleigh test (after Coblentz and Richardson, 1995. © American Geophysical Union).

the last of these parameters. The importance of fluids upon the 'strength' of rock is discussed at length in Fyfe et al. (1978) and Price and Cosgrove (1990).

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Figure 2.16 Bars, representing orientation of maximum horizontal stress, which have passed the Rayleigh test (after Coblentz and Richardson, 1995. © American Geophysical Union).

the last of these parameters. The importance of fluids upon the 'strength' of rock is discussed at length in Fyfe et al. (1978) and Price and Cosgrove (1990).

(1) Confining Pressure. As indicated in Figure 2.19, from the classic experiments by Love (1944), it can be seen that the influence of confining pressure is to increase the magnitude of the differential stress that is required to cause failure, and also that different modes of failure are induced at increasingly higher confining pressures. These tests were conducted at room temperature, on cylindrical specimens of Cararra marble (which were encased by a thin copper jacket, to prevent the fluid, in which the confining pressure was built up, from entering the rock specimen).

(2) Temperature. The simplest way of demonstrating the influence of temperature upon strength and mode of failure of a rock type, used by Griggs et al. (1960), was to conduct repeated experiments on a specific rock type, in which the duration of the experiment and the confining pressure were maintained constant. Only the temperature at which the individual experiments were conducted was changed. The results obtained for specimens of dunite are given in Figure 2.20a, while the manner in which the strength of a range of rocks and minerals varies with temperature is shown in Figure 2.20b.

(3) Strain-rate. This type of experiment, which was mainly pioneered by Heard (1963), requires patience and extremely reliable apparatus. The test specimens were held at a specific confining pressure and temperature, while an electric motor, driving through a gear box, shortened, or permitted extension of, a jacketed, cylindrical specimen, at a constant rate, which usually ranged between 10-3 to 10-75 strain per second. (Nowadays, strain-rates are controlled and maintained constant by computer-controlled hydraulic systems.) Consequently, experiments lasted from a few hours duration, for fast strain-rates, to 70-80 days for the slower strain-rates.

The results derived from this type of experiment were plotted in the form shown in Figure 2.21, which enabled the constants to be evaluated for an equation of state of the form:

de/ck = cons tarn .exp (-Q/RT) (S1 - S3)" (2.5)

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Figure 2.17a Ridge-push torque directions and the orientation of the stress bars that have passed the Rayleigh test (after Coblentz and Richardson, 1995. © American Geophysical Union).

Figure 2.17a Ridge-push torque directions and the orientation of the stress bars that have passed the Rayleigh test (after Coblentz and Richardson, 1995. © American Geophysical Union).

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Figure 2.17b Absolute plate-velocity azimuths and trend of stress bars that have passed the Rayleigh test (after Coblentz and Richardson, 1995. © American Geophysical Union). where e is strain, Q is the apparent activation energy, R is the gas constant, T is the temperature in Kelvin, (Sj-S3) is the differential stress and the exponent n is a material constant for some specific environmental conditions. It can be shown that this type of equation can be interpreted in terms of various mechanisms of crystalline deformation, so that the deformation map for a wide range of environmental conditions can be derived from a relatively limited spread of experimental data. Such a chart for olivine is shown in Figure 2.22.

Figure 2.18 Stress-strain curves of a hypothetical cylindrical rock specimen subjected to biaxial compression at various confining pressures. At zero confining pressure, brittle failure (B) occurs; at moderately large confining pressure, failure is semi-brittle (SB); at high confining pressure, the rock speciment behaves in a ductile fashion (D).

Figure 2.18 Stress-strain curves of a hypothetical cylindrical rock specimen subjected to biaxial compression at various confining pressures. At zero confining pressure, brittle failure (B) occurs; at moderately large confining pressure, failure is semi-brittle (SB); at high confining pressure, the rock speciment behaves in a ductile fashion (D).

Figure 2.19 Strength and types of failure related to magnitude of confining pressure (after Love, 1994).

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