Upper limits of power law creep of rocks

At depths below a few kilometers, most rocks probably flow in the steady-state dominantly by diffusion-assisted dislocation creep mechanisms whereby strain rate is proportional to a low power of deviatoric stress. At high stress levels for metals, ceramics and rocks, the power law breaks down (PLB) to some other strain rate-stress functional relationship and this paper explores four methods by which the breakdown stress, σb, may be estimated. For metals, alloys and ceramics, σb ≅ 10−3μ, for non-silicate rocks, σb ≅ 5 × 10−3μ, and for silicates, σb ≅ 10−2μ. For silicates, the low stress exponent power law breaks down at high stresses to an exponential dependence of strain rate upon stress or to a Dorn law, the governing flow mechanisms generally being lattice-resistance controlled dislocation glide and mechanical twinning. PLB breakdown stresses for silicates are nearly strain-rate insensitive and extrapolated values of σb are so high as to suggest that low-temperature, high-stress plasticity may be suppressed entirely in geological deformations. PLB for olivine-rich rocks is discussed in some detail and new three-dimensional deformation mechanism surfaces for wet and dry olivine polycrystals are presented.