Long-term creep experiment of rock with small deviator of stress under high confining pressure and temperature

Long-term creep tests of gabbro which have been performed with a maximum bending stress (20 bar) under a high confining pressure (1 kbar) and various temperatures, are described. Methods and techniques used in the experiment are mainly similar to those reported previously by the same authors (Itô and Sasajima, 1980) except for the application of high pressure and temperature. The techniques include the bending system, size and preparation of the sample, and the determination of its deformation by use of interference fringes of Na-D light. In order to measure a very small deformation of creep, intermittent breaks of the application of loading, confining pressure and temperature are necessary, and the creep curve is constructed from the intermittent advance of permanent deformation.The experiment has revealed two strange phenomena : one is a sinuous progress of the creep curve, and the other is that the deformation recovery shows strange behavior after the unloading. These results are discussed in close connection with the mechanism of the “turn back of creep” denoted by Itô and Sasajima (1980). The mean creep curves, at 25°C. 95°C and 150°C, obtained so far lead to viscosities of 1.6 · 10²⁰, 1.9 · 10¹⁹ and 4.8 · 10¹⁸ poise, respectively and the maximum strain rates employed in the samples were 4.2 · 10⁻¹⁴, 3.6 · 10⁻¹³ and 1.4 · 10⁻¹²/sec, respectively, which cover the geological strain rate. Although we have only three data points, the logarithm of viscosity is linearly related to the reciprocal of absolute temperature (see Fig. 7), and an activation energy for creep of gabbro is found to be 7.6 kcal/mol. It should be noted that viscosities obtained are considerably smaller than those estimated for the crust and mantle, and that the activation energy is surprisingly smaller than those obtained by high-pressure experiments of rock deformation, which have been carried out under a strain rate larger than 10⁻⁸/sec.