Towards formulations of the plastic flow properties of geological materials under general loading geometries

Many large scale natural deformation processes, ranging from the building of mountain chains and the transfer of heat and mass within the Earth's interior, to the deformation of the polar ice masses and the advance and retreat of glaciers, are accomplished by plastic flow. Consequently, the plastic deformation properties of a number of key geological materials exert a profound influence on the surface topography, internal structure, and tectonothermal evolution of the Earth and other planetary bodies, as well as having a technological importance in civil engineering projects which are sited on or within flowing media such as ice or salt bodies. The primary source of information on the plastic flow properties of geological materials comes from laboratory deformation experiments. The results of these experiments are employed in numerical models which attempt to describe the natural deformation process of interest. The rates of natural plastic deformation are generally too small to use in laboratory testing, and usually experimental samples are subjected to simpler loading geometries than those experienced during natural deformation. Consequently, if experimentally determined plastic flow properties are to be used with confidence in deformation modelling applications, it is important to establish that the flow properties obtained from laboratory experiments and extrapolated to natural deformation conditions match those implied by field measurements of naturally deforming bodies. Ice is perhaps the only material where such comparisons can readily be made, and in this case it is invariably found that naturally deformed ice is significantly stronger (up to eight times) than that deformed in the laboratory. The conclusion to which many glaciologists have been forced is that plastic flow properties are influenced by the geometry of the applied stresses in a way which has not hitherto been explored in laboratory experiments. Almost all rock and ice deformation experiments have been performed under the special case loading geometries of either pure shear (axial compression or extension) or simple shear (torsion). However, to establish flow properties under general loading geometries it is necessary to perform experiments under some combination of pure and simple shear. We propose to do this by performing a series of deformation experiments at high temperatures and pressures on polycrystalline calcite samples under simultaneously applied axial loading and torsion. The proposal takes advantage of our recently acquired capability (unique in the UK) of performing such tests, and apart from some reconnaissance-type experiments on ice which produced ambiguous results, it will be the first systematic study of its kind. Calcite will be used because among all of the volumetrically significant geological materials, its plastic flow properties are experimentally the most convenient to access, thereby allowing us to minimize the technical and data analysis difficulties associated with the experiments. The experiments will be performed at range of different temperatures using, at each temperature, loading at a range of different fixed ratios of axial strain-rate to shear strain-rate and of axial stress to torque. The programme will focus specifically on establishing the sensistivity of (a) the flow stress at large strain, and of (b) the low strain yielding behaviour to the loading conditions. In principle, the mechanical response depends not only on the loading conditions but also on the extent to which a mechanical anisotropy develops with strain. The rate at which this anisotropy develops depends on the importance of grain-size sensitive deformation processes, and so to separate the effect of the anisotropy from that of loading geometry, the experiments will be performed on two calcite starting materials with widely different grain sizes.