Micromechanisms of flow and fracture, and their relevance to the rheology of the upper mantle

Abstract
Crystalline solids respond to stress by deforming elastically and plastically, and by fracturing. The dominant response of a given material depends on the magnitude of the shear stress (0s), on the temperature (T) and on the time (t) of its application. This is because a number of alternative mechanisms exist which permit the solid to flow, and its fracture, too, occurs by one of a number of competing mechanisms. Their rates depend on 08, T and t: it is the fastest one which appears as dominant. In geophysical problems, pressure appears as an additional variable. At pressures corresponding to depths of a few kilometres below the surface of the Earth, the mechanisms of fracture are the most affected; but at depths of a few hundred kilometres, plasticity, too, is influenced in important ways. This paper outlines the mechanisms of flow and fracture which appear to be relevant in the deformation of materials of interest to the geophysicist, and the way pressure affects them. The results are illustrated and their shortcomings emphasized by using them to calculate the mechanisms of flow and fracture to be expected in the upper mantle of the Earth.

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