Strain-related weakening in deep orogenic shear zones: what factors to consider?

Quantifying strain-related weakening requires knowing which weakening processes dominate under various tectonic conditions. One commonly cited weakening factor is a change in phase topology, with a rock losing strength as weak phases become more interconnected. Accurate determinations of bulk strength requires knowledge of both the individual phase strengths and the control of microstructure on the bulk strength. Calculations described here focus first on the effects of microstructure and in particular phase topology. The calculations assume deformation by mechanisms that can be described simply by phase strengths and ignore mass transfer and grain boundary sliding. The initial motivating question is whether a constant value can adequately parameterize a given phase distribution and, if so, if that parameter value is predictable. Using a two-dimensional finite element calculation scheme for linearly viscous materials, it appears that constant values of the two microstructural parameters tested satisfactorily describe the bulk strength at viscosity contrasts less than ~50 for a given phase distribution and modal abundance. Neither tested parameter is constant with changes in mode. The values of the examined microstructural parameters do not appear to be predictable based on a qualitative analysis of the phase topology. Thus although a constant parameter value can adequately describe a given phase distribution, because that parameter is not determinable a priori, accurate measurement of the bulk strength of a composite requires a numerical calculation method. In natural samples from high strain zones from deep orogenic crust, weak phases are not necessarily interconnected and simply changing the shape preferred orientation of weak phases relative to the transport direction (i.e., transposition) can accommodate a moderate degree of weakening, particularly in the absence of large modal changes due to reactions. However, many natural shear zones also experience reactions that result in a significant change in the modes of strong and weak phases. The production of new, weak phases can swamp the effect from a topology change, indicating that the relative importance of reaction and textural weakening can vary greatly. Thus, in considering how to quantify strain-related weakening, important factors include at least (a) the relationship between phase topology and strain, (b) the relevant reactions for affected rock types, and, consequently, (c) the relative flow strengths of the product and reactant phases.