Delocalization of Deformation in a Polymineralic Material

Abstract:

Even though ninety-five volume percent of all rocks are polymineralic, numerical experiments mainly use monomineralic materials to approximate rock behavior, presuming that the rheology is mainly controlled by the weakest or the most abundant mineral phase. This assumption can be debated in regards to natural observations, experimental and theoretical studies, which show that the bulk strength of rock depends on the proportion, shape, distribution, and strength ratio of the different minerals allowing for a mixt brittle-ductile rheological response.

Here, we investigate how a polymineralic rheology affects the localization of deformation. We perform numerical metric scale shearing experiments varying the ratio of two mineral phases (feldspar and quartz), the temperature, and the strain rate. We identify four domains of deformation (brittle, semi-brittle, semi-ductile, and ductile) and three deformation types characterizing the degree of localization (localized (type I), anastomosing (type II) and delocalized (type III)). Type I deformation is expected when both phases behave in a brittle manner. In the semi brittle domain, both, brittle and ductile rheologies are mechanically active and all deformation types can be observed depending on the amount of the weak phase. In the ductile domain, deformation is independent of the phase ratio and type III is always observed. In the semi-ductile domain, both mineral phases should behave in a ductile manner. However, for a moderate amount of quartz, plagioclase exhibits brittle behavior and type II deformation is observed. It appears, therefore, that the insertion of a small amount of weak phase revert the mechanical behavior of the strong phase and lead to the formation of anastomosing shear zone (Type II) where fully ductile (Type III) behavior is expected. This highlights the importance of a bi-mineralic material on the deformation localization and furthermore on large scale deformation processes.