Mechanisms of weak phase interconnection and the effects of phase strength contrast on fabric development

To determine how deformation mechanisms and fabrics evolve during the strain weakening and localization that accompany the transition from a load-bearing framework to an interconnected weak phase, shear experiments were conducted on a fine-grained gneiss (58% quartz, interconnected; 28% plagioclase, dispersed; 13% biotite, aligned but dispersed) at 745 and 800 °C, 1500 MPa and two strain rates (2×10−5 and 2×10−6 s−1). In experiments with a high phase strength contrast (HPSC, 25× and 45×) between dispersed biotite and framework quartz grains, biotite grains become interconnected due to stress concentration at their tips that allow local semi-brittle flow of intervening quartz and cataclasis of intervening plagioclase. In the higher temperature HPSC samples, the biotite partially dehydrated in high strain zones; a single narrow shear zone formed because the PSC between the quartz and biotite/reaction product layers remained high. In the lower temperature HPSC samples there was no reaction, and a penetrative S–C′ fabric formed. The fabric is defined by the many multiply interconnected biotite strands that formed because the PSC between the quartz and biotite decreased as the biotites kinked. In slower strain rate experiments with a much lower PSC (∼10×), biotite interconnection occurs by shearing into quartz/quartz boundaries where new, weak strain-free recrystallized grains form. At low strain a weak S–C′ fabric forms, but it evolves to an S–C fabric as the PSC decreases with strain. Thus, the magnitude of the strength contrast between a weak phase and its matrix influences the mechanism of weak phase interconnection, the degree of strain localization and the fabric.