Strain weakening in crustal and upper mantle lithologies: Processes and consequences

The flow of polyphase aggregates and the mechanisms that cause strain localization and fabric development are poorly understood. I performed a series of experiments on several natural aggregates Bt+Qtz+Plag gneiss, quartzite, feldspar and two hot-pressed aggregates of biotite and olivine. The experiments were performed at a wide variety of conditions such that the deformation mechanisms varied from brittle to crystal plastic. The results of the experiments show many different processes operate during deformation of polyphase aggregates and the mechanisms operating at low strain are not usually the same as those operating at high strain. A simple concept that controls the degree of strain localization is the phase strength contrast, the difference in strengths of the different phases of the aggregate at a specific set of conditions. At high PSC the strain is strongly localized and as the PSC decreases strain is considerably more homogeneous. Additional conclusions are that reaction is enhanced by deformation and that reactions are not likely to localize strain even though they cause strain weakening of the aggregate.; The deformation mechanisms and mechanical properties of the lithospheric mantle are modeled based on low pressure low strain experiments performed in the gas apparatus. We have performed experiments in the molten salt cell at high pressures, which has excellent stress resolution, and the results of the experiments indicate that olivine aggregates are significantly weaker than predicted by current flow laws for olivine aggregates derived from low pressure, low strain experiments. We have also found that even with the addition of water, the slip system in olivine is always (010)[100], contrary to several recent papers. These results indicate that the models of the physical properties of the mantle are inaccurate and need investigation at higher pressures than the commonly used gas apparatus will allow.