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In natural shear zones, micro-porosity is found to decorate grain boundaries of rocks which have been deformed viscously. Whether porosity is formed during or after deformation is widely debated, and requires further investigations to test how micro-pores may be produced, particularly in monomineralic aggregates.

We perform shear experiments in a Griggs-type apparatus, on natural novaculite with low primary porosity. Experiments were performed at 900 °C and confining pressures of 1.2 and 1.5 GPa, where bulk strain rates were between ≃1.2×10-4 to 2.3×10-5 s-1, respectively. To the starting samples, 1 wt% of H2O was added.

During deformation, samples record a high peak of differential stress, and in some cases a significant stress drop, followed by a progressive strain weakening over several gamma of shear strain. In an experiment at 1.2 GPa confining pressure, the sample deformed above the Goetze criterion at the peak stress, which gave rise to a highly fractured sample. Microstructural observations show the production of a penetrative secondary porosity, where most pores are < 1 µm in diameter. Pores decorate most grain boundaries, which are open and easily identifiable in SEM-BSE.

An experiment at 1.5 GPa confining pressure did not experience any fracturing, and the maximum differential stress remained below the Goetze criterion (deviatoric stress = 0.8 GPa). In this experiment, a porosity of microns to tens of microns in size developed along specific bands. Electron backscatter diffraction (EBSD) reveals that quartz deformed viscously throughout the sample. However, quartz grains within pore-decorated bands have a stronger intragrain misorientation and higher lattice curvature gradients than in the surrounding quartz. A final interesting feature of EBSD maps is the lower indexation rate of quartz within these bands. TEM observations of extremely reduced grainsize suggest that this is due to grainsize smaller than EBSD step size.