Temperature dependence of grain-scale slip behavior of muscovite and implications for subduction zone seismogenesis
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Despite extensive work on the frictional or plastic strength of muscovite gouge, little is known about grain-scale processes controlling muscovite sliding. In this study we perform direct shear experiments on two muscovite crystals, shearing them parallel to their interface formed by the (001) plane, at 150°, 500° and 600°C. We conducted pressure stepping tests (270 – 170 MPa) and velocity stepping tests (0.1-10 μm/s), to provide constraints on muscovite single crystal rheology. The crystallographic orientation of the crystals was controlled to either inhibit dislocation glide, (shear parallel to the  direction) or to enhance dislocation glide (shear parallel to the  direction). It is found that the strength of muscovite single crystals at these conditions is pressure sensitive (μ = 0.055 – 0.12), rate-dependent in a velocity strengthening sense and inclined to weaken with temperature. This illustrates a pronounced difference from sliding behavior reported in studies on muscovite gouge. Microstructural analysis implies dislocation glide to be strongly affected by temperature as well as shear direction, although more work seems needed to be conclusive on the mechanical implications of this. The friction coefficient for sliding on the (001) interface between the individual crystals tested, lies in the range of 0.055 (sliding in the  direction) to 0.12, (sliding in the  direction). It is tentatively concluded that muscovite single crystals, at the range of temperatures described in this study, consistently deform through a combination of crystal plastic and frictional processes, where strength is mostly controlled by grain boundary friction. Finally, it is noted that the frictional strength of muscovite single crystals might have previously been overestimated in studies on crustal strength. The present results might therefore have important implications for studies considering crustal strength and hence seismogenesis.