In the middle crust, deformation of granitic rocks is characterized by crystal plastic deformation of quartz and cataclasis of feldspar. For common granitic rocks with textures of framework feldspar and interstice-filling quartz, brittle strength of feldspar is a close approximation of the bulk rock strength because feldspar forms the load-bearing framework. With deformation, these rocks tend to develop localized high strain zones. The high strain zones commonly consist of quartz-rich layers and feldspar-rich layers. We have examined how the strength of granitic rocks changes with the volume fraction of these layers by combining the flow laws developed by Handy et al. [Handy, M.R., Wissing, S.B., Streit, L.E., 1999. Frictional-viscous flow in mylonite with varied bimineralic composition and its effect on lithospheric strength. Tectonophysics 303, 175-191]. Our results suggest a dramatic weakening of the bulk rock strength at an early stage of the layer development. We also found that formation of muscovite in the layer structures by breakdown of feldspar can result in further weakening in the layer structure. The bulk-rock strength with mica-bearing layers is reduced by a significant amount, compared with the mica-absent layers. Our analysis suggests that layer development with syntectonic formation of mica can be a major source of weakening for granitic rocks.
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