A Granitic Mylonite Is Strongest Parallel to Lineation in a High‐Temperature Plastic Field

Abstract The effects of preexisting fabrics on the flow laws and anisotropic deformation of rocks require further study. We conducted triaxial compression experiments on a granitic mylonite parallel to lineation ( X ), perpendicular to lineation and parallel to foliation ( Y ), and perpendicular to foliation ( Z ) under a pressure of 300 MPa, temperatures of 800–1,000°C, and strain rates of ∼2.5 × 10 −6 –10 −4 s −1 using a Paterson gas‐medium apparatus. The low stress exponent ( n = 1.9–5.8), high activation energy ( Q = 325–802 kJ/mol), and macrostructures (distributed for most samples) and microstructures (such as kinked, folded and elongated biotite, elongated quartz and feldspar, microcracks within quartz and feldspar, and melt wetting and dissolution of quartz and feldspar) suggest that the deformation is dominated by dislocation creep, along with brittle regime at ≤∼850°C and likely diffusion creep at ≥900°C. Dehydration melting of biotite causes more obvious melt wetting of quartz and feldspar boundaries, lower n values at ≥950°C, and the maximum changes in n and Q along the Z ‐direction, since the biotite alignment defines the foliation. Under the same conditions, the X ‐direction samples consistently display the greatest strengths, which would have been for the Z ‐direction samples as reported previously, and most obvious deformation localization, mainly due to the alignment of the elongated quartz and feldspar along this direction. Microcracks always occur in quartz but are tensile when compressed perpendicular to the foliation plane and compressively sheared when shortened parallel to the foliation plane. These tensile microcracks further weaken the rock samples with axes perpendicular to foliation.