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

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 degrees C, and strain rates of similar to 2.5 x 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 <=similar to 850 degrees C and likely diffusion creep at >= 900 degrees C. Dehydration melting of biotite causes more obvious melt wetting of quartz and feldspar boundaries, lower n values at >= 950 degrees 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. Geological observations show that foliation (planar fabric) and lineation (linear fabric) are typical characteristics of granitic rocks that experienced ductile deformation in the deep crust. How these fabrics develop during deformation and how they affect strength and strain distributions in the crust have not received sufficient attention, limiting our understanding of rheological property changes during shear zone evolution. In particular, the effects of lineation on rock rheology remain unknown. We performed a series of triaxial compression experiments on granitic mylonite. The results show that sample deformation at 300 MPa, 800 degrees C-1,000 degrees C, and similar to 2.5 x 10-6-10-4 s-1 is dominated by dislocation creep, accompanied by brittle deformation at <=similar to 850 degrees C for compression parallel to lineation and likely diffusion creep induced by significant dehydration melting of biotite at >= 900 degrees C. Under the same conditions, the samples with axes parallel to the lineation consistently display the greatest strengths, which would have been for the samples with axes perpendicular to the foliation, as reported previously, and the most obvious deformation localization. Interestingly, tensile microcracks within the quartz grains further weaken the samples under compression perpendicular to the foliation. Deformation of granitic mylonite is dominated by dislocation creep, along with brittle regime at T <= similar to 850 degrees C and likely diffusion creep at T >= 900 degrees CThe strength (sigma) of granitic mylonite is anisotropic, with a maximum parallel to the lineation directionDehydration melting of biotite causes a large change of flow parameters normal to foliation (Z) and tensile microcracks further reduce sigma(Z)