DOES FOLIATION ORIENTATION AFFECT THE STRENGTH OF THE MIDDLE TO LOWER CONTINENTAL CRUST?

The effect of foliation orientation on the strength of rocks deformed in the upper continental crust is relatively well-understood, but very little data exists on the effects of foliation orientation on the strength of rocks deformed in the middle to lower continental crust. In order to determine how foliation orientation affects the strength of rocks deformed at middle to lower crustal conditions, we deformed cores of a fine-grained biotite + quartz + plagioclase gneiss at T = 800^oC, P = 1500 MPa, and strain rate of 2*10^-6/s with the foliation oriented perpendicular, parallel, or at 45^o to the compression direction. The micas in the gneiss are aligned, but not interconnected and sit within a dominantly plagioclase framework. The peak strengths of cores of all foliation orientations are ~500 MPa (ε = 15%) and strain weaken until the end of the experiment (ε = 30%). Thin shear zones that cross-cut the cores are observed in all samples from high strain experiments and these shear zones accommodated most of the bulk strain. Microstructures inside the shear zones include zones of fine-grained, interconnected biotite and zones of mixed phases. Microstructures outside of the shear zones include moderate kinking of biotite, undulatory extinction in quartz and plagioclase grains and small, isolated pockets of melt. The similarity between the strengths of cores with different foliation orientations indicates that foliation orientation may not significantly affect the strength of rocks deformed in the mid- to lower-continental crust.