Rock fabric controls on the failure mode of strongly deformed gneisses

The strength and deformability of anisotropic, folded metamorphic rocks strongly depend on rock mineralogy, texture and other microscopic features. Nevertheless, rock fabric controls on the failure mode of rocks which underwent severe ductile deformation are poorly understood and difficult to generalize. We studied the mechanical behaviour of gneiss samples of the Monte Canale unit (Central Alps, Italy). About 60 rock samples were tested under uniaxial and triaxial compression, and indirect tension tests. The samples revealed exceptionally low unconfined compressive strength and Young’s modulus. Rock samples broke according to four failure modes, from shear failure along foliation to the development of centimetre-scale brittle shear zones, both at low and high confining pressure. X-ray Computed Tomography imaging and micro-structural analysis suggest that failure of strongly deformed metamorphic rocks is controlled by several anisotropies related to micro-fabric, not always readily detectable at meso-scale