Fabric Controls on the Failure Mode of Strongly Deformed Metamorphic Rocks with Multiple Anisotropies

Classical models of rock fracture in geodynamics, seismology, and rock engineering are built on basic modes of failure (e.g. tensile, shear, leaning) in isotropic homogeneous material. Nevertheless, the strength, deformability and mode of failure of anisotropic, often folded metamorphic rocks forming significant parts of both the shallow and deep Earth's crust strongly depend on rock mineralogy and fabric. Although a large body of literature exists on the effects of single planar anisotropy on rock failure (e.g. strength dependencies and slip tendency), fabric controls on the behavior of rocks which underwent severe ductile deformation (e.g. crenulation folding) are still poorly understood and further knowledge is required to improve existing models. In this perspective, we studied experimentally the mechanical behaviour of gneisses (Monte Canale unit) and phyllites (Ambria Unit) from the Italian Central Alps and Southern Alps. Rock petrography and fabric were characterised through standard optical microscopy and XRD techniques. About 60 gneiss samples and 40 phyllite samples from available drill cores were prepared and tested under uniaxial and triaxial (multistage) compression, and indirect tension (Brazilian) tests. Laboratory tests allowed reconstructing the stress-strain behaviour and the main mechanical properties of intact rock, as well as observing the fracture patterns and estimating fracture energies of tested samples. Rock samples revealed low and scattered values of unconfined compressive strength (UCS) and Young's modulus. Rock samples broke according to four failure modes, from the well-known shear failure along foliation to the development of centimetre-scale brittle shear zones, both at low and high confining pressure. Moreover, no clear dependence of rock strength on the foliation direction could be identified. Fracture patterns were thus further investigated and quantified in 3D through X-ray Computed Tomography imaging, performed at different resolutions (MicroCT: 40-60 μm; medical CT: 625 μm) and micro-structural analysis of thin sections. Investigation results suggest that the failure of strongly deformed metamorphic rocks is controlled by the occurrence of multiple anisotropies related to micro-fabric, not always characterised by clear meso-scale expression, including crenulation folding, shape preferred orientation, intracrystalline deformation microstructure. Different failure modes dominate depending on the geometrical arrangement of both foliation and fold axial surfaces, in turn affecting the values of rock strength and deformability. The results of this study point to the need of accounting for the effects of multiple, geometrically complex anisotropies in setting up realistic models of rock fracturing at different scale and for different geological and engineering applications.