Slip-tendency analysis as a tool to constrain the mechanical properties of anisotropic rocks

The mechanical strength of foliated rocks is typically anisotropic because it varies with the orientation of the foliation relative to the applied principal stresses and commonly depends on phyllosicate content and phyllosicate physical interconnectivity. We constrain the degree of mechanical anisotropy associated with pre-existing planar discontinuities, such as metamorphic foliations and inherited faults, by combining paleostress analysis and meso- and microscale characterization of brittle failure modes in different phyllosilicate-bearing rocks outcropping in the Sierras de Córdoba (SDC) of Central Argentina. The SDC show evidence of a long brittle deformation history from Early Triassic – Present with three distinct brittle deformational events. Each phase caused new strain increments accommodated by the formation of newly-formed faults or by the reactivation of inherited discontinuities. Structural investigations reveal that gneisses and phyllites deformed by different failure modes during the different events. Therefore, we were able to use a conceptual field-scale triaxial experiment by applying a stress model based on normalized slip-tendency analysis. We constrained the friction coefficient for slip along the foliations (μs) and along pre-existing faults (μf) to 0.2 to 0.3 and 0.4, respectively. These values fit independent estimates for similar rocks confirming the potential of our approach for other case studies.