On the material dependence of experimental shear fracture orientation

It is not uncommon to observe shear fractures in ductile rocks oriented at more than 45° with respect to the maximum compression direction. Since these orientations cannot be explained with the classic Mohr-Coulumb or Tresca yield criteria, Zheng et al. (Journal of Structural Geology, 35: 1394–1405, 2011) proposed the maximum effective moment (MEM) failure criterion. This rule suggests that shear fractures in ductile rocks form at ~55° with the maximum compression axis and that this orientation is material-independent and, therefore, universal. Zheng et al. (Science China: Earth Sciences, 57(11): 2819–2824, 2014) used data from our own experiments as supporting evidence of their failure criterion. In this contribution we discuss why shear fracture formation in ductile rocks indeed strongly depends on the mechanical properties of the deforming medium, and why experimental data should not be taken to prove the validity of the MEM criterion. The formation mechanisms and orientations of shear fractures in our experiments significantly vary depending on the material strength and degree and type of anisotropy (composite and intrinsic). We therefore demonstrate using experimental data that a universal failure angle in ductile and anisotropic rocks does not apply. Additionally, we highlight some inconsistencies of the MEM criterion.