The role of pre-existing anisotropies in fault mechanics: experimental insights from triaxial saw-cut experiments

Fault orientation and fault rock mechanical properties exert a major control on fault reactivation. Slip along faults that lie at high angle to the maximum compressive stress requires restrictive conditions, such as very low friction coefficient and/or near-lithostatic pore fluid pressure. However, the mechanism allowing a misoriented fault to initiate is still a matter of debate. Nevertheless, misoriented faults, such as the San Andreas fault, low-angle normal faults and sub-horizontal décollements, are recognized throughout the world. On a smaller scale, field observations show that mesoscale faults in mechanical multilayers often propagate within clay-rich layers at high angle to the maximum principal stress. These observations suggest that the presence of pre-existing anisotropies, not only resulting from faulting but also due to inherited foliation or sedimentary layering, plays a key role in determining fault geometry. To investigate this problem, we present a suite of triaxial experiments in which we deformed cylindrical samples of sandstone with pre-imposed saw-cuts with variable orientations respect to the axial stress. A layer of powdered marl or shale was placed within the saw-cut to simulate a pre-existing anisotropy. We performed: 1) conventional triaxial experiments to evaluate the failure envelope of the sandstone, and 2) biaxial experiments to evaluate the frictional strength of the marl and shale. Then, 3) we conducted triaxial experiments at constant confining pressure with saw-cuts oriented at different angles to the axial stress, from 30° (favourably oriented) up to 80° (severely misoriented). Microstructural observations on reactivated saw-cuts were conducted to investigate the deformation processes with increasing fault deformation. Finally, we compared the resulting deformation mode, i.e. saw- cut reactivation vs. new fracture development, with theoretical predictions based on sandstone strength and marl/shale friction. Our results show a complex mechanical behaviour that results from the interplay between the stress field orientation and the contrast in mechanical properties between the gouge and the surrounding. Reactivation occurs only in saw-cuts at angles to the axial stress that are lower than 60°-70°, consistently with theoretical predictions. Preliminary laboratory data show that under dry conditions the reactivation is a multi-stage process: after an initial compaction phase and a linear-elastic phase, yielding of the fault gouge occurs. Given the same boundary conditions, yielding occurs at the same differential stress independently of saw-cut orientation. After the yielding, a second linear-elastic phase precedes stick-slips followed by stable sliding. Integrating mechanical data and microstructural investigation we suggest that the first inelastic yielding occurs when the Mohr circle is tangent to the frictional reactivation criterion, obtained from friction experiments (2). Then, stick-slip and stable sliding are associated to localization of deformation along shear planes. We infer that they occur only when the stress path of the saw-cut intersects the frictional reactivation criterion.