Brittle faulting of ductile rock induced by pore fluid pressure build‐up

Under upper crustal conditions, deformations are primarily brittle (i.e., localized) and accommodated by frictional mechanisms. At greater depth, deformations are ductile (i.e., distributed) and accommodated by crystal plasticity, diffusion mass transfer or cataclastic flow. The transition from the brittle to the ductile domain is not associated with a critical depth, but rather varies in time and space. One main parameter controlling the variation of this transition is the pore fluid pressure. On the one hand, a pore fluid pressure increase reduces the effective stresses and possibly increases the strain rate, bringing the system closer to brittle conditions. On the other hand, pore fluid can favour ductile mechanisms, mostly via chemical effects, by facilitating intra‐crystalline plasticity, enhancing fluid‐solid diffusion and fracture healing/sealing. We report triaxial laboratory experiments that investigated the effect of pore fluid pressure increase during the ductile deformation of Tavel limestone. Three injection rates were tested: 1, 5 and 10 MPa/min. We demonstrate that: 1) Under initially ductile conditions pore fluid pressure increase immediately turns the system from compaction to dilation. 2) Dilation is due to the development of localized shear fractures. However, the macroscopic localisation of the deformation is not instantaneous when the ductile to brittle transition is surpassed; a transient creeping phase is first needed. 3) To reach macroscopic brittle failure of initially ductile samples, a critical dilatancy is required. 4) Injection rate controls the final fracture distribution. We demonstrate that pore pressure build‐up in a rock undergoing ductile deformation can induce shear fracturing of the system.