Experimental Insights Into Fault Reactivation in Gouge‐Filled Fault Zones

Faults in the brittle crust constitute preexisting weakness zones that can be reactivateddepending on their friction, orientation within the local stressfield, and stressfield magnitude.Analytical approaches to evaluate the potential for fault reactivation are generally based on theassumption that faults are ideal planes characterized by zero thickness and constant friction. However,natural faults are complex structures that typically host thick fault rocks. Here we experimentallyinvestigate the reactivation of gouge‐bearing faults and compare the resulting data with theoreticalpredictions based on analytical models. We simulate preexisting faults by conducting triaxial experimentson sandstone cylinders containing saw‐cutsfilled with a clay‐rich gouge and oriented at different angles,from 30° to 80°, to the maximum principal stress. Our results show the reactivation of preexistingfaults when oriented at 30°, 40°, and 50° to the maximum principal stress and the formation of a newfracture for fault orientations higher than 50°. Although these observations are consistent with the faultlock‐up predicted by analytical models, the differential stress required for reactivation strongly differsfrom theoretical predictions. In particular, unfavorable oriented faults appear systematically weaker,especially when a thick gouge layer is present. We infer that the observed weakness relates to the rotationof the stressfield within the gouge layer during the documented distributed deformation that precedesunstable fault reactivation. Thus, the assumption of zero‐thickness planar fault provides only an upperbound to the stress required for reactivation of misoriented faults, which might result in misleadingpredictions of fault reactivation.