Fabric induced weakness of tectonic faults

Mature fault zones appear to be weaker than predicted by both theory and experiment. One explanation involves the presence of weak minerals, such as talc. However, talc is only a minor constituent of most fault zones and thus the question arises: what proportion of a weak mineral is needed to satisfy weak fault models? Existing studies of fault gouges indicate that >30% of the weak phase is necessary to weaken faults ‐ a proportion not supported by observations. Here we demonstrate that weakening of fault gouges can be accomplished by as little as 4 wt% talc, provided the talc forms a critically‐aligned, through‐going layer. Observations of foliated fault rocks in mature, large‐offset faults suggest they are produced as a consequence of ongoing fault displacement and thus our observations may provide a common explanation for weakness of mature faults.