A comparison between joints and faults as brittle structures used for evaluating the stress field

A simple genetic model is proposed for the formation of extensional joints and thus relationships between joints and the three principal axes of a stress field. The basic assumption is that joints are purely extensional and have no shear component on them. Consequently they can form only parallel to principal planes. The model also explains why joints usually create two roughly orthogonal sets. The model is tested by comparison with associated fault systems. Using well-tested methods of structural analysis for faults and a recently proposed one for joints, some case studies are considered, both at the scale of a whole site and at a smaller scale. In both cases the results confirm the model. In conclusion, a close relationship between faults and joints has been brought into focus and the importance of joint systems as brittle mesostructures useful for evaluating a stress field is emphasized.