6.1 Joint Zones‌

Joint zones, also known as joint clusters, fracture corridors (or swarms), refer to more than two (commonly many) individual joints localized in a narrow zone, as shown in Figure 13. These features are significant conduits for fluid flow. Joint zone widths measured in the field vary from centimeters to tens of meters. Figure 13a shows a joint zone about 60 to 80 cm in width that is composed of several subparallel joints in the Entrada Sandstone at Arches National Park, Utah, USA. The photograph in Figure 13b shows a joint zone that is localized in a specific horizon and associated with several sheared vertical joints behind the person placed for scale (Cruikshank and Aydin, 1995). The formation of joint zones may be controlled by the velocity of the joint growth (Olson, 2004). When this mechanism is active, clustering can occur for very high values of the subcritical index. The subcritical index relates the stress intensity factor (which predicts the stress state near the fracture tip) to the fracture propagation velocity.

Other possible fracture clustering mechanisms involve specific tensile stress patterns (in such cases tension halos develop around fractures) and mechanical interactions between fractures repeated through time (de Joussineau and Petit, 2021). These examples indicate that there are different mechanisms for joint zone formation, including shearing. Further information related to the geometrical properties of fracture corridors is provided by de Joussineau (2023).

As noted earlier, once formed, almost all joint zones represent weak zones prone to shearing, which improves the connectivity between neighboring fractures in the zone as well as between the zone and the neighboring joints or joint zones.