7.1 Single Joint Sets‌

The aerial photograph in Figure 22 shows a set of joints in the Moab member of the Entrada Sandstone at Arches National Park, Utah, USA. There, the dominant joint set is parallel to the axis of the Salt Valley Anticline. The critical properties of joint sets such as those reported in Figure 22 are their spacing and the continuity of individual elements in the array. The spacing of joints in siliciclastic rocks depends on the thickness of units in which they occur as well as the loading. The simplest common case is the proportionality between bedding thickness and joint spacing. However, the magnitude of strain in the direction perpendicular to the set is also an important factor.

Figure 22 – Bird’s eye view of a systematic joint set and other associated fractures in the Moab member of Entrada Sandstone exposed on the northern limb of Salt Valley Anticline, Arches National Park, Utah, USA. The width of the photograph, taken from an airplane, is about 300 m.

There are two main concepts related to the growth of joint sets. The first is the notion of fracture saturation, which corresponds to a critical level of joint production beyond which the number of joints does not change appreciably. In this case, any amount of additional extension is accommodated by increasing joint apertures. In layered rocks, the joint spacing to layer thickness ratio appears to level off around unity for fairly well-developed joint systems. In such systems, the spacing of joints is controlled largely by the height of the joints, the basis of which is the stress shadow zone that forms around joints (Lachenbruch, 1961). The second important concept is the interaction between closely spaced joints (Pollard and Aydin, 1988). A particular geometry referred to as en échelon (here echelon for simplicity) and relays, implies the possibility of gaps in the lateral continuity of the joints. The continuity of joints or the lack thereof is important for fluid flow. As mentioned earlier, even though the lineaments such as those in Figure 22 may appear to be continuous, a close inspection may reveal gaps along them or, in other words, space between the consecutive echelons. In general, the segmentation of joints in both lateral and vertical directions is the rule and not an exception. A wide range of observations tells us that it is very difficult for joints (or any fracture type) to propagate within a plane over a long distance. The main reasons for this are the presence of widespread material inhomogeneities in rocks and the interactions among adjacent fractures during ongoing deformation.