As noted earlier, three orthogonal sets of pressure solution seams commonly form the background structures in carbonate rocks in fold and thrust belt environments. Although the appearance of more than three sets of pressure solution seams were reported in the literature, for example, by Alvarez et al. (1978), any set beyond the three fundamental orthogonal sets must have occurred at the expense of shearing of one or more of the initial three sets. We here consider shearing of pairs of orthogonal sets and their products.

Consider a pair of bed-parallel (red) and bed-perpendicular (blue) pressure solution sets (Figure 1a) as observed in the platform carbonates of Majella Mountain, Italy. In this case, the bed-perpendicular set trends along the strike of tilted beds. When sheared in response to flexural slip, each of these sets experiences slip sense as shown on the map and results in splay pressure solution seams (black) oblique to both sets as shown. If deformation continued, further shearing utilizes the inclined splay pressure solution seams as well as one or both of the initial orthogonal sets (Figure 1b). The final product is one or two sets of mature faults grown by linkage from one bed level to the next by cataclastic deformation (Figure 2).

Figure 1. Shearing of two orthogonal sets of pressure solution seams; one bed-parallel (red) and the other bed-perpendicular (blue) but along the strike direction. A flexural slip results in shearing along the two sets in the sense marked on the map and produces splay pressure solution seams inclined to both orthogonal sets. Continued deformation generates faults which take advantage of both the inclined splays as well as one or both sheared orthogonal sets. From Graham et al. (2003).

Figure 2. Development of two sets of normal faults by shearing of two orthogonal pressure solution sets with bed-parallel and bed-perpendicular with strike-parallel configurations. From Aydin et al. (2010).

In case the initial system has two bed-perpendicular orthogonal systems of pressure solution seams in strike and dip directions and both are subjected to shearing as depicted in a highly schematic diagram in Figure 3, then, two shear zones may eventually develop with a dihedral intersection angle (Figure 4), each utilizing the strike- and dip-parallel sheared pressure solution seams and their splays. The sense of shearing may be dictated by longitudinal variation in the amount of transport of thrust sheets.

Figure 3. Shearing of two ped-perpendicular orthogonal pressure solution sets and potential products. Rotation of stresses about a vertical axis would possibly induce these shears but their senses would vary along a thrust front.

Figure 4. Strike-slip faults produced by shearing of the orthogonal sets of pressure solution seams and their splays in the platform carbonates above the leading edge of the Majella Thrust, Italy. From Aydin et al. (2010).