Similar to the multiple joint sets categories, we consider multiple pressure solution seam sets as (1) orthogonal and (2) non orthogonal. The simplest of these is comprised of two sets of orthogonal pressure solution seams as shown in Figure 1 which is a photograph of a core slab from a clastic rock.

Figure 1. Two orthogonal pressure solution seams (marked by full arrows) in a core slab recovered from a North Sea sandstone reservoir. From Aydin et al. (2006).

More often, however, three sets of mutually orthogonal pressure solution sets occur as an assemblage as labelled on the photograph in Figure 2. One set occurs along the bedding interfaces and the other two are perpendicular to bedding and perpendicular and parallel or nearly parallel to the axial plane of the fold. These mutually orthogonal pressure solution sets labelled as PS1, PS2a, and PS2b in Figure 3 form the background structures in many fold and thrust belts, for example, in the platform carbonates of the Apennines in Italy.

Figure 2. Three sets of mutually orthogonal pressure solution sets in a folded terrain of clastic rocks of southwestern Ireland. One of the sets occurs along the bedding and the other two are mutually orthogonal and are perpendicular to bedding and generally sub-parallel to the fold axis. Slightly revised from Nenna et al. (2010).

Figure 3. Three mutually orthogonal pressure solution seam sets (bed-parallel, PS1, and bed-perpendicular, PS2a and b) plus a diagonally oriented set (PS3) in platform carbonate rock of the central Apennines, Italy. The former sets as depicted in the diagram in Figure 2 above are pervasive background structures in carbonate terrains of major fold and thrust belts of the world. The latter is due to shearing of one or more of the sets of the orthogonal system. From Agosta and Aydin (2006).

The outcrop shown in Figure 3 contains more than three sets of pressure solution seams. PS3 in the photograph is oblique to bedding and any of the three mutually orthogonal sets of pressure solution seams. The outcrop shown in Figure 4 also includes more than three sets of pressure solutions. At this outcrop, there is evidence for bedding plane slip based on the striations on them. Obviously, the formation of additional sets of pressure solution seams besides the three mutually orthogonal ones requires a different mechanism which will be addressed later under 'Mechanisms and Mechanics of Non-orthogonal PSS Sets' and the evidence for slip on some sets provides a clue for it.

Figure 4. Three sets of pressure solution sets in platform carbonate rock outcropping at the Vallone Santo Spirito, Maiella Mountain, central Apennines, Italy. The bedding is due to a set of bed-parallel pressure solution seams and is mechanical in origion. Highlighted is one pressure solution seam normal to bedding and along the dip direction, which has many pressure solution seams (blue) at splay orientation. These seams are also in splay orientation to the bed-parallel set.