Both sets of joints in an orthogonal pattern may be subjected to shearing if there is a rotation of the stress system or tilting of the medium. Whether one or both joint sets would slip depends on the resolved shear and normal stresses on the joint planes in two orientations and frictional resistence to slip along the joints. Figure 1a shows a case in which splays are formed diagonally to the up-and-down set on a wave cut pavement of flagstone cropping out in Caithness, northern Scotland. Figure 1b is a broader view of the pavement with incipient faults utilizing both the splays and the initial joints to form through-going small faults.

Figure 1. Sheared orthogonal sets of joints and diagonal splays (a), and coalescence of the sheared joints and sheared splays to form numerous through-going faults (b) on a wave cut platform, Caithness, Scotland. Photographs courtesy of Neil Grant of ConocoPhillips.

Figure 2 depicts schematically the initial orthogonal joint pattern (a) and the subsequent shearing of both sets eventually leading to two through-going faults with dihedral intersection angle (b). Due to the angular geometry between adjacent segments, larger deformation will break the rock into smaller fragments via cataclastic deformation which may eventually lead to the formation of larger faults with more continuous slip surfaces.

Figure 2. Schematic diagrams depicting an orthogonal joint pattern (a) and formation of two faults (shown in blue and orange in (b)) through shearing of initial joints, formation of splays in diagonal orientation (red), and their shearing.