Faults represent the response shown by brittle rock to applied stresses. If suitable measurements are taken on faults observed in the field it is possible to estimate the nature of the palaeostresses involved, or at least to place broad limits on the possible orientations of those stresses.

Conjugate pairs of faults

Conjugate faults are broadly contemporaneous faults which formed under similar stress conditions. Known from the results of experimental rock deformation and the deformation of rock analogue materials, such faults are arranged in a symmetrical fashion in relation to the principal axes of the applied stresses (Fig. 29b). Pairs of faults are difficult to designate as conjugate in isolation. The conjugate relationship is established when two sets of faults are present and where members of one set exhibit inconsistent cross-cutting relations with members of the other set, thereby suggesting contemporaneity (Fig. 29a). In addition, the slip direction on each fault belonging to a conjugate pair should be at right angles to the line of intersection of the two faults.

Once recognized, the directions of the principal stresses responsible for the formation of conjugate faults are found as follows.

1. 1 The line of intersection is found by plotting the great circles for each fault plane. The line of intersection is taken as the direction of the intermediate stress axis σ2 (Fig. 29c).

2. […]