Some gold deposits may present several types of faults. For example, a reverse fault that is one along the hanging wall has moved upward on the foot-wall, which is commonly considered as having been forced under the overlying mass by compression. Such faults are less likely to be simple fractures than normal faults, and are frequently accompanied by parallel sheeting, or by a folding of the strata or rock masses that form their walls. Faults due to torsional stresses are commonly accompanied by differential movement, the displacement over one part of the fault being greater than over other parts, resulting in a tilting of the faulted block. Such faults frequently follow curved lines of strike, and are often branching.
Faults are sometimes referred to as strike faults, or as dip faults, according to their direction as compared to the strike or dip of enclosing strata or associated gold veins. A series of approximately parallel faults of similar class and displacement are called step faults; if of opposite displacement, they are called compensating faults. Two faults, or two systems of faults, parallel in strike but intersecting in dip, are occasionally formed by the same compressive or torsional stresses, and are known as conjugate faults or fault systems.
Faults formed at slight depth below the surface are likely to be less regular and less persistent than deep-seated faults. The occurrence of many small, irregular fractures that become less in number and more regular in direction as depth is gained, is indicative of faulting under light load and at slight depth; a further indication of shallow formation is the presence of friction breccia along a fault rather than the pasty gouge that is characteristic of deep-seated movements under heavy loads of overlying rocks. The upward branching of a fault, or a marked change in dip, is a sign of shallow dislocation. Surface detritus matter is occasionally found in a fault filling, conclusively proving a shallow depth at the time of faulting.
It has been noted that where a fold was formed by compressive stress beyond the capacity of the rock to withstand, a fault is developed along the axis, or the plane bisecting the angle between the component limbs of the fold. Folds' not infrequently pass into faults along their strike, and in a region that has been subjected to both folding and faulting these expressions of stress are likely to be parallel. The term fracture denotes a break in a rock mass in importance intermediate between a joint and a fault, as these latter terms are generally used by gold prospectors. Also, a flexure is a sharp bend in a series of strata or in a rock mass, without the development of a continuous fracture, the result being a displacement similar to that of a monocline.