Fault

Fault: A fracture in rock along which there has been an observable amount of displacement. Faults are rarely single planar units; normally they occur as parallel to sub-parallel sets of planes along which movement has taken place to a greater or lesser extent. Such sets are called fault or fracture-zones.

Various terms are used to describe the attitude of the fault plane and the nature of the movement upon it:
A fault plane may be vertical, or may have a dip. The hade is the angle which the fault plane makes with the vertical plane - the complement of the angle of a dip.

The term strike is applied to a fault plane in precisely the same way as to a bedding plane. However, it is customary to describe a fault according to the relationship of the fault-strike and bedding-strike:

• Fault-strike parallel or sub-parallel to the bedding-strike = strike fault.
• Fault-strike approximately at right-angles to the bedding-strike (i.e. nearly parallel to the dip) = dip fault.
• Fault-strike making a well-defined angle with the bedding-strike = oblique fault.

If the direction of movement on the fault plane is parallel to the dip of the fault (i.e. upwards or downwards) the fault is said to have a dip-slip movement. If the direction of movement on the fault plane is parallel to the strike of the fault (i.e. sideways), the fault is said to have strike-slip movement. If the direction of the movement along the fault plane is in any other direction, the fault is said to have an oblique-slip movement. An oblique slip can always be resolved into dip-slip and strike-slip components.

The blocks on either side of a fault display relative movement with respect to each other. It is convenient to talk about upthrown or downthrown sides of a fault - the throw being the measure of the vertical component of the displacement of the upthrown and downthrown blocks. It is emphasised that, in general, it is very difficult to determine which side of a fault has actually moved, or whether, in fact, both sides have moved. The horizontal component of the displacement is known as the heave. In a dipping fault, the surface of rock along the fault plane which has rock above it is called the hanging wall. Similarly the footwall is the surface of rock along the fault plane which has rock below it. Originally these two terms were used by Cornish miners when working an inclined vein.

Fracture having appreciable movement parallel to the plane of the fracture. The following are the major types of fault:

Normal fault A fault with a major dip-slip component in which the hanging wall is on the downthrow side. Normal faults are sometimes referred to as tension or gravity faults, but the use of these terms is undesirable, since they imply an origin which cannot always be justified.
Reverse fault A fault with a major dip-slip component in which the hanging wall is on the upthrow side. If the dip of such a fault is low, then the term thrust or thrust fault is used (see below). Reverse faults are sometimes called compressional faults, but the use of this term is also undesirable (see 'normal fault', above).
Tear fault (Wrench fault, transcurrent fault) A fault in which the movement is dominantly strike-slip (i.e. horizontal). The terms dextral and sinistral are applied to tear faults to describe the apparent direction of movement. If one stands on the outcrop of a particular rock, facing the fault plane, then for dextral movement the corresponding outcrop on the other side of the fault will be displaced to the right hand, and for sinistral movement will be displaced to the left hand.

Rotational movement between the two fault blocks will result in varying throws being recorded along the strike. Two main types may be recognised:
Hinge fault, where the displacement increases from zero to a maximum along the strike (this type often passes into a flexure of some kind).
Pivot fault, where one block appears to have rotated about a point on the fault plane such that for part of its length the fault is normal with a decreasing throw, and for the remainder of its length is a reverse fault with an increasing throw, the position of no displacement being the point around which rotation appears to have taken place.
Thrust faults should perhaps be called slides - a more general term for both thrusts and lags. This classification of near horizontal dislocations is based upon the concept of their having been derived from overturned or recumbent folds, the slide plane replacing one of the limbs of the fold. If the uninverted limb of the fold is eliminated, the slide is termed a lag, and if the inverted limb is eliminated the slide is termed a thrust, which is the more common case. Where several thrust planes develop in parallel sets, a series of high-angle reverse faults may also develop between pairs of thrust planes, giving rise to what is known as imbricate or schuppen structure. A thrust which develops otherwise than from a recumbent fold is termed a shear thrust, while the rather rare occurrence of a thrust plane reaching the surface, so that rocks slide forward over it, is termed a surface thrust.

Patterns of faults develop according to the stress system involved, and may be parallel, en echelon, radial, concentric, or in two directions (intersecting) to give a trellis pattern.

A downthrown area between two parallel faults is produced by trough faulting, and if this occurs on a large scale a rift valley or graben results. An upthrown area between two parallel faults is called a horst. A parallel series of faults with a repeated downthrow in the same direction is termed step faulting.

Riedel faults (fractures) are branching faults developed in the cover by movement in the basement. Faults which are intimately associated with the development of folds are termed longitudinal crestal faults, cross faults and marginal faults.

The terms antithetic and synthetic are sometimes applied to fault planes which dip in the opposite direction to, or in the same direction as, the bedding. These terms have, however, now been replaced by a simple statement concerning the direction and amount of dip or hade of the fault plane.