Faults occur in a great variety. It is possible to classify them into different types on the basis of some common characters. We will discuss here only the few very common types of faults and the characters on which they are grouped together.
Following factors are more commonly considered important in classification of faults:
1. Apparent movement as basis;
2. Attitude of fault as basis;
3. Slip as basis;
4. Mode of Occurrence as basis.
1. Apparent Movement as Basis:
In a faulted sequence of rock layers, one part may appear to have moved up or down, or moved right or left with respect to the other part of the same layer. The emphasis in this case is only on appearance because actually it may require to be established which of the two parts, the hanging wall and the foot wall, has moved during faulting and by how much.
Three fundamental types of faults are commonly distinguished on the basis of apparent movement:
1. Normal faults,
2. Reverse faults,
3. Strike slips faults and
4. Hinge Faults.
1. Normal Faults:
Such a fault in which hanging wall has apparently moved down with respect to foot wall is classified as a Normal Fault.
In this definition it is clearly implied that nothing can be said with certainty whether it was the hanging wall which moved down or the foot wall which moved up or both the walls moved down, the hanging wall moving more than the foot wall and hence the appearance. Whatever the case, when the fault satisfies the definition of hanging wall standing at a lower position with respect to the foot wall it may be classed as a normal fault.
In normal faults, the fault plane may be inclined at any angle between horizontal and vertical, but most commonly, the fault angles are between 45° and vertical. Further, due to the inclined nature of the fault plane and downward displacement of a part of the strata, normal faults cause an extension in the crust wherever they occur.
Normal faults are also often termed as gravity faults especially when it is established that the hanging wall has actually moved down with respect to the foot wall.
Horst and Graben:
These are physiographic features caused by normal faults occurring in pairs.
When two normal faults appear on either side of a central wedge shaped elongated block in such a way that the central wedge appears raised high up with respect to the sides, the outstanding structure is called a Horst. Sometimes it may be high and extensive enough to be called a BLOCK MOUNTAIN.
In many horsts, the border faults are almost parallel in strike and very high angled. The movement of blocks in horsts is assumed to be relative but the effect is that the central block is left raised relative to the side blocks. Numerous small and big examples of horsts are found in major mountain systems such as Alps, Central Europe and East Africa.
It is almost reverse of a horst in structure and appearance. A graben may be described as an elongated wedge shaped central block, which appears to have moved downward with respect to the side blocks along two downward converging normal faults. The bordering faults are almost parallel in strike and high angled in character.
In their occurrence, the grabens are generally associated with horsts, which often form their immediately flanking highlands.
The origin of horsts and grabens is believed to be due to lateral tension in the crust in most cases. Faults involving extensive blocks and resulting in horsts and grabens are often called as block faults and the process as block faulting.
Faults in which the fault plane is vertical or nearly so and the resulting movement of blocks is also in a vertical direction are termed as Vertical Faults. It is customary to group vertical faults along with normal faults while discussing their origin.
2. Reverse Faults:
It is such a type of fault in which the hanging wall appears to have moved up with respect to the foot wall. In reverse faults, the fault plane is generally inclined between horizontal and 45 degrees although reverse faults with steeply inclined fault surface have been also encountered. By virtue of their inclination and direction of movement, reverse faulting involves shortening of the crust of the Earth (compare with normal faults).
These are, broadly speaking, such varieties of reverse faults in which the hanging wall has moved up relative to the foot wall and the faults dip at angles below 45 degrees. Faults dipping above 45 degree with hanging wall having gone up are then called as reverse faults.
The thrust faults or simply thrusts are of very common occurrence in folded mountains and seem to have originated as a further step (after folding) in the process of adjustment of rocks to the imposed stresses.
Thrusts are sometimes further distinguished into two sub-types: the over thrusts and the under thrusts. In the former it is the hanging wall that seems to have been actively and actually displaced with respect to a passive foot wall. In the under thrusts, reverse is believed to be the case- the hanging wall seems to have remained passive while the foot wall has been active in the displacement.
The Himalayan Mountains in the Indian sub-continent present numerous examples of thrust faults developed all along its extension from northwest to southeast. In the outer Himalayas, the MAIN BOUNDARY FAULT is the most important example of thrust faults, which extends from Punjab to Assam along which the older Murree formations have been thrust up and overlie the younger Siwalik formations for several hundred kilometers.
This term is used for extensive blocks of rocks that have been translated to great distances, often ranging to several hundred kilometers, along a thrust plane. The large-scale movement may be attributed to a major over thrusting or a recumbent folding followed by thrust faulting. When a series of thrust faults occur in close proximity, thrust blocks are piled up one above another and all fault surfaces dip in the same direction, the resulting interesting structure is known as an Imbricate Structure.
In the Himalayan Mountains, many well defined nappe zones have been recognized among which may be mentioned the Kashmir Nappe, the Nappe zone of Simla Himalayas and the Nappes of the Garhwal Himalayas. In the case of Kashmir Nappe Zone, it is believed that the Kashmir Nappe, composed of pre Cambrian sediments has been thrust for a great distance over a horizontal thrust called the Panjal Thrust.
3. Strike-Slip Faults:
This is the third major category of faults known to occur in nature and on a very large scale. These may be defined as faults in which faulted blocks have been moved against each other in an essentially horizontal direction. The fault plane is almost vertical and the net slip may be measured in great distances. There are some other terms used for strike slip faults such as lateral faults, transverse faults, wrench faults and transform faults. Of these, the transform faults are very common and denote strike slip faults specially developed in oceanic ridges.
Strike-Slip faults are further distinguished into right handed (dextral) or left handed (sinisteral) depending on the direction of movement of the block with respect to an observer- it is a left-handed fault if the left block appears to have moved towards the observer and a right handed fault if the right block seems to have moved towards the observer.
The best example of a strike slip fault is the great San Andres Fault of California. It extends for almost about 1,000 km in a NW-SE direction. The strata is believed to have suffered displacement varying between 50 km to 200 km.
Another important feature of this fault is that it is actually made up of zone of intensely sheared and crushed rock, often mylonised, but maintains its straight extension for greater part. Although the latest recorded movement along the San Andres fault occurred on April 18, 1906 during the San Francisco Earthquake, this fault has remained active during last 25 million years.
4. Hinge Faults:
These are also called pivotal faults or rotational faults. A hinge fault is characterised by a movement of the disrupted blocks along a medial point called the hinge point. The movement is, therefore, rotational rather than translational (as in the first three cases). In such faults, the amount of displacement increases away from the hinge point. These are rather rare type of faults.
2. Attitude of Fault as Basis:
The mutual relationship of attitude (dip and strike) of fault and of the disrupted rock has also been used in some cases for classifying faults into three types:
1. Strike faults,
2. Dip faults and
3. Oblique faults.
1. Strike Faults:
These are the faults that develop parallel to the strike of the strata. In other words, the strike of the fault and that of the disrupted layers are essentially parallel. Sometimes the faults are developed along the bedding planes; in such cases they are aptly called bedding faults.
2. Dip Faults:
These are the faults which develop parallel to the dip of the strata. In other words, the fault strike is parallel to the dip of the layers broken and disrupted by the fault.
3. Oblique Faults:
These are sometimes called diagonal faults. In such a fault, the fault strike makes an oblique angle with the strike of the rocks in which it has caused the displacement.
3. Slip as Basis:
The direction of slip forms the most important basis for classifying the faults into three main types:
I. The strike-slip faults;
II. The dip-slip faults and
III. The oblique-slip faults
I. Strike-Slip Faults:
These are those faults in which the net slip is essentially parallel to the strike of the faults, the slip along the dip being almost absent. These are the most important and widely developed faults in the crust of the earth, which have been observed both on the continental and oceanic environments. Eventually a number of other names depending upon the environment under which these faults have been developed have been specially used for strike-slip faults.
The following are some of the examples:
(i) Wrench Fault:
It is a strike slip fault in origin in which the fault plane has developed transverse to the regional structure and even the net slip has also taken place in the same manner. The dip of the fault is very steep, nearly vertical. These are also sometimes referred as transverse faults.
(ii) Transform Faults:
These are strike-slip faults occurring in oceanic ridges on an extensive scale. Their importance lies in the fact that it was from the study of development and distribution of transform faults that the theoretical concept of Plate Tectonics got sound physical evidence. The occurrence of transform faults establishes the boundaries of oceanic plates in a satisfactory manner.
(iii) Tear Faults:
These are also strike-slip faults, occurring in groups in continental regions that divide an originally extensive block into blocks of smaller and convenient dimension that are translated during the process of regional faulting. It is believed that these are similar in nature to the transform faults developing on the continents rather than in the oceans.
II. Dip-Slip Faults:
All those faults in which the net slip has taken place parallel to the dip of the fault are classified as Dip-slip faults. They are often also called normal-dip-faults.
III. Oblique-Slip Faults:
These may be defined as faults in which the direction of net slip is neither parallel to the dip of the fault nor to the strike of the fault but is inclined to both these directions.
4. Mode of Occurrence as Basis:
In their actual existence faults may occur in groups and show a variety of relationship with each other, offering another basis for their classification.
Some common types recognized on this basis are:
1. Parallel Faults:
A group of faults occurring in close proximity, having their fault planes striking essentially in the same direction and having parallel and equal dips form what are commonly called parallel faults. In some cases, the intervening blocks are down thrown in the same general direction so that viewed from one side; the group gives a step-like appearance in the structure. These are then called STEP FAULTS.
2. Enechelon Faults:
These may be defined as a group of small sized faults that overlap each other in the region of their occurrence. A second fault appears on the surface at a distance before the first fault ends and so on.
3. Peripheral Faults:
When in any given region the majority of faults are concentrated along the border or margin of the area, the faulting is termed peripheral. In such a case the individual faults are generally arcuate in character.
4. Radial Faults:
A group of faults that appear emerging outward from a common central region are classed as Radial Faults. The area is divided into blocks with inwardly tapering ends.