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Types and classification of faults

Definition of a Fault:

We can define a fault as a fracture along which blocks of rock have been displaced relative to each other. This planar discontinuity originates by tectonic forces acting regionally. The displacement along a fault may be less than a meter, several hundred meters, or maybe many kilometers. Both tensional and compressional forces may result a fault.

Types of faults:

Faults can be classified on six bases; (detail of these six bases)

  1. Classification based on the apparent movement of the fault.
  2. Classification based on the relationship of a strike and the attitude of strata.
  3. Classification based on the degree of dip.
  4. Classification based on the relationship between the direction of slip and the attitude of the fault plane.
  5. Classification based on the forces responsible for the formation of faults.
  6. Classification of faults on the basis of their pattern.

(1). Classification made on the basis of apparent movement the faults

On the basis of apparent movement, the faults are classified into two groups.

  1. Normal Fault: A normal fault is one in which the hanging wall appears to have moved downward relative to the footwall. In this case, the fault plane dips toward the downthrow side. Generally, normal faults are produced by tensional forces and they are also called “gravity faults”. These faults indicate the lengthening of the earth’s crust. The subdivision of the crust into blocks by normal faults is called “block faulting”. Normal faults usually have a high angle dip.
  2. Reverse Faults: A “reverse fault” is one in which the hanging wall appears to have moved upward relative to the footwall. In the angle faults. In this case, the fault plane dips toward the up-throw side. The reverse faults are usually high-angle faults. They are produced by compressional forces. These faults indicate the shortening of the earth’s crust.

Normally reverse faults have dips of the order of 45o or more. Intense compression produces low-angle reverse faults called the “thrusts”. Thrust fault may curve at the bottom to merge with a bedding plane. A horizontal or low angle thrust fault in which the displacement is large is called the “overthrust”. The displacement of “overthrust” may be measured in kilometers. The sheet of rock that has moved forward along the thrust plane is called “nappe”. Isolated parts of overthrust rock mass resting on the underlying strata are known as the “nappe outlier” or Klippe”.

 

(2). Classification made on the basis of the relationship between the strike of a fault and the attitude of strata

This classification is based on the relationship between the strike of a fault and the attitude of strata. There are five types of fault.

  1. Dip Fault: A fault, which strikes approximately parallel to the dip direction of beds, is called the “dip fault”.
  2. Strike Fault: A fault, which runs parallel to the strike of strata, is called the strike fault. A strike fault is termed as a “longitudinal fault” when it runs along the strike of the regional structure such as major fold axes, etc.
  3. Bedding Fault: A bedding fault is one, which occurs along a contact between beds of different or same lithology. The bedding faults are difficult to recognize.
  4. Oblique Faults: A fault, which runs obliquely to the strike and dip directions of strata is called the “oblique fault”. When a fault strikes diagonal to the dip of the regional structure, it is known s the “transverse fault”.
  5. Tear Fault or Wrench Fault: In strike-slip faulting the relative displacement of the block is horizontal. The fault plane of a strike-slip fault is more or less vertical and often extends for long distances. Because these faults frequently strike across the trend of folds, they are called “transcurrent”, “tear” or “wrench” faults.

(3). Classification based on the degree of dip

This classification has been made on the basis of the degree of dip. On the basis of degree of dip there are two types of faults.

  1. High angle Faults: The “high angle faults” are those which have a di greater than 45o . Normal faults are commonly are the “high angle faults”.
  2. Low Angle Faults: The “low angle faults” are those which have a dip less than 45o . The thrust faults are commonly “low angle faults”.

(4). Classification made on the basis of the relationship between the direction of slip and the attitude of the fault plane

This classification has been made on the basis of the relationship between the direction of slip and the attitude of the fault plane. On the basis of this phenomenon, faults are classified into three types.

  1. Strike-Slip Faults: In “strike Slip faults” the movement is essentially horizontal along the strike of the fault. These faults usually have very steep to vertical dips.
  2. Oblique Slip Faults: A fault in which the direction of movement is diagonal to both the dip and strike of fault, is called the “oblique-slip fault”.
  3. Dip Slip Fault: A fault in which movement is essentially downward along the dip of fault is called the “dip-slip fault”.

(5). Classification on the basis of force responsible for the formation of faults

This classification has been made on the basis of the force responsible for the formation of faults. According to this phenomenon, there are four groups of faults.

  1. Gravity or Tension Faults: These faults are formed by tensional forces, which pull the earth’s crust apart. Normal faults are the example of this group.
  2. Compressional Faults: These faults are caused by the compressional forces, which produce folds as well as faults. Reverse faults and thrust faults belong to this group, and they are commonly associated with folding.
  3. Transcurrent Faults: These faults are caused by the lateral thrust. Tear or Wrench faults are included in this type of fault. Transcurrent faults exhibit mainly horizontal movement.
  4. Pivotal Faults: These faults are caused by rotational stress and movement. These faults are the result of rotational ground movement.

(6). Classification based on the pattern of the faults

Faults usually occur in groups. When a number of faults occur together, they form a fault system. This classification of the faults is based on the pattern of the faults. On a pattern basis, there are seven types of faults.

  1. Parallel Faults: A series of faults that have the same strike and dip are called the “parallel faults”. Such faults run parallel to one another and all hade in the same direction with the same angle.
  2. Step Faults: As the term implies the “step fault” are those, in which the down-throw is always in the same direction. In these faults, the down-thrown blocks produce a step-like outcrop.
  3. Graben or Rift Faults: Grabens are long and relatively narrow fault troughs bounded by parallel high angel faults. This structure is produced when the two parallel normal faults hade towards each other and the rock beds between them are thrown down under the influence of gravity forming a topographic low. Tensional crustal forces, which pull the crust apart are responsible for the formation of rift faults. Graben typically along crests of upwarps.
  4. Horst: When two parallel normal faults hade away from one another and the rock block between them is uplifted to form a ridge, the structure is called a “horst”.
  5. Enechlon Faults: These are relatively short faults, which overlap each other.
  6. Peripheral Faults: The curved faults which have nearly circular or arc-like outcrops on level surfaces, are called “peripheral faults”.
  7. Radial Faults: A number of faults exhibiting a radial pattern on the ground surface are known as the “radial faults”.

 

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