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Faults; (fault terminology, classification of faults, effects of faults on outcrop)

What is a fault?

A fault may be defined 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. Faults result from tensional as well as compressional forces.

Types of faults:

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

  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.

Fault terminology:

  1. Fault Plane: The fracture surface along which relative movement has taken place, is called a ”fault plane”. The word fault includes both the fault plane and the displacement that has occurred along it. A fault plane may be markedly curved or in some cases undulating. Instead of one clearly defined fault plane, there may be a number of parallel shear fractures along which the fault movement is distributed. such a fracture zone is called a ”shear zone”.
  2. Hade: The hade of a fault plane makes with a horizontal surface.
  3. Dip: The dip is the angle plane makes with a horizontal surface.
  4. Strike: The strike of a fault is the direction of its continuity on the ground surface. it may be defined as the trend of the line formed by the intersection of a fault plane and a horizontal plane, such as the ground surface of a map surface.
  5. Throw: The vertical component of the displacement of fractured rock blocks is called the throw of fault. The side on which the strata appear to have been thrown down is called the “down throw side”, while the other side on which they appear to have gone up is called the ”upthrow side”.
  6. Heave: The horizontal component of the displacement of a fault is called the ”heave”. It is a horizontal shift of strata as seen in a section of a fault.
  7. Net Slip: The total displacement measured along the fault plane is called the “net slip”. On a fault, the distance between two formerly adjacent points on either side of the fault, measured on the fault surface or parallel to it. It defines both the direction and relative amount of displacement.
  8. Strike-Slip and Dip Slip: Dip slip is the movement parallel to the direction of dip of the fault plane. The movement, which is parallel to the strike of the fault plane is called “strike-slip”.
  9. Fault Scarp: A fault scarp is a cliff formed initially along the upthrow side of the fault. 
  10. Hanging Wall and Foot Wall: If the fault plane is not vertical, the block of rock laying above it is known as the ”hanging wall”, and the block below the fault plane is known as the ”footwall”. Vertical faults have neither hanging wall nor footwall.
  11. Fault Zone: Most fault planes are associated with a zone of crushed or altered rocks. This zone is called the ”fault zone”. When several parallel faults occur close together, the resulting zone of broken and crushed rock is called the ”shear zone”. A shear zone differs from a fault zone in that no distinct fault planes can be detected because here the local deformation is ductile. The fault zones and shear zones vary greatly in width and in some cases they are measured in hundreds of meters.

Effect of Faults on the outcrop:

Different types of faults have different effects on the outcrop. Effect on outcrop is given in the following lines as per the type and nature of a fault.

(1). Effect of Dip Fault: 

The effect of a dip-fault is to cause a lateral displacement in the outcrops of the disrupted strata. The amount of displacement becomes less with the increase in the dip of rock beds and in the vertical strata the displacement of outcrop will be nil.

(2). Effect of Strike Fault on outcrop:

The effect of the strike fault is either to cause the repetition of the outcrops of disrupted strata or to eliminate the outcrops of some of the beds altogether. Repetition of outcrop occurs when a strike fault hades in the direction opposite to that of the dip of strata. Omission or concealment of outcrop occurs when a strike sault hades in the same direction as the dip of strata.

(3). Effects of Faults on Foldes Strata: 

In a syncline, the outcrop of the limbs comes closer on the up-throw side, while the reverse applies in case of an anticline.

Significance of Folds and Faults:

Folds and faults are of major significance to industrial geologists because they often form structural traps for valuable mineral deposits. Faults that develop above an intrusive granite allow mineralizing fluids to pass into the overlying rocks. The deposits of minerals such as lead, tin, zinc, and copper ores are formed in this way. Faults that do not reach the surface many form channels through which oil and gas can rise. In synclines where porous sand beds overlie impermeable clays and shale, collection of water forum reservoirs which produce artesian springs.

Correct interpretation of folds and faults is essential in mining. For example, recumbent folding and reverse faulting can cause coal seams to be repeated vertically, while normal faulting can cause a horizontal gap. A coat seam may thus be passed through several times in drilling or alternatively missed altogether.

Along a fault zone, highly crushed and sheared rocks are met with. These zones being weak, unstable, and highly permeable, pose expensive problems in civil engineering construction, such as dams, reservoirs, tunnels, and highways.

Related articles:

  1. Dip and Strike (geological structure)
  2. Folds (elements of folds, types of folds,)
  3. Mechanics of Folds
  4. Recognition of a Fold
  5. Classification/types of the Faults

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