Fault

Faults are the fractured surface in the Earth’s crust. During the formation of a fault, the vertical displacement of a rock block may occur up to several 100 meters and horizontally the rock blocks may be displaced up to several kilometers.

A fault represents weaker zones of the earth where crustal movements become operative for a longer duration.

Different Components of a Fault:

Fault
  • Fault Plane is that plane along which the rock blocks are displaced by tensional and compressional forces acting vertically and horizontally to form a fault. It plane may be vertical, inclined, horizontal, curved or of any other form.
  • The Dip is the slope of a fault.
  • Upthrown Block represents the uppermost block of a fault.
  • Downthrown Block is the lowermost block of a fault.
  • Hanging wall represents the upper wall of a fault.
  • The Footwall is the lower wall of a fault.
  • Fault Scrap is the cliffs that represent the edge of a vertically displaced block.

Types of Faults:

Faults can be generalized into four principal types based on the direction and angle of movement.

(1) Normal Faults are formed due to the displacement of both the rock blocks in opposite directions. A normal fault results from tension stresses (pulling apart or extension) in the crust.

(2) Reverse or Thrust Faults are formed due to the movement of both the fractured rock blocks towards each other. A reverse fault is produced from compression stresses, with the upthrown block rising steeply above the downthrown block so that the fault scrap would be severely over steepened if erosion did not act to smooth the slope somewhat. Landslides often accompany reverse faulting.

When the compressive force exceeds the strength of the rocks, one block of the fault overrides the other block and the resultant fault is called an ‘Overthrust Fault’, wherein the fault plane becomes almost horizontal.

Overthrusting occurs frequently in mountain building, resulting in unusual geology relationship such as older strata being piled up on top of younger rocks. Both reverse and overthrust faults are commonly associated with subduction zones and continental collision zones.

(3) Lateral or Strike-Slip Faults are formed when the rock blocks are displaced horizontally along the fault plane due to horizontal movement. Strike-slip faults are a consequence of shear stresses. In the majority of the cases, there are no scarps in such faults, if they occur at all, they are very low in height.

Transform Fault special case of a strike-slip fault making up the boundary of two moving lithospheric plates. Most transform faults are found along the mid-oceanic ridge where seafloor spreading is in progress. Transform faults are associated with a great deal of seismic activity, commonly producing shallow-focus earthquakes.

(4) Step Faults are formed when a series of faults occur in any area in such a way that the slopes of all the fault planes of all the faults are in the same direction. It is a prerequisite condition for the formation of step faults that the downward displacement of all the downthrown blocks must occur in the same direction.

Landforms Associated with Normal Faults:

(1) Block Mountains- The internal forces that work within the Earth’s surface cause compression as well as tension. When the forces of tension act on the rocks, they create faults. Sometimes due to vertical movements of the earth, the land between two parallel faults moves upward. This result in the formation of block mountain. The classic example of a block mountain range is the Black Forest in Europe and California’s Sierra Nevada.

(2) Horst- A block of high ground which stands out because it is flanked by normal faults on each side. Horst is the result of the land on both sides being downward rather than the block itself being uplifted. The horst may take the form of a plateau or a mountain mass with two steep, straight sides.

(3) Graben- A block of land bounded by parallel faults in which the block relatively downthrown producing a distinctive structural valley with straight, steep-sided fault scraps on both sides.

(4) Rift Valleys- Where are divergent plate boundary develops within the continent, the resulting down-faulted grabens occasionally extend for extraordinary distances as linear valleys enclosed between steep fault scraps. Such lengthy troughs are called rift valleys and they comprise some of the earth’s most notable structural alignments, particularly the great rift valley in East Africa.

Landforms Associated with Strike-Slip Faults:

A wide variety of landforms can result from strike-slip faulting.

(1) The surface trace of a large strike-slip fault may be marked by a ‘Linear Fault Trough or Valley’ formed by repeated movement and fracturing of rock within the fault zone.

(2) Small depression known as ‘Sags’ developed through the setting of Rock within the fault zone and may become filled with water to form ‘Sag Ponds’.

(3) There may be the development of ‘Linear Ridges and Scarps’ trending parallel to the trace of the fault and occur along the fault.

(4) The most conspicuous landform produced by Strike-Slip Fault is an ‘offset Stream’. Streams flowing across the fault are displaced by periodic fault movement or diverted when a ‘Shutter Ridge’ is faulted in front of a drainage channel.