Fault in Geography

12 months ago

Atiqur Rahman

9 minutes

Faults reflect ruptures or crakes in the earth, along which one side is relatively displaced with reference to the other side. Sometimes they are also known as disjunctive dislocation. In size and total displacement, they may range from a few inches to hundreds of kilometres long.

The above picture shows that since they are formed due to great stresses, they reflect brittle deformation. The brittle deformation results in fracturing and faulting. You can also notice brittle deformation if you drop a glass (but by mistake!) on a hard floor. You will notice that it breaks into several pieces.

Similarity crakes in buildings formed due to earthquakes, gravitational effect, and subsidence also denote brittle deformation. Geomorphologists and geologists recognize the fault by identifying the omission of strata, dislocation of structure or discontinuity, geometry of fault plane, and sudden changes in topography.

Basic Fault Geometry

Wall: Rock adjacent to a fault surface is the wall of the fault.

Fault block: the body of rock that moved as a consequence of a slip on the fault is a fault block.

Hanging-Wall Block: It is a fault block or rock body above the fault plane.

Foot-Wall Block: The fault block below the fault plane is recognized as a foot-wall block.

Strike: It denotes the trend of a horizontal line in the fault plane.

Dip: generally, we recognize a fault as steep or vertical to tell about the image of a fault, which in turn reflects the dip of the fault. The dip represents the angle between a horizontal surface and the fault plane. It makes a right angle to the strike of the fault.

Hade: is the complement angle of the dip, i.e., Hade = 90° – Dip

Throw: Normally, under the gravitational pull during the faulting vertical displacement of rock occurs. The figure (below) shows that this vertical displacement is recognized as Throw. In a nutshell, it denotes the vertical component of dip separation.

Heave: The above figure shows that the apparent horizontal displacement of the fault block is known as heave. It denotes the horizontal component of dip separation.

Types of Faults

Like folds, faults can also be classified in several ways.

(A) Faults Based on Apparent Movement

There are three types, i.e., Normal Fault, Thrust or Reverse Fault and Strike-Slip Fault.

1. Normal Fault:

The normal fault is also known as gravity, tensional, or extensional. They occur when the vertical stress is greater than the horizontal stress. The picture shows that in normal fault, primarily displacement or movement is vertical. In this case, the downthrown block is the hanging-wall block which is influenced by gravity, so the normal fault is also recognized as gravity or tensional fault.

On a global level normal fault system leads to the lengthening of the earth’s crust. This process occurs along the rift valleys and on either side of mid-ocean ridges. Normal faults are also visible along the edge of mountain ranges in California. The cliff formed by faulting is commonly called a fault scarp or escarpment.

The process of Normal Faulting can give rise to the following geomorphic features:

(i) Horst: An upstanding fault block bounded by two normal faults is regarded as a horst. The figure (below) shows that, in appearance, it looks like a block plateau or mountain, generally flat on top but with steep sides. Horst may be formed by upliftment of block between two normal faults or may be left upstanding due to subsidence of either side of the middle block. The Shillong Plateau is an example of horst.

(ii) Graben: Graben is from the German word for “trough”. The figure shows that graben is a valley-like depression or trough representing a sloped fault block bounded by normal faults. It is like a trench with straight parallel walls created by tensional force. On the contrary, the horst represents a narrow elevated block between two normal faults.

(iii) Half Graben: The figure (below) shows that in the Half Graben, the upper part of the hanging wall block is tilted towards the fault due to the rotation of the displaced block along the normal fault line. This type of displaced rotational movement creates half-graben depression. Half graben are common in the Basin and Range Province of the Western United States.

(iv) Fault Scarp: From the figure, it is apparent that the steep straight cliff-like topography created along the normal fault plane is called fault scarps. In length, they may go up to 300 kilometres. The fault scarp’s height may range from a few meters to a few hundred meters.

(v) Block Mountain: The long ridge-like horst is called block mountain. Vosges Black Forest in Germany is also a well-known example of Block Mountain.

(vi) Rift Valley: A long narrow fault trough created along the divergent plate boundary (due to tectonic activity) is called a rift valley. The East African Rift Valley is the well know example of rifting. The map shows the areal extent of the East African Rift Valley along the divergent plate boundary. This belt extends from Ethiopia to Tanzania. This rift valley is also the site of volcanic activity. The volcano Kilimanjaro in Tanzania is a well-known example of this belt. As rift valleys open, water flows into the new lowlands. The Red Sea and the Gulf of California are examples of this process; they are actually confined in larger rift valleys.

Similarly, rifting due to tensional stress can also occur at the local level. For example, Narmada and Tapti rivers in India flow throw rift valleys.

(vii) Step Fault: Sometimes, parallel series of faults are formed due to normal faulting. They appear like giant steps of stairs. They are known as step faults. They are generally common in rift valley regions.

2. Thrust or Reverse Fault:

Unlike normal faults, thrust faults are created by compressional stress. From the figure (Below), it is apparent that they are formed when the hanging wall block has overridden the footwall block generally at a very shallow angle. That is why they are also known as “contractional faults”.

It should be noted that in the thrust fault, maximum stress is horizontal and minimum stress is vertical, which leads to the shortening of the earth’s crust. There is a minor difference between thrust and reverse faults. In the former, the dip angle is less than 45°, and in the latter, it is more than 45°.

The subduction zones along the convergent plate boundaries are characterized by thrust faults. In such tectonic settings, thrusting occurs in combination with the formation of folds. In the Alps and the Himalayas, several such overthrusts result from compressional stresses of the ongoing collision between different plates.

(i) Ramp Valley: A valley-like depression formed by compressional stress may create a ramp valley. The figure shows that in this category of the thrust fault, the two hanging blocks move side upward, and the one-foot wall block appears to remain stationary. The Brahmaputra valley between the Himalayas and Shillong Plateau is an example of a ramp valley.

3. Strike-Slip Fault:

As the name suggests, in this fault category, the relative displacement of fault blocks remains mainly parallel to the strike of the fault. They are formed when rocks are torn by lateral-shearing stress. On the basic standing position of the observer they are divided into two categories which are as follows:

(i) Right Lateral Strike-Slip Fault: From the above diagram, it is clear that in this type of fault, the displacement appears on the right side of the observer.

(ii) Left lateral Strike-Slip Fault: If a person stands at the fault and looks across to see that a block or a portion of land has been displaced to the left-hand direction, it is designated as a left-lateral strike-slip fault.

(iii) Transform Fault: Transform faults are associated with plate tectonics. The Transform Fault represents the plate boundary at which the lithosphere is neither created nor destroyed. Therefore, they are devoid of stunning landform features compared to convergent and divergent boundaries.

San Andreas Fault in California is the best-known example of this category. One can also identify the trace of the transform fault through offset roads and fences. It is also noteworthy that any relative sudden displacement of rocks along the transform fault may cause an immense loss of life property.

Fault in Geography

Basic Fault Geometry