Tectonic processes originate within the earth itself and produce movements of the earth’s crust. There are basically two types of pressures exerted on the crust: compression and tension.
Compression results in the wrinkling of the crust, while tension causes the crust to crack or fracture. While compression has a tendency to shorten the earth’s surface, the tension tends to stretch or expand the surface.
Folding refers to the wrinkling of the crust which occurs because of slow lateral compression. When layered flat strata are subjected to compressional forces, they are bent and deformed.
Now, it is believed that convergent plate margins worldwide intensely compress rocks. The term ‘fold’ is used in case of smaller structures which result from the bending of layers of rocks.
They may be simple or very complex in form, due to the varying intensity of the tectonic forces that produced them. There is great variation in the length and width of folds produced. Various shapes and forms of folding are the following:
Monocline is the simplest type of fold. It involves only one inclination of the rocks. It refers to the terrace-like structure of the rocks. It produces the terrace-like surface topography. In a monocline the fold has only one limb and layers are bent in one direction only.
When the pressure exerted on a rock is not great, a series of simple folds are produced. In such folds the arches or up-folds are called anticlines.
In a simple fold, the troughs or down-folds are called synclines. The sides of the folds between the anticlinal crests and synclinal troughs are called the fold limbs.
It is to be noted that in a normal anticline the beds on either side dip in opposite directions away from the crest or axial plane of the fold. The tops of anticlines are generally fractured and are easily eroded.
Contrary to an anticline, the sides of a syncline dip towards each other. After being eroded for long period of time, synclinal structures often underlie hills and ridges. Synclines in such cases are more resistant to erosion than are the anticlines.
These structures are found in folded mountains. The form of an anticlinorium resembles that of a large anticline with minor synclinal and anticlinal structures incorporated in it. In anticlinorium the forces are exerted unevenly and the rocks yielded unequally.
It refers to a complex syncline of great lateral extent in which many minor folds occur.
In this fold one limb is partly doubled under the other and the axial plane is inclined.
If one side of the fold is steeper than the other, it is said to be asymmetrical.
If an asymmetrical anticline is pushed right over, a recumbent is formed in which the axial plane lies in a horizontal position. This is actually an overturned fold.
It is a simple fold in which both the limbs incline uniformly. This type of fold is formed when compressive forces are of moderate intensity.
Thrust and thrust planes:
When lateral compression is very intense, the rocks break and their movement takes place on the gently inclined planes of rupture. Such dislocations associated with folding are called thrusts and the actual planes of movement of the broken pieces of rocks are called thrust-planes.
If the pressure exerted on a recumbent fold is very great, the rocks may then fracture, and a great mass will be thrust forward, often to great distances (sometimes many kilometers) along the plane of fracture called the thrust-plane.
These over-thrust masses are forced to move away from their roots. These masses are called nappes (after the French word for table-cloth).
It was in the Alps that nappes were seen first. Such nappes are found in the Kashmir Himalaya, Shimla Himalaya, Garhwal Himalaya and also in Kumaun Himalaya.
Axis and axial Plane:
The central line of a structural fold (either the crest or the trough) from which the strata dip (downwards in an anticline, upwards in a syncline) in opposing directions is called the axis of the fold.
The imaginary plane that bisects a fold is a called the axial plane, and the axis passes through it. The inclination of the axis from the horizontal plane is called the plunge.
Rigid rocks fracture under stress, and are displaced by faults. In other words, when rock strata are strained beyond their ability to retain their solid state as a unit, they fracture.
So when a fracture occurs and the rocks are displaced on either side of it relative to one another, the result is known as a fault.
This process is known as faulting. Faults often occur in groups along a fault zone. The San Andreas Fault zone is an example. The fault zones are characterized by crustal movement.
Faulting is closely associated with displacement along the fault plane. Some faults displace rocks up and down. This is called vertical displacement. Some faults cause lateral displacement or sideways movements.
The actual vertical change of level of the strata is known as the throw. The vertical height ranges from a few centimeters to several meters. Where the fault is inclined, there is some horizontal displacement. This is known as heave.
When there are sudden slippage movements, it generates earthquakes. A single fault movement may result in slippage ranging from a centimeter to about 15 meters.
Such slippage may occur in quick succession or may occur after gaps of several decades or centuries. The cumulative displacement may involve as much as hundreds of kilometers.
The following technical terms are associated with the process of faulting:
(i) Fault Plane:
The fault plane refers to the plane along which rocks are displaced by tensional or compressional forces. The displacement of rocks may be vertical or horizontal. A fault plane may be vertical, horizontal or inclined.
(ii) Fault dip:
Fault dip means the angle which the fault plane makes with the vertical plane or horizontal plane.
(iii) Fault Strike:
The term strike is applied to a fault plane, as it is in case of a bedding plane.
(iv) The upthrow side:
This term refers to the uppermost block of a fault.
(v) The downthrow side:
This term is applied to the lowermost block in a fault.
(vi) Fault scarp:
It is a small cliff formed by the displacement of a recent fault. Remember that it is just a temporary landform which is soon modified by denudation.
(vii) Fault-line scarp:
It is a scarp which is produced by differential erosion going on, on either side of a fault-line when rocks of different hardness are found side by side.
(a) Resequent fault-line scarp:
If at the level of the present erosion surface the softer rock is on the down throw side, this side will form lower ground and the fault line scarp will be like the original fault-scarp in form. It is called a resequent fault-line scarp.
(b) Obsequent fault-line scarp:
This type of scarp forms when the harder rock is on the down-throw side. In this case the surface level will be inclined in the reverse direction to the throw of the fault.
If the direction of movement is parallel with the dip of the fault (upwards or downwards), it is termed a dip-fault.
(ix) The hanging wall and fool-wall:
In a dipping fault, the rock surface above the fault plane is called the hanging wall and that below is termed foot wall.
Types of Faults:
Out of several types of fault, only the more important types have been briefly described as under:
(i) The tension fault or normal fault:
A normal fault occurs where tension causes fracture in rocks that are being pulled apart. The rock above the fault plane moves down relative to the rock beneath the fault plane.
(ii) The reverse fault:
Reverse fault occurs where compressional force causes the upper block of rock to be pushed over the lower. Over-thrusting, where a reverse fault of a very low angle is involved, is a common phenomenon when compressional forces have been very powerful.
The term ‘thrust fault’ is also applied to a reverse fault. As a result of compressional forces there is shortening of the crust.
(iii) The strike-dip fault:
Strike-dip fault is also known as tear-fault. Though this fault represents a vertical fracture, there is no vertical movement of one side relative to the other. In this fault the displacement of one block of rock against the other is horizontal along the fault line.
The classical example of such a fault is the Great Glen of Scotland in Britain. In this fault the horizontal displacement to the strata is about 104 km. It is interesting to note that faults are generally found in groups.
A single fault is a rare physical phenomenon. However, one of the longest single faults is the San Andreas Fault which can be traced out on the surface for about 480 km. It runs parallel to the coast.
Cross-faulting refers to a phenomenon in which a number of faults intersect each other almost at right angles. The dissected plateaus in East and South Africa show the effects of cross-faulting on a large scale.