Wave-like bands are formed in the crustal rocks due to tangential compressive force resulting from horizontal movement caused by the endogenetic force originating deep within the earth. Such bands are called ‘Folds’ wherein some parts are bent up and some parts are bent down.
Different Components of a Fold:
- The up folded rock strata in the arch-like form are called ‘anticlines’.
- While the down folded structure forming through-like feature is called ‘syncline’.
- The inclination of rock beds to the horizontal plane is termed as ‘dip’.
- The strike of an inclined bed is the direction of any horizontal line along a bedding plane. The direction of dip is always at the right angle to the strike.
Types of Folds:
The nature of folds depends on several factors e.g. the nature of rocks, the nature and intensity of compressive forces, duration of the operation of compressive forces, etc.
The elasticity of rocks largely affects the nature and the magnitude of the folding process. The softer and more elastic rocks are subjected to intense folding while rigid and less elastic rocks are only moderately folded.
The difference in the intensity and magnitude of compressive forces also causes variations in the characteristics of folds. Based on the inclination of the limbs, folds are divided into five types:
(1) Symmetrical Folds are simple folds, the limbs (both) of which incline uniformly. These folds are an example of an open fold and formed when compressive forces work regularly but with moderate intensity. Symmetrical folds are very rarely found in the field.
(2) Asymmetrical Folds are characterized by unequal and irregular limbs. Both the limbs incline at different angles. One limb is relatively larger and the inclination is moderate and regular while the other limb is relativity shorter with steep inclination. Thus, both the limbs are asymmetrical in terms of inclination and length.
(3) Monoclinal Folds are those in which one limb inclines moderately with regular slope while the other limb inclines steeply at the right angle and the slope is almost vertical. It may be pointed out that vertical force and movement are held responsible for the formation of monoclinal folds.
Monoclinal folds are also formed due to unequal horizontal compressive forces coming from both sides. There is every possibility for the splitting of the limbs of such folds because of intense folding. Splitting of limbs gives birth to the formation of faults.
(4) Isoclinal Folds are formed when the compressive forces are so strong that both the limbs of the fold become parallel but not horizontal.
(5) Recumbent Folds are formed when the compressive forces are so strong that both the limbs of the fold become parallel as well as horizontal.
Some other Types of Folds:
Overturned Folds are those folds in which one of the fold is thrust upon another fold due to intense compressive forces. Limbs are seldom horizontal.
Fan Fold represents an extensive and broad fold consisting of several minor anticlines and synclines. Such fold resembles a fan. Such type of folded structure is also called as ‘anticlinorium’ or ‘synclinorium’.
Anticlinorium refers to those folded structures in the regions of folded mountains where there are a series of minor anticlines and synclines within one extensive anticline. Anticlinorium is formed when the horizontal compressive tangential forces do not work regularly. Consequently, due to the difference in the intensity of compressive forces such structures are formed.
Synclinorium represents such a folded structure which includes an extensive syncline having numerous minor anticlines and synclines. Such structure is also formed due to the difference in the intensity of compressive forces.
Open Fold are those in which the angle between the two limbs of the fold is more than 90° but less than 180°. Such open folds are formed due to wave-like folding because of the moderate nature of the compressive force.
Closed Folds are those folds in which the angle between the two limbs of a fold is acute. Such folds are formed because of intense compressive force.
Landforms Associated with Folds:
Nappes are the result of complex folding mechanism caused by intense horizontal movement and resultant compressive force. Both the limbs of a recumbent fold are parallel and horizontal. Due to further increase in the continued compressive force, one limb of the recumbent folds slides forward and overrides the other fold. This process is called ‘thrust’.
When the compressive force becomes so acute that it crosses the limit of the elasticity of the rock beds, the limbs of the fold are so acutely folded that these break at the axis of the fold and the lower rock beds come upward. Thus, the resultant structure becomes reverse to the normal structure.
Due to continued horizontal movement and compressive force, the broken limb of the fold is thrown several kilometers away from its original place and overrides the rock beds of the distant place.
Such type of structure becomes unconformable to the original structure of the place where the broken limb of the fold of the other place overrides the rock beds. Such a broken limb of the fold is called ‘nappe’.
In most of the localities, the overriding nappes have been eroded because of dynamic wheels of denudational processes and thus buried basic structure has been exposed. When the portion of lower nappe is seen because of denudation of overriding nappe, the resultant open structure is called ‘structural window’.