When one or a stack of originally flat, level surfaces such as sedimentary strata, are bent or curved as a result of pressure and high temperature, folds are formed. Folds may be defined as undulations or bends or curvatures developed in the rock of the crust as a result of stresses to which these rocks have been subjected. The folds may develop in any type of rock and may be of any shape and geometry or any types of folds. The process of development of folds in the rock is called Folding. It is a very slow Geological process and indicates an effort of the rock in a particular environment to adjust themselves to the changing force fields operating on, within or around them. In general, Folding in a ductile type of deformation experienced by the rock compared to the brittle deformation where the rock actually gets broken and displaced when stressed. The basic cause is likely to be some aspect of plate tectonics. Folds may be observed directly or may be inferred from various kinds of data.
Parts of a fold:
Axis- The line of intersection of the axial plane with the upper and lower surface of any of the constituent beds.
Limb– The Stretch of the rock beds lying between any crest and any of the adjacent troughs on either side is known as limb of the fold.
Axial Plane– It is an imaginary line which divides the fold as symmetrical as possible is described as its axial plane. It is the surface connecting all the hinges. It may be a simple plane or a curved surface.
Crest And Trough- the line joining through the highest points in the uparched side of the fold is called Crest. Simerly line joining through the lowest points in the downarched side of fold is called Trough.
Hinge- The line along which a change in the amount or direction of dip takes place is known as Hinge Line. It can also be expressed as the line of maximum curvature.
CLASSIFICATION OF FOLD
Folds have been classified into various types, on the following basis:
- Appearance in cross –Section
- Symmetry of fold
- Thickness of limb
- Inter limb- angle
- Attitude of the fold
I- Appearance in Cross – Section:
- Antiform- Any upwardly convex structure is termed as an Antiform.
- Synform- Any upwardly concave structure is termed as an Synform.
- Anticline- It is generally convex upwards where the limbs commonly slope away from the axial plane.
- Syncline– It is a fold with concave upwards and usually dip towards the axial plane.
- Anticlinorium- It is a large anticline with secondary folds of smaller size developed on it.
- Synclinorium- It is a large syncline with secondary folds of smaller size developed on it.
- Anticlinal Bend- It is due to local steepening of a bed, whereby there occurs a sudden increase in the dip of a bed which is originally horizontal to a near vertical position but the original bedding remains as before.
- Synclinal bend- In case of a dipping bed, due to local flattening of the beds at a particular spot, the beds acquires horizontality and then again follows their original dip without any change in the direction of dip.
II- Symmetry of fold:
Six types of folds have been recognised on the basis of symmetry of fold.
- Symmetrical Fold- when the axial plane is vertical and bisects the fold is said to be asymmetrical or upright fold. They may be Anticlinal and synclinal in nature.
- Asymmetrical Fold- If the axial plane has dip, the fold is described as ‘inclined or asymmetric’ fold. In this case both the limbs dip at different angles and the axial plane cannot divide the fold into two symmetrical halves.
- Over turned Fold– Here the axial plane is inclined and boththe limbs dip in the same direction usually at different angles. In this one limb occurs in the normal position while the other appears to have been rotated or may be completely overturned from its original position.
- Recumbent Fold- In this type of fold the axial plane is horizontal or more or less so. In it one limb lies exactly under the other limb. Other parts of this fold are named as follows:
- Root or the root zone
- Isoclinal Fold– In it the axial planes are essentially parallel i.e. the corresponding limbs are dipping at equal amounts. Both the limbs have the same amount of dip, towards the same direction.
III- Thickness of the limb:
- Parallel Fold– These are also known as the concentric folds as here the concentric semicircles have the constant centre and regularly increasing radius. In it the anticlines become sharper with depth but broader and more open upward. Similarly the synclines become broader and more open upwards.
- Similar Fold-In this the shape of the fold may vary along the axial plane and at right angles to the fold axis. Every bed is thinner in the limbs and thicker at the hinges.
(A) Similar Fold (B) Parallel Fold
- Suprataneous Fold– In this the strata is thinnest at the crest and thickest at the trough of the synclines. When the sediments got deposited over a ridge and may also develop through differential compaction of sediments around such ridges.
IV- Inter limb Angle:
|Type of Fold||Interlimb angle|
|Ø Open Fold or Gentle Fold||Greater than 70°|
|Ø Closed Fold||30° to 70°|
|Ø Tight Fold||Below 30°|
V- Attitude of Fold:
- Plunging Fold– A fold is said to Plunge when the axial plane of the fold is horizontal. The amount of plunge being the angle between the axis and a horizontal line lying in a common vertical plane.
- Non-Plunging Fold– When the axis of the fold does not dip in any direction.
- Doubly Plunging Fold– In this type of fold the direction of plunge reverses.