Competitive Exams: Folds
Folds are wave-like bends formed due to tangential compressive forces resulting from horizontal movement caused by the endogenetic force originating deep within the earth. The two sides of a fold are called limbs of the fold. The limb which is shared companion syncline is called middle limb. The plane which Dissects the angle between the two limbs or middle limb of the syncline is called the axis of fold or axial plane. On the basis of anticline and syncline, these axial planes are called as axis of anticline and axis of syncline respectively. The inclination of rock beds with respect to horizontal plane is termed as ‘dip’ the angle of dip is measured with an instrument called clinometers. The strike of an inclined bed is the direction of any horizontal line along a bedding plane. The direction of dip is always at right angle to the strike.
Components of Folds
Anticlines: The unfolded rock beds are called anticlines.
Synclines: Downfold rock beds due to compressive forces caused by horizontal tangential forces are called synclines.
Anticlinorium: It 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.
Synclinorium: It represents such a folded structure which includes an extensive syncline having numerous minor anticlines and synclines.
Types of Folds
- Monoclinal folds: These are those in which one limb inclines moderately with regular slope while the other limb inclines steeply at right angle and the slope is almost vertical.
- Isoclinal folds: When the compressive forces are so strong that both the limbs of the fold become parallel but not horizontal.
- Recumbent folds: These are formed when the compressive forces are so strong that both the limbs of the fold become parallel as well as horizontal.
- Overturned folds: These are those folds in which one limb of the fold is thrust upon another fold due to intense compressive forces. Limbs are seldom horizontal.
- Plunge folds: These are found when the axis of the fold, instead of being parallel to the horizontal plane, becomes tilted and forms plunge angle which is the angle between the axis and the horizontal plane.
- Fan folds: These with anticlinorium or obtuse angle.
- Open folds: These are those in which the angle between the two limbs of the fold is more than 90 degree but less than 180 degree
- Closed folds: These are those folds in which the angle between the two limbs of a fold is acute angle. Such folds are formed because of intense compressive force.