Fold Classification: Symmetry and Tightness

Classification of Folds Based on Symmetry:

• Symmetrical folds have limbs of equal length and dip at equal angles but in opposite directions.
• Asymmetrical folds possess limbs of unequal length and dip at different angles.

Classification of Folds Based on Tightness (Interlimb Angle):

• Interlimb angle is the internal angle between the two limbs of a fold.
• It can also be defined as the angle between lines tangent to the curve at the inflection point.
• Open folds are broad with limbs dipping gently away from the crest.
• Tight folds have limbs that dip steeply and tightly together.
• Isoclinal folds feature parallel limbs.

Fold Classification by Interlimb Angle:

• Gentle folds exhibit an interlimb angle greater than 120 degrees.
• Open folds have an interlimb angle ranging from 70 to 120 degrees.
• Closed folds show an interlimb angle between 30 and 70 degrees.
• Tight folds have an interlimb angle between 0 and 30 degrees.
• Isoclinal folds feature an interlimb angle of 0 degrees.

Classification Based on Orientation of Axial Plane:

• Upright folds have a vertical axial plane and symmetric limbs.
• Overturned folds have a moderately inclined axial plane, with one normal limb and one overturned limb.
• Recumbent folds have a near-horizontal axial plane and one inverted limb.

Classification Based on Bed Thickness:

• Concentric folds have layers of uniform thickness, with thickness remaining constant, also known as parallel folds.
• Isogons converge downward.
• Similar folds display significant thickening on the hinge and thinning on the limbs.
• Isogons are parallel to one another and to the fold's axial plane.
• Suprateneous folds exhibit thinning in the crestal part and thickening in the limb.