Classification of Folds Based on Dip Isogons
Folds are fundamental structures in structural geology, resulting from the ductile deformation of rock layers. Their classification based on dip isogons provides a detailed and quantitative method to understand their geometry and kinematic evolution. Dip isogons are lines connecting points of equal dip on the upper and lower boundaries of a folded layer. Their behavior throughout a fold allows for a rigorous classification.

Dip Isogons
A dip isogon is a line that joins points of equal dip (slope) on the inner and outer boundaries of a folded layer. By observing the convergence, divergence, or parallelism of these isogons, geologists can classify folds into distinct categories, which in turn reflect the deformation mechanisms at play.

Classification of Folds
Based on the behavior of dip isogons, folds are classified into three main classes, with further subdivisions:

• Class 1 Folds (Convergent Isogons)
  In Class 1 folds, the dip isogons converge towards the inner arc of the fold. This indicates that the layer thickness, measured perpendicular to the bedding (true thickness), decreases from the hinge to the limbs. This class is further subdivided based on the degree of convergence.
  Mechanism: Typically formed by ductile flow or passive folding where material moves from the limbs to the hinge zone, or by flexural slip with significant layer-parallel shortening in the hinge.

• Class 2 Folds (Parallel Isogons - Similar Folds)
  In Class 2 folds, the dip isogons are parallel to the axial plane of the fold. This implies that the layer thickness, measured parallel to the axial plane (axial plane thickness), remains constant around the fold. True thickness, however, varies significantly, being thickest at the hinge and thinnest at the limbs.
  Mechanism: Formed by passive flow or shear folding, where deformation occurs through homogeneous simple shear or flow of material within the layers. This is common in highly ductile rocks or at high temperatures and pressures.

• Class 3 Folds (Divergent Isogons)
  In Class 3 folds, the dip isogons diverge towards the inner arc (or converge towards the outer arc). This signifies that the true thickness of the layer increases from the hinge to the limbs. The curvature of the outer arc is greater than that of the inner arc.
  Mechanism: Can be associated with complex rheological behaviors or specific strain distributions where limb thickening occurs.