On the basis of dip isogons describe the classification of folds with neat diagrams.

Understanding Dip Isogons and Ramsay's Classification

In structural geology, the morphology of folds is systematically characterized using geometric tools. Among these, the concept of dip isogons, introduced by J.G. Ramsay in 1967, is particularly significant. It provides a robust framework for classifying folds based on the variation of layer thickness across the fold profile.

What is a Dip Isogon?

A dip isogon is an imaginary line connecting points of equal dip on the inner and outer bounding surfaces of a folded layer. In essence, it tracks how the inclination of the layer changes from one surface to the other within the fold. Its behavior (convergent, parallel, or divergent) with respect to the axial surface forms the basis of Ramsay's classification.

The classification also considers the ratio of the orthogonal thickness (t_α) measured perpendicular to the layering at a given dip angle (α) to the thickness at the hinge (t₀).

Ramsay's Classification of Folds Based on Dip Isogons

Ramsay's classification divides folds into three primary classes, with Class 1 further subdivided into three subclasses, based on the patterns formed by dip isogons and the relative curvature of the inner and outer arcs of the fold. This classification is geometric and does not directly imply a specific formation mechanism.

1. Class 1 Folds (Convergent Isogons)

In Class 1 folds, the dip isogons converge towards the inner arc (or core) of the fold when traced across the layer. This indicates that the curvature of the outer arc is generally less than that of the inner arc. The layer thickness varies significantly within these folds.

• Curvature Relationship: Curvature of the outer arc (C_outer) is less than the curvature of the inner arc (C_inner).
• Dip Isogon Behavior: Isogons converge towards the core of the fold.

Class 1 folds are further subdivided into:

• Class 1A (Strongly Convergent Isogons):
  • Dip isogons converge strongly towards the inner arc.
  • The orthogonal thickness (t_α) decreases significantly from the limbs towards the hinge, meaning the limbs are thicker than the hinge.
  • The smallest true thickness and vertical thickness are at the hinge.
  • This type is common in folds developed by buckle folding where deformation is concentrated in the limbs.

  Diagram: Imagine a folded layer where lines drawn perpendicular to the top and bottom surfaces, at points of equal dip, converge sharply towards the fold's center. The hinge area appears notably thinned compared to the limbs.

• Class 1B (Parallel/Concentric Folds - Moderately Convergent Isogons):
  • Dip isogons are moderately convergent, often appearing perpendicular to the layering.
  • The orthogonal thickness (t_α) remains constant throughout the fold, meaning the true thickness is preserved.
  • These are commonly known as parallel folds or concentric folds.
  • The axial surface bisects the interlimb angle.
  • These folds are prevalent in upper crustal settings where deformation involves flexural slip folding.

  Diagram: Depict a fold where lines connecting points of equal dip on the outer and inner arcs are roughly perpendicular to the layer boundaries, and the layer maintains a consistent thickness throughout the fold.

• Class 1C (Weakly Convergent Isogons):
  • Dip isogons converge weakly towards the inner arc.
  • The orthogonal thickness (t_α) is maximum at the hinge and decreases towards the limbs, implying the hinge is thicker than the limbs.
  • The vertical thickness is minimum at the hinge.

  Diagram: Show a fold where dip isogons converge slightly towards the core, with the hinge zone appearing slightly thickened relative to the limbs.

2. Class 2 Folds (Parallel Isogons - Similar Folds)

In Class 2 folds, the dip isogons are parallel to the axial plane trace. This signifies that the curvature of the outer arc is equal to the curvature of the inner arc. These folds are often referred to as similar folds.

• Curvature Relationship: Curvature of the outer arc (C_outer) is equal to the curvature of the inner arc (C_inner).
• Dip Isogon Behavior: Isogons are parallel to the axial surface.
• Thickness Variation: The layer thickness measured parallel to the axial plane (axial trace thickness) remains constant, but the orthogonal thickness (t_α) varies, being maximum at the hinge and minimum at the limbs (hinge thickening, limb thinning).
• These folds are common in metamorphic terrains where deformation occurs by extensive ductile flow (passive folding).

Diagram: Illustrate a fold where lines connecting points of equal dip on the top and bottom surfaces are parallel to each other and parallel to the axial plane. The limbs will appear thinned and the hinge thickened.

3. Class 3 Folds (Divergent Isogons)

In Class 3 folds, the dip isogons diverge towards the inner arc (or core) of the fold. This indicates that the curvature of the outer arc is greater than that of the inner arc.

• Curvature Relationship: Curvature of the outer arc (C_outer) is greater than the curvature of the inner arc (C_inner).
• Dip Isogon Behavior: Isogons diverge towards the core of the fold.
• Thickness Variation: The orthogonal thickness (t_α) is minimum at the hinge and increases significantly towards the limbs, meaning the hinge is thinner than the limbs.
• The largest true and vertical thickness are at the hinge.

Diagram: Show a fold where lines connecting points of equal dip on the outer and inner arcs diverge significantly towards the fold's core, with the hinge region appearing markedly thinned.

Summary Table of Ramsay's Fold Classification

Fold Class	Dip Isogon Behavior	Curvature (Couter vs. Cinner)	Orthogonal Thickness (tα) Variation	Common Name/Characteristics
Class 1A	Strongly convergent	Couter < Cinner	Decreases from limb to hinge (limbs thicker)	Hinge thinning
Class 1B	Moderately convergent (perpendicular to layering)	Couter < Cinner	Constant (tα = t0)	Parallel or Concentric folds
Class 1C	Weakly convergent	Couter < Cinner	Increases from limb to hinge (hinge thicker)	Hinge thickening, limb thinning less pronounced than Class 2
Class 2	Parallel to axial surface	Couter = Cinner	Varies (max at hinge, min at limb); axial trace thickness constant	Similar folds (Hinge thickening, limb thinning)
Class 3	Divergent	Couter > Cinner	Increases from hinge to limb (limbs thicker)	Strong hinge thinning

Neat Diagrams:

**(Note: As an AI, I cannot directly draw diagrams. However, I will describe them conceptually, and an aspirant should draw clear, labeled cross-sectional diagrams for each fold type. Each diagram should show the folded layers, the axial trace, and several representative dip isogons.)**

Diagram for Class 1A Fold: A tightly folded layer with a sharp hinge. Multiple lines (dip isogons) connecting the inner and outer arcs converge sharply towards the core of the fold. The layer appears thicker in the limbs than at the hinge.

Diagram for Class 1B Fold (Parallel Fold): A fold where the layer maintains a consistent thickness throughout. The dip isogons are drawn perpendicular to the bedding surfaces at various points along the fold, showing their moderate convergence towards the core but maintaining layer-perpendicularity.

Diagram for Class 1C Fold: A fold with a slightly broadened hinge. The dip isogons converge gently towards the core. The layer appears slightly thicker at the hinge and thinner in the limbs compared to 1A.

Diagram for Class 2 Fold (Similar Fold): A fold where the thickness of the layer measured parallel to the axial plane is constant. The dip isogons are drawn parallel to the axial plane across the fold. This results in significant thickening at the hinge and thinning along the limbs.

Diagram for Class 3 Fold: A fold with a pronouncedly thinned hinge and thickened limbs. The dip isogons diverge outwards from the core of the fold, indicating that the outer arc has a greater curvature than the inner arc.