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Fold Geometry (3)

Fold Geometry (3)

• Classification of folds (cont.)• Fold system

Classification of folds (cont.)

Based on :5 shape of hinge zone6 fold profile7 dip isogons Hinge Zone Shapes

Rounded Angular Box Fold Monocline

Classification of folds based on hinge zone shape

A . Constant Bed Thickness perpendicular to Fold surface

B. Thickness constantparallel to axial

surface

C. A common center of curvature

D. Planar limbs and sharp angularhinge zone

Classification of folds based on the fold profile Similar folds(J.G.Ramsay,1962)

2

Similar folds

Detachment zone

Concentric fold

Kink bands Markedly asymmetric chevron with a very long limb and a very short limb

The band is the zone in which the common short straightlimb bounded by the two adjacent fold surfaces the perpendicular to

the axial surface

Method of measuring variation in layer thickness tα in a fold.Measurement is made at the point where the tangent

to the outer curve makes an angle α with the perpendicular to the axial surface (Park,1997)

Dip isogon, axial trace thickness (Twiss & Moores)

(Ramsay, 1967)

t0 is the layer thickness at the hinge

t’- a curves

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Fold Classification Based on Dip Isogones

Construction of dip isogons The isogons join points on successive fold surfaces with the same inclination α. The

isogons converge to the fold core here (Ramsay, 1967)

Fold classification based on dip isogons Class 1: fold with convergent isogons; Class 2: fold with parallel isogons; Class 3: fold with divergent isogons. Class1A: strongly convergent isogons.Class 1B: parallel folds with isogons perpendicular to fold surface. Class 1C: weakly convergent isogons.

(Ramsay, 1967)

Class 2:Similar folds

Class 1A:Thickened limbs

Class 1B:Parallel folds

Class 1C,Class 2,3:Thinned limbs

DESCRIPTION OF FOLD SYSTEMS

• Fold symmetry• Fold vergence• Parasitic folds• Harmonic and disharmonic systems• Conjugate and polyclinal systems

Symmetric and asymmetric folds

Symmetric folds with limbs of equal length. Asymmetric folds with Limbs of unequal length

Fold vergence: easterly vergence, or verging east according to the Shorter limbs of the antiforms of the set of asymmetric folds.

(Park, 1997)

WE

PARASITIC FOLDS

Z – SHAPED FOLDS S –SHAPED

FOLDS

M – SHAPED FOLDS

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Enveloping surface drawing through the hinge lines of all folds can show the overall shape of the main structure

(Park, 1997) Harmonic folds affect layers for many times the half-wavelength along the axialsurface tracein the profile plane (Twiss & Moores, 2007)

λ

Disharmonic folds, nearby layers fold independently of one another(Twiss & Moores, 2007)

Zone of contact strain (A), Disharmonic folds (B), Harmonic folds (C) and Complex harmonic folds (D)

Harmonic folds: correspond with each other in wavelength, symmetry and general shape.

Disharmonic folds: wavelength and shape of folds in adjacent layer are quite different

(Ramsay, 1987)

Disharmonic foldsThe wavelength of the inner,thinner layers is

much shorter than that of the outer layer(Park, 1997)

Conjugate folds(A,B) with opposite sense of asymmetry and polyclinal folds(C) with

variable axial surfaces(Park, 1997)

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Disharmonic folds, Northwest Sichuan, China (Zuoxun Zeng, 1997) Angular folds and kink bands (Zuoxun Zeng, 1997)

Parasitic foldsDisharmonic folds

Disharmonic folds

Problems for review 1. List the geometric elements of folds2. Distinguish between a synform and a

syncline3. Distinguish between a antiform and a

anticline4. Distinguish between a antiformal syncline

and a synformal anticline5. What does ‘fold facing direction’ mean?6. How do you determine the facing direction of

a fold?7. Distinguish between different classifications

of folds

Problems for review (cont.)

8. What is the significance of the enveloping surface of folds.

9.Could you explain the formation of symmetric and asymmetric folds?

10.How do you use the parasitic folds in geologic mapping?

11. How do you distinguish between harmonic and disharmonic folds?

12. What are similar folds and parallel folds?13. What is a kink band?