This document discusses different types of folds that can form in the Earth's crust. It describes how folds are caused by plastic deformation from compressive forces acting under conditions of stress, pressure, temperature gradients. There are three main mechanisms of folding: flexural folding where both competent and incompetent beds are bent, shear folding which involves slip along fractures, and flow folding where rocks fold like a fluid at high temperatures and depths. Common fold types include anticlines, synclines, and monoclines. Folds can form from tectonic processes like horizontal compression or mantle convection, or from non-tectonic processes like hillside creep or collapse structures.

1. FOLD DEVELOPMENT
Introduction
Causes
Mechanics
Folding in primary & secondary
rocks
Common types of folds
Origin of folds
Chingshom N
Konyak
Roll no. 172031

2. FOLDS
Undulations or waves in the stratified rocks of the Earth’s crust formed by
bending of strata by permanent deformation.
A fold is measured by its strike,
dip and direction of fold axis.
Axial Plane: Imaginary plane that divides
the fold as symmetrical as possible
Fold axis: Line of intersection of axial
plane and ground surface
Plunge: Angle fold axis makes with the
horizontal

3. SIZE OF FOLDS
Few centimetres or
less
To
Several kilometres across
Mt. Head, Canada

4. CAUSES OF FOLDING
Plastic deformation by
compressive forces under
varied conditions of:
 Stress
 Hydrostatic Pressure
 Pore Pressure
 Temperature Gradient
Synsedimentary fold:
Generated by the
liquefaction of the soft
sediment prior to lithification.

5. PLASTIC DEFORMATION
Plastic deformation causes
permanent deformation.
CONDITIONS
 Ductility of material (e.g.,
clay, gypsum)
 Deform under heat and
pressure
 Confining pressure does
not exceed internal
strength
 Deformation must be
slow.
A: Tough(elastic) C: Ductile(plastic)
B: Brittle X: Plastic limit

6. MECHANICS OF FOLDING
1. Flexure folding
2. Shear folding
3. Flow folding

7. 1. Flexure folding
-True folding
Rocks are sufficiently
plastic
Both competent and
incompetent beds
Beds are bent
Concave side thickens &
convex side thins
Also called flexural-slip
fold
Can interpret drag folds

8. Rainbow Basin syncline in
the Barstow Formation near Barstow,
California
Cape Fold Belt, South Africa

9. 2. SHEAR FOLDING
-Slip folding
Minute
displacement along
closely spaced
fractures
Fractures across
bedding planes
Realignment of
the fractures due to
friction
These small
fractures give rise to
a major fold
May be
temporary in case
the rock is
recrystallised

10. 3. FLOW FOLDING
-Passive folding
At high temperature and/or great depth
Fluid nature of rocks. e.g., rock salts, deep buried
rocks
Little resistance is offered
Strata appear shifted undistorted but thin with
displacement
Take the shape off surrounding rocks

11. Folded schist
Sedimentary rock layers at Ardalanish Beach Park, Scotland
Folded schist and quartzite layers on the side of
Mt. Monadnock, New Hampshire.
Folding in primary &
secondary rocks

12. TYPES OF FOLD

13. ORIGIN OF FOLDS
TECTONIC
Result more or less directly from forces operating within
the earth.
NON-TECTONIC
Result of superficial processes

14. TECTONIC PROCESSES
1. Horizontal compression (as in Appalachians & Jura Mountains)
 Compressive force acting parallel to the surface of the earth
 Greatest stress axis is horizontal

15. 2.Mantle convection
Sub crustal convection
When the lower parts are sufficiently heated
This currents drag along the base of the crust
Overlying sediments are folded

16. 3. Vertical uplift & Gravity sliding
Large open folds that
are ten to hundreds of
miles across
Dipping only a few
degrees
Took millions of years
to form
Cincinnati & Nashville
domes
Michigan basins

17. 1. Hillside creep
Slow downward progression of rock and soil down a low grade slope
Differential dip in a stratum
In incompetent rocks like shale
NON-TECTONIC PROCESSES

18. 2. Collapse structure
A type of gravity sliding

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