NAME: Mustahsan Shuaib ((مستحسن عباسي

Reg.No: FA10-ERS-076

TOPIC: FOLDS AND THEIR CLASSIFICATION

Folds

Wave like features that form due to curving, buckling, and bending of originally planar rock layers ( e.g beds, foliations) through ductile deformation

Folds and their classification

Parts of the folds Limbs Axial plane Axis Hing Hing line

Row 1 Row 2 Row 3 Row 40

2

4

6

8

10

12

Column 1

Column 2

Column 3

Descriptions of main parts

Limb

The stretch of rock beds

Axial Plane

Imaginary plane which divides the fold as symmetrically as possibe

Axis

The intersection of Intersection of axial plane

Hing

Line along which a change in amount of dip takes place is called hing line.

Appearance in cross-section Symmetry of fold Thickness of limb Interlimb-angle Attitude of the fold

Criteria for Classification

Appearance in cross-section Antiform

Any upwardly convex structure is termed as antiform

Synform

Any upwardly concave structure is termed as synform

Anticline

It is convex upward where the limbs slope away from the axial plane.In this older beds are present at centre of curvature.

Syncline

It is a fold which is concave upward and the dips towards the axial plane. The younger beds are present in centre of curvature of fold.

Anticlinorium

A large anticline with secondary folds of smaller size develope on it.

Synclinorium

It is a large syncline with secondary folds of smaller size developed on it.

Anticlinal bend or Monocline

It is due to local steeping of a bed, whereby there occurs a sudden increase in the dip of a bed which is originally horizontal to a near vertical postion. But the original bedding remains as before.

Symmetry of fold

Symmetrical fold Asymmetrical fold Recombent fold Isoclinal fold Over-turned fold Homocline

Symmetrical fold

When the axial plane is vertical and bisect the fold, the fold is said to be symmetrical or upright fold. It could be anticline or syncline.The amount of dip is same.

Asymmetrical fold

If the axial plane has dip, the fold is described as 'inclined' or 'asymmetrical' fold. In this case both the limb dip at different angles and the axial plane can't divide the fold into two symmetrical halves. There may be asymmetrical anticlines as well as synclines.

Recumbent fold

It is an overturned fold, in which the axial plane is horizontal or more nearly so. The strata inverted limbs are usually much thinner than the corresponding beds in normal limbs.

Over-turned fold

These are also known as 'over folds'. Here the axial plane is inclined and both the limbs dip in the same direction, usually at different angles. There may be anticline as well as synclines.

Isoclinal fold

In an over-turned fold when the both limbs have same amount of dip, toward the same direction, it is known as isoclinal fold. It may be vertical , inclined as well as recumbent isoclinal fold.

Homocline

These can be said as tilted beds, where all the beds have same amount of dip in same direcion.

Homocline

Thickness of limb

Parallel fold Similar folds Supateneous folds

Parallel folds or Concentric folds

These are folds with the successive semi-circles have a constant centre and a regularly increasing radius. Here the thickness of beds does not change during the folding and the anticlines become sharper with depth but broader and more open upward. Similarly the synclines become broader with depth but sharper upward.

Similar folds

In the case of shape of the folds may vary along the axial plane and right angles to fold axis. Her every bed is thinner in limbs and thicker near the hinges. For this, there must be consideable plastic movement of material away from the limbs and towards the hinges.

Supratenous folds

Here the strata are thinnest at the crust of the anticlines and thickest at the trought of synclines.These are produced when depostion takes place over a ridge.

Interlimb-angle Open or gentle fold

When inter-limb angle is greater than 70' Closed fold

When inter-limb angle is between 30'-70' Tight fold

When inter-limb angle is below than 30' Cylindrical fold

When the profile is essentially semi-circle and remains constant when traced along thier axes.

Tight Fold

Attitude of the fold Plunging fold

A fold is said to be plung if the axis is not horizontal. The amount of plung being the angle between the axis and a horizontal line laying in common vertical plane.

Non-plunging fold

When the axis of fold does not dip in any direction it is said to be non-plunging fold.

Doubly-plunging fold

When the fold reverses its direction of plunge within the limits of the area under consideration.

Some special types of folds Chervon folds

These are angular folds having straight limbs and sharp hings. These are also known as zig-zag, concertina or accordion fold.

Fan foldings

If in any fold both the limbs are overturned the fold assumed the shape of fan where the crests and troughs are suffeciently ounded.

Box fold or 'coffer folds'

These are rectangular in cross-section. In this crest is broad and flat , two hings are present one on either side of flat crest.

Kink-fold

These are narrow bands usually a few inches or few feet wide, in which the beds assume a dip which is steeper than the adjecent beds.

Geosyncline

These are elongated large basins which are found submereged beneath the sea water and contain very great thickness of sediments.

Geoanticline

It is broad uplifted area brodering a geosyncline, which supplies sediments for its filling.