folds and their classification
Post on 14-Oct-2014
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DESCRIPTIONFolds are the most prominent geological structures.Here is the full presentation of the folds and their classification that was prepared by me.
NAME: Mustahsan Shuaib () Reg.No: FA10-ERS-076 TOPIC: FOLDS AND THEIR CLASSIFICATION
FoldsWave 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 classificationParts of the folds 12
Limbs Axial plane Axis
8 Column 1 Column 2 Column 3
Hing line2 0 Row 1 Row 2 Row 3 Row 4
Descriptions of main parts
Limb Axial Plane
The stretch of rock beds
Imaginary plane which divides the fold as symmetrically as possibe
The intersection of Intersection of axial plane
Line along which a change in amount of dip takes place is called hing line.
Criteria for Classification
Appearance in cross-section Symmetry of fold Thickness of limb Interlimb-angle Attitude of the fold
Appearance in cross-section
Any upwardly convex structure is termed as antiform
Any upwardly concave structure is termed as synform
It is convex upward where the limbs slope away from the axial plane.In this older beds are present at centre of curvature.
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.
A large anticline with secondary folds of smaller size develope on it.
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
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.
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.
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.
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.
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.
These can be said as tilted beds, where all the beds have same amount of dip in same direcion.
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.
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.
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.
Open or gentle fold Closed fold Tight fold Cylindrical fold
When inter-limb angle is greater than 70'
When inter-limb angle is between 30'-70'
When inter-limb angle is below than 30'
When the profile is essentially semi-circle and remains constant when traced along thier axes.
Attitude of the 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.
When the axis of fold does not dip in any direction it is said to be non-plunging fold.
When the fold reverses its direction of plunge within the limits of the area under consideration.
Some special types of folds
These are angular folds having straight limbs and sharp hings. These are also known as zig-zag, concertina or accordion fold.
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.
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.
These are elongated large basins which are found submereged beneath the sea water and contain very great thickness of sediments.
It is broad uplifted area brodering a geosyncline, which supplies sediments for its filling.