Folds when crustal rocks are subjected to stress over a period of time, they compress and form folds. These compressed beds appear like undulations, showing sharp bends and curvatures.
Thus the final form and extent of a fold will depend on the rock type, direction, magnitude and duration of stress acting on it.
Therefore as the applied stress increases these beds form anticlines, syncline, isoclines and recumbent fold. Further stress will lead to overturned folds.
Parts of a fold:
The number of terms used to describe the morphology or shape of a fold. Some of the most common terms are:
1. Limns of folds
A simple fold has two limbs each of which lie on either side of the axial plane. In case of complex folds a middle limb will be common to the two adjacent folds.
It is a point of maximum curvature in the folded surface more commonly the hinge is a zone called a hinge zone. If all the points of maximum curvature are jointed the resulting line is known as a hinge line
3. Axial plane
It is an imaginary plane or surface which divides a fold into two halves.
4. Axis of fold
An axial of fold is defined as the line of intersection between the axial plane and the surface of any of the constituent rock beds.
Types of folds
Firstly anticlines are folded in which the limb dip away from each other at the crest and convex upwards.
The shape of the anticline resembles letter A . In simple cases i.e. in the area which not so tectonically active, the older rocks occupy a position in the interior of the fold, oldest at the core of the fold.
Firstly these folds are the reserve of anticlines in all details and may be described as those folds in which the strata concave downwards . the limb of a syncline dip towards each other.
Younger beds occupy the central portion of the fold and the older rocks move away from the center its Lastly.
In symmetrical anticline and symmetrical syncline, the axial plane is vertical. The limbs being of the same length dip equally in opposite direction from the center.( hinge)
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