Folds Geology, Lecture notes of Geology

Download Folds Geology and more Lecture notes Geology in PDF only on Docsity!

FOLDS

ANATOMY, GEOMETRY,

MECHANICS & CLASSIFICATION

PRESENTATION

MADE BY:

EZAZ FAROOQ

FOLDS

ANATOMY, GEOMETRY, MECHANICS

& CLASSIFICATION

FOLD DEFINITION

Folds are wave-like structures that result from deformation of bedding, foliation, or other originally planar surfaces in rocks. Fold can be very broad to tightly compressed. They may occur as isolated folds or extensive fold trains of different sizes.

ANATOMY OF FOLDS

Crest – The highest point on the cross section of a fold Trough – The lowest point on the cross section of a fold Limbs – The straighter portion of the fold connecting the hinge zones Hinge Zones – Exhibit the greatest curvature of a fold

ANATOMY OF FOLDS

Hinge Line – The line joining points of greatest curvature on a folded surface Axial Surface – When many hinge lines (from multiple folded surfaces) connect forming a plane Axial Plane – The plane that joins all the points of maximum curvature. For a symmetrical fold, this axial plane is perpendicular to the horizontal plane that contains the strike lines of the fold. For asymmetrical and overturned folds, the axial plane is itself inclined. Plunge – When the fold hinge is inclined to the horizontal Fold Axis - The fold axis is the line that forms where the axial plane intersects the fold. For a horizontal fold, the fold axis is horizontal.

3D ILLUSTRATION OF FOLD

ANATOMY OF FOLDS

First-Order Folds – The master (largest) fold Second-Order Folds – Smaller folds on the flanks of first-order folds Enveloping Surface – Enables the relation of small to large scale folds

SCALE OF FOLDS

Microscopic – Need magnification Mesoscopic – Occur from hand specimens to outcrop scales Macroscopic – Occur on the map scale or larger

FOLD ANGLE AND FOLD AXIS

FOLD CLASSIFICATION

Based on:

• Appearance in Cross Section
• Symmetry of fold(Attitude of axial plane)
• Inter-limb angle
• Attitude of fold axis
• Shape of hinge zone (Special folds)

A2. APPEARANCE ON CROSS SECTION

CLASSIFICATION

Anticlinorium – A large anticline with secondary folds of smaller size develop 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 position. But the original bedding remains as before.

B. SYMMETRY OF FOLD(ATTITUDE OF AXIAL PLANE)

CLASSIFICATION

Symmetrical fold - When the axial plane is vertical and bisect the fold, the fold is said to be symmetrical or upright fold. Amount of dip is same. Asymmetrical fold - In this case both the limb dip at different angles. Recumbent fold - It is an overturned fold, in which the axial plane is horizontal or more nearly so. Over-turned fold - Here the axial plane is inclined and both the limbs dip in the same direction, usually at different angles. 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.

B. SYMMETRY OF FOLD(ATTITUDE OF AXIAL PLANE)