The bent and curved rock strata are known as folds. The rock beds tend to yield by folding as a result of the compressional stress applied slowly under high confining pressure. During mountain building, flat-lying sedimentary rocks are bent into folds. The result of folding is a shortening and thickening of the earth’s crust.
What are Folds:
”Fold” may be defined as a curved or zig-zag structure shown by rock beds. In other words, wavy undulations in the rock beds are called “folds”. They consist of arches and troughs in an alienated manner. They are best displayed by the sedimentary rocks. The size of folds varies greatly. The width of some folds is measured in kilometers, while those of others in meters or centimeters.
Elements of Folds:
A fold is a complex geological structure, which mainly consists of the following elements/ parts.
(1). Anticline and Syncline: An anticline is an up-fold, where the limbs dip away from the axis of the fold on either side. A syncline is a down-fold, where the limb rises (upward) toward the axis of the fold on either side. The highest point of the anticline is known as the crest or hinge and the lowest part of a dip is known as the trough of the dip.
A typical anticline is convex up in which the crest (hinge) is the location where the curvature is greatest, and the limbs are the sides of the fold that dip away from the hinge. Anticlines can be recognized and differentiated from antiforms by a sequence of rock layers that become progressively older toward the center of the fold. Therefore, if age relationships between various rock strata are unknown, the term antiform should be used. Folds in which the limbs dip toward the hinge and display a more U-like shape are called synclines.
They usually flank the sides of anticlines and display opposite characteristics. A syncline’s oldest rock strata are in its outer limbs; the rocks become progressively younger toward its hinge. A monocline is a bend in the strata resulting in a local steepening in only one direction of dip. Monoclines have the shape of a carpet draped over a stairstep. An anticline is known as a breach or scalped anticline if has greatly eroded at the center. When too much erosion takes place at the anticlinal top, the anticline is, then, known as the ”anticlinal valley”.
(2). Limbs of Fold: The sloping parts of the fold extending away from an anticline or syncline are known as the limbs of the fold. The top crest of the fold is known as the anticline, while the lowest deepened concave-shaped part of the fold is known as the syncline of the fold. The limb is the sloping side between a crest and trough of the fold. In other words, the sloping part of a fold present between the highest and the lowest part of the fold is known as the limb of the fold, as shown in the following figure.
(3). Axial plane: It is an imaginary plane or surface which divides a fold into two equal halves.
(4). Axis of Fold: An ”axis of fold” is defined as the line of intersection of the axial plane with the surface of any of the constituent rock bed. Folds are generally classified according to the attitude of their axes and their appearance in cross-sections perpendicular to the trend of the fold. The axial plane of a fold is the plane or surface that divides the fold as symmetrically as possible.
The axial plane may be vertical, horizontal, or inclined at any intermediate angle. An axis of a fold is the intersection of the axial plane with one of the strata of which the fold is composed. Although in the simpler types of folds the axis is horizontal or gently inclined, it may be steeply inclined or even vertical. The angle of inclination of the axis, as measured from the horizontal, is called the plunge. The portions of the fold between adjacent axes form the flanks, limbs, or slopes of a fold.
(5). Plunge of Folds: Folds having inclined axes are called “plunging folds”. The angle of inclination of a fold axis with the horizontal is called the ”angle of plunge”. The direction in which this axis is inclined is called the direction of plunge. Here in the following diagram both the plunging and non-plunging folding strata are drawn to make the concept fully comprehensive. When the fold axis is horizontal and not inclined, the hinge/crest or trough of the fold will also remain horizontal. Such a fold is known as a non-plunged fold. On the other hand when the fold axis is not horizontal or inclined from one side. The trough and crest of the fold will also be inclined and dipped into the ground from one side. Such folds are known as the plunging/plunged folds.
Types of Folds:
Folds are classified into many types based on their shape and structure. The most common types of folds are:
(1). Symmetrical Folds: A ”symmetrical fold” is one where the two limbs dip at the same angle but in opposite direction. One limb of the fold dips in an upward direction, while the other dips in a downward direction, but the angle of dipping in both the limbs is the same. In this case, the axial plane is vertical and it passes through the crest or trough quite vertically.
(2). Asymmetrical Folds: An ”asymmetrical fold” is one where the limbs dip at unequal angles in opposite directions. In this case, the axial plane is inclined and it does not necessarily pass through the crest line. In the case of the asymmetrical fold, the axial plane is always inclined to one side.
(3). Overturned Folds: The overturned fold is a type of asymmetrical fold, whose one limb is turned past the vertical. In this case, the axial plane is inclined, and both the limbs dip in the same direction. In the overturned fold the lower limb is turned upside down.
(4). Recumbent Fold: In this type of fold the folding is so intense that both the limbs become almost horizontal. In this case, the axial plane also becomes nearly horizontal and the lower limb gets overturned. In recumbent folds fractures usually develop across bends to produce overthrusts.
(5). Isoclinal Folds: Folds that have parallel limbs are called ”isoclinal folds’. In this case limbs dip at athe same angle and in the same direction. Isoclinal folds are of three types:
- Inclined isoclinal folds: where the axial plane is inclined
- Vertical isoclinal folds: where the axial plane is vertical
- Recumbent isoclinal folds: where the axial plane is horizontal or nearly horizontal
(6). Non-Plunging and Plunging Folds:
The axis of the fold may be horizontal or inclined. In the case of a fold having an inclined axis with one end submerged in the ground, the fold is called plunging or plunged fold. On the other hand, if the axis of a fold has a horizontal axis, and no end of the axis is plunged into the ground, is known as a non-plunging or non-plunged fold. An outcrop of a plunging anticline and syncline is shown in the following figure. The plunge causes the outcrop to turn and double back producing a ”nose”. The nose is important as it indicates the direction of the plunge. In the case of an anticline, the plunge is always in the direction of the nose, whereas in the syncline, it is away from the nose.
(7). Anticlinorium and Synclinorium Folds:
These are the complex large folds of general anticlinal and synclinal forms. An ”anticlinorium” is a large anticline with a number of secondary folds of smaller sizes developed on it. While a ”synclinorium” is a large syncline of similar nature. The syncline of the fold consists of many small-sized folds.
(8). Open and Closed Folds: In ”open folds” the folding is mind and therefore, the limbs meet at bends at an obtuse angle. In this case, the thickness of the constituent beds remains unchanged everywhere. While in a closed fold the folding is so tight that the incompetent strata flow plastically towards the crests and troughs. This causes thinning at the flanks and thickening at the crests. The close folds develop under great stress.
(9). Dome and Basin: When the strata have been subjected to folding in two directions at right angles, each anticline is converted into a dome and each syncline into a basin. These folds are about as wide as they are long. Thus a “dome” may be defined as an up-fold where the beds dip radially outward in all directions from the center. In such a case the strata are said to have a ”quaquaversal dip”. A ”basin” may be defined as a down-fold, where the beds dip radially inward towards the center.
(10). Fan Folds: As the name indicates, it is a fold, whose anticline is ”fan-shaped”. A ”fan-fold” is an upright fold in which both the limbs are overturned. In anticlines, limbs dip towards the axial plane, while in synclines they dip away from the axial plane. The beds within the anticline are much compressed below while they open out above. The crests and troughs of fan folds are generally sufficiently rounded.
(11). Chevron Folds: The folds which have straight or nearly straight limbs, their crests and troughs become sharp and angular. Such zigzag folds are called “chevron folds”.
(12). Homocline Folds: When rock strata dip uniformly with the same angle and in the same direction for a distance of the order of a kilometer or more, the structure is known as a ”homoclinic”.
(13). Monocline: A local wrapping in the horizontal strata is called the ”monocline”. In this case, the rock beds lying at two levels are separated by a limb that shows a steep inclination. A monocline is formed by vertical movement. A fault is generally found below most monoclines. The width of monoclines is often several kilometers.
(14). Structural Terrace: A local flattering in an otherwise uniformly dipping bed is called the ”structural terrace”/ In this case the dipping strata become horizontal at a particular spot and then they continue to follow their original dip.
(15). Drag Folds: The ”drag folds” may be defined as a minor fold developed within the body of incompetent beds during the process of major folding. When a sequence where incompetent beds are interbedded between competent beds is subjected to folding, drag folds develop in the incompetent layer. During folding the competent beds lie over one another without deformation while the weak and incompetent layer is contorted into small secondary folds. Drag folds are important because their shape indicates the direction of relative movement. These folds may be used to determine the top and bottom of highly inclined strata as their transverse axes are approximately parallel to that of the major fold.
The ”incompetent rock beds” are those which become mobile and flow plastically during folding. Such beds show variations in thickness from place to place. An example of incompetent rock is shale. The ‘competent beds”, on the other hand, show greater turgidity and their thickness remains unchanged during folding. An example of competent rock is sandstone.