Classification of Folds | Rocks | Geology

Folds show a great variety of forms; some may be quite simple, whereas others may be highly complex and complicated in their geometry and morphology.

In fact, in most cases folds may be simple or complex modifications of two basic types of folds:

i. Anticlines and

ii. Synclines.

i. Anticlines:

Anticlines are defined as those folds in which:

(i) The strata are uparched, that is, these become CONVEX UPWARDS;

(ii) The geologically older rocks occupy a position in the interior of the fold, oldest being positioned at the core of the fold and the youngest forming the outermost flank (provided strata show normal order of superposition), and,

(iii) The limbs dip away from each other at the crest in the simplest cases.

Symbolically, an anticline may be indicated by two arrows diverging from the central point, as

←∙→

Synclines:

These folds are the reverse of anticlines in all details and may be described as those folds in which:

(i) The strata are downarched, that is, these become CONVEX DOWNWARDS;

(ii) The geologically younger rocks occupy a position in the core of the fold and the older rocks form the outer flanks, provided the normal order of superposition is not disturbed,

(iii) In the simplest cases in synclines, the limbs dip towards a common center. Symbolically, a syncline may be indicated by two arrows pointing towards a central point, the hinge point.

→∙←

Some structural geologists prefer to use the terms Antiforms and Synforms for uparched and downarched folding respectively unless the stratigraphic order is completely established. This implies that the terms anticlines and synclines shall always be used with reference to geological age of the rocks- if the curvature is towards the direction of younger rocks, the fold is anticline and if it is in the direction of older rocks, the fold is a syncline.

As per this notion, there can occur a synformal anticline and an antiformal syncline. Such situations are very rare, as these require overturning of the normal order of superposition before folding. Hence, anticlines and synclines are favourite terms with the geologists and students of geology.

In their actual existence folds are far from simple and have been variously classified on the basis of following parameters:

(i) Position of the axial plane;

(ii) The degree of compression;

(iii) Behaviour with depth;

(iv) Relative curvature of the outer and inner arcs;

(v) Plunge of the folds;

(vi) Profile of the outer surface;

(vii) Mode of occurrence;

(viii) Miscellaneous types;

We shall illustrate only a few common types of folds encountered in the field that can be said to fall in one and/or other category from the above list. However, it must be mentioned here that in view of great variety of these structures developed in the crust, the types and classes mentioned below are only illustrative and in no way exhaustive.

(i) Position of Axial Plane:

Depending upon the nature and direction of the stresses, the axial plane in a resulting fold may acquire any position in space, that is, it may be vertical, inclined or even horizontal.

Following main types are recognized on the basis of position of the axial plane in the resulting fold:

1. Symmetrical Folds:

These are also called normal or upright folds. In such a fold, the axial plane is essentially vertical. The limbs are equal in length and dip equally in opposite directions. Basically, it may be an anticline or syncline and when classified, may be described as symmetrical anticline/syncline as the case may be.

2. Asymmetrical Folds:

All those folds, anticlines or synclines, in which the limbs are unequal in length and these dip unequally on either side from the hinge line are termed as asymmetrical folds. The axial plane in an asymmetrical fold is essentially inclined.

3. Overturned Folds:

These are folds with inclined axial planes in which both the limbs are dipping essentially in the same general direction. The amount of dip of the two limbs may or may not be the same.

Over folding indicates very severe degree of folding. One of the two limbs (which are termed the reversed limb) comes to occupy the present position after having suffered a rotation through more than 90 degrees. The other limb is known as the normal limb.

In certain cases, both the limbs of a fold may get overturned because of very high lateral compression. It may be originally either an anticline or a syncline but the extreme compression from opposite sides results in bringing the limbs so close to each other that the usual dip conditions may get reversed—anticlinal limbs dip towards each other and the synclinal limbs dip away from each other.

Such a type of fold is commonly referred to as a fan fold. In such folds, the anticlinal tops are said to have opened up into a broad, fan-shaped outline due to intense compression in the lower region.

4. Isoclinal Folds:

These are group of folds in which all the axial planes are essentially parallel, meaning that all the component limbs are dipping at equal amounts. They may be made up of series of anticlines and synclines.

5. Recumbent Folds:

These may be described as extreme types of overturned folds in which the axial plane acquires an almost horizontal attitude. In such folds, one limb comes to lie exactly under the other limb so that a drill hole dug at the surface in the upper limb passes through the lower limb also. The lower limb is often called the inverted limb or the reversed limb.

Other parts of a recumbent fold are sometimes named as follows:

(i) The arch, which is zone of curvature corresponding to crest and trough in the upright folds;

(ii) The shell, which is the outer zone made up mostly of sedimentary formations;

(iii) The core, which is the innermost part of the fold and may be made mostly of crystalline igneous or metamorphic rocks;

(iv) The root or the root zone, which is the basal part of the fold and may or may not be easily traceable; once traced it can throw light whether the fold was originally an anticline or syncline that has suffered further inversion.

6. Conjugate Folds:

In certain cases a pair of folds that are apparently related to each other may have mutually inclined axial planes. Such folds are described as conjugate folds. The individual folds themselves may be anticlinal or synclinal or their modifications.

7. Box Fold:

It may be described as a special type of fold with exceptionally flattened top and steeply inclined limbs almost forming three sides of a rectangle.

(ii) Degree of Compression:

Layers of rocks may be compressed only slightly or very severely during the process of folding depending upon the intensity of forces acting on the rocks. In the first case, a bend or fold may be caused without causing any appreciable variation in the thickness of the rock anywhere. But when folding is due to very severe forces, the process may actually involve plastic movement of the rock masses resulting in thinner limbs and thicker crests or troughs.

Sometime ‘fold angle’ is made on the basis of classification of a fold into- Gentle folds (170°-180°); Open folds (90°-170°); and Tight folds (10°-90°). These folds in which the thickness of the rocks is not affected during the process are termed as Open folds, and the other type with thickened crests or troughs and thinner limbs as Closed folds.

(iii) Behaviour with Depth:

Folding may involve strata for considerable depth showing conspicuous characters that may be useful for their further classification.

Two major types of folds recognized on the basis of thickness of the layers as a measure with depth are the concentric (or parallel folds) and the similar folds:

(1) The Concentric Folds:

The strata in concentric or parallel folds have reacted to the effecting tangential compression in such a way that the thickness of the involved layers remained uniform and constant even after folding. The folding has been due to layer parallel slip that took place along the least cohesive bedding planes involving little or no distortion of mass making the layers.

This particular mechanism of folding is responsible for a typical character of the concentric folds in which:

(i) Anticlines become sharper with depth;

(ii) Synclines become sharper upwards.

(2) Similar Folds:

Those folds in which the degree of folding is observed to be similar for indefinite depths are grouped as similar folds. In such folds, the axial regions are thicker than the limb regions indicating a plastic type of movement of the material of rocks from the limb regions to the axial regions during the process of folding.

The similar folds are taken to be characteristic of the zone of flowage below the surface, taking place in softer strata confined under overburden. It is typical of such folds that thickness of strata as measured in limb regions shows variation from point to point, the smallest change being observed at the outer margins of the limbs. The thickness of the strata at the hinge regions suffers the maximum change.

(3) Supratenuous Folds:

These are folds showing differences in thickness at the crestal and the trough regions, not induced by folding process but essentially being due to erosional and depositional processes operating in the folded regions. In such cases, the crestal regions of the anticlinal folds may become thinner due to excessive erosion on the outstanding areas and the trough regions may become thicker due to deposition in these low lying areas. Such folds are often described as supratenuous folds.

(iv) Relative Curvature:

Ramsay divides all types of folds in three main classes on the basis of relative curvature of the outer and the inner arcs of a fold.

The three classes are:

Class 1 Folds:

This class includes all those folds in which the degree of curvature in the outer arc of the fold is less than that of the curvature of the inner arc.

Class 2 Folds:

All those folds in which degree of curvature as measured in the outer arc and the inner arc is equal, are placed in class 2.

Class 3 Folds:

These are just the reverse of class 1 folds; in these folds, the degree of curvature as measured on the outer arc is greater than that of the inner arc.

For the purpose of this classification, the given fold is assumed to take the shape of an UPRIGHT ANTICLINE and the curvature both at the outer and the inner arc determined. Determination of dips may be made and then lines of equal dips drawn. These are called ISOGANS. From the position of these isogans, the class of a fold is determined. Thus, in Class 1 folds, the Isogans converge inwards whereas in Class 3, these converge upwards. Obviously, in Class 2 folds, the isogans would run parallel.

(v) Plunge as Basis:

Only two main types are recognized- the plunging folds and the non-plunging folds.

Plunging Fold:

Any fold in which fold axis is NOT HORIZONTAL, i.e. it makes an angle with the horizontal, may be described as a plunging fold; to be specific, it may be described as a plunging anticline or syncline; to be further specific, it may be described as an anticline or syncline (or whatsoever type it may be) plunging at a particular angle in a particular direction.

A Non-Plunging Fold:

Any fold in which the axis of fold is essentially horizontal, i.e. the folding continues indefinitely in the direction of the axis of the fold, is specifically described as non- plunging fold of whatever main type it may be.

(vi) Profile of the Fold Surface:

Folds may develop distinctly different profiles as seen in cross sections- sharp angled, broadly curved, flat topped, semi-cylindrical, cylindrical and even elliptical and so on. Hence, shape of the fold is also made a convenient basis for classification of folds.

Chevron Folds:

These are characterized with well-defined, sharp hinge points and straight planar limbs.

Conjugate Folds:

These are composite folds characterized with two hinges and three planar limbs in which the central limb is exceptionally flattened, in some cases giving a typical box-like appearance to the fold. The last type of folds is sometimes specially referred as Box Folds.

Cuspate Folds:

The limbs of the folds are not planar, they are quite clearly curved becoming concave upwards in the case of anticlines and concave downwards in the case of synclines. The hinge zones are, therefore, not very sharp.

Cylindrical Folds:

The Cylindrical folds resemble sections of pipes and have very well defined axes of folds repeated parallel to each other. In fact, folds in which repetition of axes parallel to themselves is not possible, are classed as non-cylindrical.

(vii) Mode of Occurrence:

Folds rarely occur singularly- more often they occur in groups. In such groups individual members may show similarities as well as some dissimilarity. Hence mode of occurrence has also been made a basis for classification of folds.

1. Anticlinorium:

It is a system of exceptionally large sized folds running often for several hundred kilometers in length and several kilometers in width. The general trend of the folded system, when determined on an averaged bending, is anticlinal in nature, that is, the strata as a whole have been uparched.

Locally, the system may show development of numerous small-scale folds of various types, such as anticlines and synclines, overfolds and even recumbent folds. Many mountain systems of the world may be broadly described as anticlinoria.

2. Synclinorium:

It is the reverse of anticlinorium and may be defined as an extensive system of folds having a clearly down arched (synclinal) folding trend. Locally, however, the strata may show numerous types of small-scale folds of all types. The depressed or basin type general outline of the strata indicates a major downward bending of the crust along the extent of the synclinorium.

In both the anticlinorium and synclinorium, presence of large number of secondary folds, faults and fracture systems is a characteristic feature. Similar folding but signifying still larger bending and uplifting of strata on sub-continental scales are expressed by the terms GEANTICLINES AND GEOSYNCLINES respectively.

The geosynclines are believed to serve as depositional fields or basins of sedimentation to which sediments derived by the erosion of the adjoining geanticlines get accumulated and compacted. This material is then compressed and uplifted in the second stage of orogeny, (the mountain building activity) to gradually take the shape of mountain systems.

3. Domes and Basins:

Domes are a group of strata centrally uplifted in such a way that seen from the top, these dip away in all directions. In any two cross sections drawn mutually at right angles to each other in a dome, a fold of anticlinal character will be seen to emerge. As such, a dome may be considered as a compound anticline.

Diapiric Folds are anticlines or domes in which uparching of strata is attributed to the rising of viscous magma from below; the force associated with the magma is often strong enough to cause a rupture at the dome when this is specially referred to as a diapiric fold.

Basins are the reverse of the domes and may be defined as a group of strata that are centrally depressed in such a way that the involved layers dip towards a common central point from all the sides. Unlike domes, two cross sections drawn at mutually right angle directions in a basin would show clearly synclinal characters. Hence a basin may be called a compound syncline.

(viii) Miscellaneous Types:

Besides the various types explained above, some other types of folds that may be observed in the field are as follows:

1. Monocline:

It may be described as essentially a localized warping in which case otherwise horizontal strata show a single bend for a limited length and attain the horizontal attitude once again. Sometimes the effect of monoclinal bending is so conspicuous that a great difference of elevation is induced between the same strata on either side of the bend.

2. Homocline:

A homocline actually describes a sequence of strata dipping in the same general direction at a uniform angle, especially when such structure is established to be a limb of a major fold. An anticline or a syncline of big magnitude, for instance, will each show two homoclines, one on either side of the hinge.

3. Drag Folds:

These may be defined as minor folds developed within the body of incompetent (or weaker) rocks surrounded on both the sides by layers of competent or stronger rocks. These derive their name from cause of origin- they develop due to drag effect suffered by the soft and ductile type material of the incompetent rock when the surrounding competent rocks move apart them in opposite directions during lateral compression or shearing process.

Much importance is attached to the drag folds in the interpretation of the major folding and faulting in the area of their development.

It has been established that:

(a) The axes of the drag folds are parallel to those of the major folds;

(b) The drag folds plunge in the same manner as the major folds;

(c) The layers on the upper side of the drag folds slide away from the synclinal axis thereby indicating the possible location of the synclinal axis of the major folds.