What are Folds?

The earth’s center is not in a state of equilibrium. It is synamic with changes in pressure,

temperature resulting in tectonic forces, which wave like causes ripple-effects at the surface of

the earth. The magnitude of these forces are such that even at the outermost ripple, it changes the

contour of the surface of the earth by compression here and tensional forces there. Geology has

named these changes as folds ( as in folding of a sheet of paper). As these forces are not

neccessariliy symmetrical and nor co-axial, the folds on the earh’s surface display different

geometrical shapes.

What is the significance of Folds?

The geometric patterns of folds can be either standard or complex. In the field of structural

geology, folds are due to the plastic deformation of rocks when acted upon by tectonic forces.

With the advent of the tectonic forces, the hard rocks resist the attempts and ultimately break,

without any deformation. And the other rocks which show a plastic deformation fold, sometimes

Along fault lines and sometimes not and gives risre to new faults. It may be worth mentioning

here that although all the activity described in this chapter above in the present continuous,these

upheavals which lead to formation of faults and folds, takes place not instanteously but in the

geological time scale.” Assuming a probable value of the Hubble constant as 18 km/s per

1,000,000 l.y., we get an age of 16.5.109 years. The decimal figure is illusory however, because

the error is plus or minus a few billion years” C.Emiliani 1992. The significance of folds is they

can be used to know about the tectonic history from which ductile deformations can be identified

and probable locations of the reserves of hydrocarbon, coal and other ore deposits can be

assumed.

Plasttic deformation in rocks, which give rise to folds, follow no boundaries of size . They can be

of very small size like small crinkles to enormous mountain ranges. An image of a small crinkle

fold is shown below with the pickax placed near so that the reader can have of the relative size.

Again, fold formation of a huge mountain range is seen in the adjacent illustration.

Folding and Stress:

What stresses can contribute to folding phenomena? The answer here will be the three main

kinds – which are compressive, tensile and shearing or any combination of them there of. In fact,

it is to rare for a single foce to work alone without its co-perpetrators. As will be evident, for

that to happen, the main responsible force will be required to act perfectly along the axes of a 2D

model which has a pathetic probability of taking place.

The force which contributes to formation of major folds is compressive. Shear force combined

with any of the others and tensile forces do not have that much of an effect on fold formation, as

compared to compressive forces.

This is best understood by an illustration as the one above. As can be seen above, these excerting

tectonic forces are more pronounced in the case of compression then tension, the latter mainly

resulting in lowering the thickness of the landscape at the center. Shear forces again result in fols

with lateral and vertical movement, and in some cases both of unequal magnitude.

The Phenomena of Folding

A. Geometric Parts : Terms in common usage.

It would be helpful to list down some commonly used terms in Fold Geometry as follows

Inflection Line: A point or points at which the sense of curvature changes (e.g. from convex

up to convex down ).

Fold Train : a series of folds having alternating senses of curvature.

Folds that have opposite senses of curvature such as convex upward( Antiforms) and those which

are concave outwards(Synforms). A set of folds with identical geometry are considered to be of

common orign and are termed Fold Systems. A Hinge is a lineon the folded surface alomg which

the curvature is the maximum. Limbs/Flanks are the regions with the lowest curvature. Fold

profile is the view of the seen when viewed parallel to its hinge. Anticline is a fold in which the

older rock layersare on the cicave surface of a bedding surface and the younger layers are on the

convex side.A Syncline is a fold where the older layers are on the concave side and the older

layers are on the convex side of the bedding surface.

Rock Cleavage and its relation to folding is discussed in detail by Price and Cosgrove (Analysis

of geological Scrutures;1990)and the different types of rock cleavages are classified by citing

from other earlier Sources. Sedgewick (1835) was the first to discover the geometrical

relationship between folds and Rock cleaveges’. Darwin (1846) and Rogers (1856) also pointed

out that certainly a general parallelism exists between axial folds and rock clevages. This is

mentioned clearly by Price and Cosgrove in their book. This rock cleavages have also been

found to contain minerals in large quantities.

So, to recap we have found folds to be a phenomena and are formrd due to the tectonic forces

that occur because of the inherent instability of the earth’s core. Folds are of varied geometric

shapes and designs as they come about because the form is dependent on the force applied on a

landscape which can vary in many ways.It has been seen that it is easier to locate ores, coal and

hydrocarbons below the earth’s surface by making a studyb of the adjacent folds which helps in

forecasting presence of other piles. However, once these fold have been studied in detail, and as

different geometric shapes are co-related to the presence of an external mineral, the geologist’s

work of forecasting the approximate locations of minerals can be done by looking at the

geometric profiles of the folds in adjacent landscape. It is therefore logical enough to know about

the geometric profiles of folds, frm this stage onwards.

To do this, it is clear that such a system of classification should be highly practical and never

analytic, becayse the in-field-geologist has to say by looking at the fold or even by measuring it,

that whether the site can be clubbed with the ‘with potential group’ or the ‘no potential group’.

He has no time and nor the infrastructure to test analytically and find out what secrets the rocks

hold under them. As Ramsay and Huber themselves say they had focused in “..practical methods,

which give rapid, accurate and useful geometric descriptions…….to define precisely and in

numerical terms, the exact geometric features of folds.

Ford Classification as per Shape:

While we have earlier defined antiforms ans synforms,there are some folds which do not close

either upwards or downwards but have a tendency to side close. These structures having a

tendency to side close are called neutral folds and if all surfaces of a neutral fold are inclined

vertically, then these structures are called vertical fold or vertical neutral fold.

Geological Folds can be classified into different types, the criteria behind such classification

being not only geometric forms but also to define stratigraphic relaaationships in a fold, i.e. older

rocks and younger rocks, the ‘ages’ of course taken in the context of geological time. (However,

these classifications are by no means absolute and the potential always exists of further

divisions.). These are as given under.

1.Monoclines, Neutral and Vertical Folds

A Monocline is a Neutral Fold which has one its limbs horizontal.

2. Synclines and Anticlynes: These have been discussed previously. It can also be added

here that Anticlyne Folds form figures somewhat like arches and Synclines can be said

as trough-like structures. These two folds can be found solitarily but in most cases, they

are generally found in pairs. This is obvious from the mechanics of their formation.

The illustration above will further make the matter clear.

3. DOME, and

4. BASIN

A Dome is described as an ovaaal shaaaped bulge of rock layers while a Basin is an ovaaal

shaaaped bedrock depression. Ramsay & Huber defines the ssame aaas “ such changes of

plunge lines set up Domes (aaaantiform with hinge line culmination) , Basins (synform

with hingeline depression), Saddles with (antiform with fepression) or Inverted Saddles

with (synform with culmination. In certain regions, these changes of hinge line orientation

may be very strong 9greaaater then 90o, and the dome or basin morphologybecomes

extremely prominent. Such folds have been termed eyed folds or “Sheath Folds””

Other aspects of Fold geometry are concerned with such properties like “symmetry” and

“erosion”. These are fairly simple to understand, the symmetry talked about being the

symmetry of the fold along its ‘Axial Plane’being either symmetric or assymetric. It is

evident that most folds will be assymetrical in nature aaalthough, when conditions are

favourable, a majority of symmetrical folds over an area can be possible too. Another

definitive term for a fold is ‘Overturned Folds’. These folds aaare non symmetrical with

aaaall the axial planes tipping at one side and of these , one axial plane is tipped aaat

beyond the verticel plane.

Illustration of an OVERTURNED FOLD

Erosion and Weathering:

One of the most interesting asssspects of Fold Moerhology is erosssion. Let us consider an

uncomplicaaated fold with a plunging characteristic. As decades and centuries of exposure

to the elements take place, the rock deformation due to weathering and erosion. The rate and

places of erosion depend upon the contours of the adjacent landscaaapes, its position ( a

anticline fold at the middle of a mountain range will suffer less from erosion as it is

protected for a certain time by its big brothers on both sides.). The landscape thaat we find

all around us today is not planned. Rather, the contours existing are the resut of upheavels

and tectonic shifts inside our plant, magma dried after being pushed out of the earth’s

bowels andthen subjected to erosion, settling of fine particles away from their place of orign

to deserts or rapid wear due to favourable conditions of erosion…all these make the

The angle of dip from the axial plaaane is much higher then the other, although both the folds dip in the same direction from the fold axis.

phenomena of folding very interesting and the study of this science maks us feel our puny

forms when compared to these huge forces of nature.

For simplicity, we take an example of a symmetric anticline without any plunging along its

axial plaaane. Assuming that there is no bias of erosion on the fold, the illustration below

highlights the probable shape today of such a fold. The original dip angle has been taaaken

as 450.

1. (Oldest Rock Formation) Brown Sandstone 2. Red Sandstone

3. Shale 4. Limestone 5. Conglomerate (Youngest Formation)

GEOLOGICAL MAP

Cross-Section post Erosion of the fold

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