Essay on Classification of Fold Structure

Folds are the waves in the bedded rocks on the crust of the earth. These rocks were mainly formed from a deposit on the flat horizontal sheets, but sometimes in many places, strata are no so horizontal, but they were wrapped. Sometimes the wrapping is so appeased that the disposition of the layers is hardly detectable, or the wrapping may be pronounced that the layers of the two flanks often be necessarily parallel or lie near the flats. Flods are high in size, and these sizes are varying from others; some of them are on the distance of hundred kilometers, some of them are on several kilometers, and some of them are a few centimeters away. The upper surface of the ample folds mainly appears on the surface of the earth.

Figure: Geological folds

Structure of Folds

The formation of the folds would be found in many sizes and shapes, and some of them are found in complicated forms. The nomenclature form of the fold is also very difficult in nature. The folds can be observed on the samples of rock, seismic images, outcrops, aerial photographs, and satellites. Folds are seen on millimeter scales in the thin sections. The proper picture of the structure of the fold can not be adequately viewed on the outcrop due to non-exposure. That's why the analysis of the fold structure is essential to understand the main elements or their anatomy.(Neville J. Price, 1990)

·         Hinge and limbs

·         Anticlines, Synclines, and monoclines

Figure: Geological fold’s Rock structure

Hinge and limbs fold

A fold is the structure of the rock, which had two surface limbs and hinge lines; the limbs are connected with the hinge line, or if we are speaking practically it is attached with a hinge zone. The maximum range of the curvature is called Hinge. The line of hinge is straight; in most of the cases this line makes a cylindrical fold, or in some other types, they may be vertical among the intersecting line and hinge line, which provide with folds in the the non-cylindrical form. As per the plunge hinge line, fold can be classified into a horizontal plunge.

The massive Folds with long hinge line riffles the strike, which will have the apogee and the downs. Plunge folds are away from apogee and plunge against the depression. The double plunging fold is the form of a hinge line that plunges in two opposite directions. The most critical anticlines are the double-plunging fold because they give the four-way dip closure for the accumulations of gas and oil.

The straight lines of the vertical and horizontals considered as the axis fold. The fold axis is a geometrically straight line that moves parallel to develop the shape of the fold. The Non-cylindrical folds do not have the fold axes. It is necessary to divide the folds into a further cylindrical shape, each with the straight hinge line and relatively short. We can represent the hinge for each layer of the rock in the structure of fold in the folded stack axel plane connected with the hinge lines. All the trace of the axel surface from where the fold passes by the hinge points,these lines are known as axil trace of the fold. By strike and dip the orientation of the axel surface can be measured. The fold can be inclined, upright,and the recumbent by the axel plane dips.

The direction toward the axel plan of the fold is called Vergence. Or we may be called this as the displacement of the lower limb with the upper limb of the fold is vergence. For example axial plane of the fold may have the strike of N25 E, and the dip is at the angle of 30 SE. Thus this is a north-west vergence.(McClay, 2013)

The symmetric fold is the upright fold, which is inclined as asymmetric. In symmetric fold, both dip limbs have the similar directions from varying angles. In the said position, forth_limb remains in his usual shape with back limb, and this is rotated more than 90, suppose to be inverted or reversed.the extreme case of overturned fold is where the recumbent fold is lie-down in the position of the axial plane. Folds that are highly overturned and had huge magnitudes are named as structures of nappes or the fold-nappes. These naps are found in the collisional mountains, and they are also found in Himalayans. The folds that had the parallel dips and two limbs with axial planned, which inclined or upright is called the isoclinal fold.

Figure: Hinge and Limbs

Anticlines, Synclines, and monoclines fold:

The antiform fold is convex upward and limbed dips downward. The fold which is concave upward and limbs dip is also upward is referred to as the synform. If we are aware of all these terminologies of the folds, then we quickly understand and use the terms synclines and anticlines. For the anticline category, the rocks tend to become older as compared to the core of the fold, and stones, still younger, are synclines folds. It is possible only when the fold antiform his shape into strata, but even younger from core, it is termed as antiformal syncline. As far as the fold with the synoform is concerned it looked older from the base is known as synformal anticline. And then fold in which all the above forms are not presented is called neutral fold.

The green belts have the regional synclines and anticlines. When the significant anticlines of the limbs are more pleated for the second & the third order anticlines is known as the anticlinorium. In the synclinorium the two major limbs of the syncline pleated for the second & the third order synclines. The parasitic folds have the second, third, and higher-order folds because they create the regional structures of the fold.

The folds that have the though crust in the symmetric shape, this crust resembles the hinge of the anti-form, and the synform’s flap. For asymmetric fold, the mountain’s coat is on the highest surface of the earth and the lowest surface of the planet. The synforms and the anti forms are often respectively called domes and basins.(Professor David Pollard, 2005)

Monoclines are the horizontal sequence of strata in the local steepening. The monocline is the sub-cylindrical fold having only one limb. For any of the structure which has the same dips and strike is called homoclines. The monocline fold is sharply bent with connected strata, which depend upon the different levels, such as horizontal, vertical with the flexure line, and this is the popular feature of the monocline. And it is also had a high surface in the many parts, including west.

Bends buckle and the flow are represented as the different classification of the folds according to their formation and mechanism. Folds are further categorized as the flexural folds; in this type of fold, layers are mechanically divided between the segments, which play the dominant role, which means the layers are moderate or low. In passive folding the layers are profoundly affected by anisotropy, and in the quasiflexural folding the fold seems to be flexural in overall folds.

Figure: Anticlines, Synclines, and monoclines

References of Classification of Fold Structure

 Henrik Bohr, S. B. ( 1994 ). Protein Structure by Distance Analysis. IOS Press.

Henrik Bohr, S. B. (1994). Classification of fold structure. Protein Structure by Distance Analysis, 335 pages.

McClay, K. R. (2013). The Mapping of Geological Structures. John Wiley & Sons.

Neville J. Price, J. W. (1990). Analysis of Geological Structures. Cambridge University Press.

Professor David Pollard, D. D. (2005). Fundamentals of Structural Geology. Cambridge University Press.