Folds, faults and joints are the main types of structural deformation in the Earth’s crustal rocks. Of these, folds often create the most spectacular geological scenes. Petroleum exploration has historically been associated with folds more than with any other geological structure. In 1861, two years after Drake’s successful discovery well in Pennsylvania, E.B. Andrews, a professor at Marietta College, Ohio published an article in the American Journal of Science, in which he observed: “I have recently found a most interesting line of uplift and dislocation… As seen in Ohio it presents a well-marked anticlinal axis but with the eastern slope more steep than the western. Near the anticlinal axis are the oil and gas springs.” In the same year, T. Sterry Hunt of the Geological Survey of Canada also proposed that oil accumulations “occur along the line of a low broad anticlinal axis which runs nearly east and west through the western peninsula of Canada” (Smithsonian Institute Annual Report, 1861). Thus was born the ‘anticlinal theory of oil accumulation.’ The industry’s fascination with folds still continues.

Fold structures are found in various shapes and sizes, and can be very complex. Nomenclature of folds can also be confusing. We can observe folds on rock samples (hand specimens), outcrops, seismic images, and on satellite and aerial photographs. Millimeter-scale folds can be observed in thin-sections. The complete picture of a fold structure may not be visible in an outcrop due to erosion or non-exposure. Therefore, to reconstruct and analyse fold structures it is important to understand their basic elements – their anatomy.

Hinge and Limbs: Fold Geometry

Chevron folds on Ribbon Chert (deepwater radiolarian chert, part of the Franciscan accretionary complex), Oregon. Source: Dr Marli Miller/University of OregonFor simplicity, let us consider a single fold. A fold is a rock structure in which two curved surfaces, or limbs (flanks), are joined at a hinge line (or a hinge point on a 2D profile) or practically speaking a hinge zone. The hinge is the line of maximum curvature. The hinge line may be straight, in which case it forms a cylindrical fold, or it may have a plunge (vertical angle between the hinge line and intersecting horizontal line) which creates a non-cylindrical fold. According to the plunge of the hinge line, we classify folds as horizontal (negligible plunge up to 10°), plunging (10–80°) or vertical (80–90°).

Large folds with long hinge lines undulating along the strike will have culminations and depressions. Folds plunge away from culminations and plunge toward depressions. A doubly-plunging fold is one in which the hinge line plunges in two opposite directions. Large doubly-plunging anticlines are especially important because they provide four-way dip closures for oil and gas accumulations.

Horizontal and vertical folds have straight hinge lines which can be considered as fold axis. A fold axis is a geometric (imaginary) straight line which when moved parallel to itself through space generates the shape of the fold. Non-cylindrical folds (with curved hinge lines) do not have fold axes, and for the purpose of detailed structural analysis (for example, stereographic representation), it is necessary to subdivide them into several cylindrical folds, each with a relatively short, nearly straight hinge line.

M.J. Fleuty (Geologists’ Association Proceedings, 1964) has proposed a classification of folds based on dip of axial plane and plunge of hinge line. This scheme is useful to characterize the geometric position of a fold.For each rock layer in a folded structure we can represent a hinge. The axial plane connects all the hinge lines in a folded stack. The axial plane is also called axial surface because it may be a curved plane. In profile (cross-section) view, the trace of the axial surface of a fold passes through all the hinge points; such line is called the axial trace of the fold. The attitude (orientation) of an axial surface is measured by its strike and dip (inclination). According to axial plane dips, a fold may be upright (dips of 90–80°), inclined (80–10°) or recumbent (10–0°).

Vergence is the direction toward which the axial plane of fold has tilted. In other words, it is the sense (direction) of displacement of the upper limb relative to the lower limb of the fold. For example, a fold axial plane may have a strike of N 25° E, and dip at angle of 30° SE, and thus a north-west vergence.

An upright fold is also a symmetric fold; inclined folds are asymmetric. An overturned fold is an inclined (asymmetric) fold in which both limbs dip in the same direction but with different angles. In this case, the backlimb (the gentler limb) retains the normal stratigraphic position while the forelimb (the steep limb), that has rotated more than 90°, possesses overturned (reversed or inverted) stratigraphy. A recumbent fold, where the axial plane is in the ‘lie-down’ position, is an extreme case of an overturned fold. Highly overturned and recumbent folds of large dimensions are sometimes called fold nappes or nappe structures. They are found in collisional mountains like the Alps and the Himalayas. An isoclinal fold is one in which the two limbs have parallel dips irrespective of whether the axial plane is upright or inclined.

Anticlines, Synclines and Monoclines

Source: Rasoul SorkhabiA fold that is convex upward, that is the limbs dip down, is called antiform, while one that is concave upward, that is the limbs dip up, is synform. If we know the stratigraphy of the folded layers, then we can respectively use the terms anticlines and synclines. In an anticline, the rocks become older toward the core of the fold; in a syncline the rocks become younger. It is also possible for the fold to have the shape of an antiform but strata become younger toward the core; it is then called an antiformal syncline. Or the fold has the shape of a synform but strata become older toward the core; it is then called a synformal anticline. A fold that is neither antiform nor synform is called neutral fold. Examples include vertical plunging folds and recumbent folds.

Orogenic belts usually have regional anticlines and synclines. When the limbs of a major anticline are further folded into second-order and third-order anticlines (composite anticlines), it is called an anticlinorium. Similarly, when the limbs of a major syncline are further folded into second-order and third-order synclines (composite synclines), it is called a synclinorium. The second, third and higher order folds are also called parasitic folds because they develop on the main, regional fold structures.

Tight folds on rocks near Moruya, New South Wales, Australia. Source: D.M. Vernon, 2006, WikipediaFolds have crests and troughs. In a symmetric (upright) fold, the crest corresponds to the hinge of the antiform, and the trough to the hinge of the synform. But in an asymmetric or overturned fold, the crest is the highest topographic part of the fold and the trough its lowest topographic part. Circular antiforms and synforms are sometimes called domes and basins, respectively.

A monocline is a local steepening of an otherwise horizontal sequence of strata. A monocline is thus a sub-cylindrical fold with only one inclined limb. Homocline (‘same inclination’) is a general term for any structures that have the same attitude (strike and dip), for example beds tilted in a parallel direction, one limb of an anticline or syncline, an isoclinal fold, or monoclines.

This is Part I in a two-part series by author Rasoul Sorkhabi. The second part – where we answer the question, how do stiff, solid rocks fold? – is available here, Part II.

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