folds and folding

folds and folding An interest in geology may be sparked by a first contact with the products of geological processes, and the resulting sense of aesthetic pleasure may lead to a lifetime's study of the Earth. A well-exposed fold can trigger this process. Something about the appearance of a fold appeals to those who have a taste for geometry, lines, or symmetry (Figs 1 and 2). Besides, how can something as ‘hard’ and ‘rigid’ as rock possibly fold?

For a rock to be folded, it must have a set of parallel surfaces and preferably layering. The deflection of this layering defines the folds (Figs 1, 2, and 3). Because most folds look different from one other, a few basic geometric terms are necessary to distinguish fold appearances (Fig. 3). Determining fold appearance in outcrops serves two essential roles. First, the geometry and style of small folds can reflect these aspects of larger associated folds of the same age. These larger folds are usually poorly exposed, and so their appearance, size, and location are therefore inferred from their smaller or parasitic brethren, which commonly are better exposed. A knowledge of larger folds is important because by virtue of their size they contribute more to rock deformation and the geometry of rock layers in the Earth.

Secondly, appearance can indicate the origin of a fold, although a particular fold morphology can be the product of more than one possible folding history. The preservation of geological features as end products that lack clear evidence for their formative history poses a constant problem in geology. As with fold geometry and origin, the final appearance of a geological feature offers geoscientists many opportunities to leap to conclusions about its formation. Many times, they subsequently discover, often with the ‘aid’ of their colleagues, that the rocks lack sufficient evidence to prove their interpretation conclusively. This problem is analogous to attempting to deduce how a book was written, while having only the final printed copy before you as a source of data.

Let us establish the vocabulary of a few basic geometric terms (Fig. 3). The zone of maximum curvature in the fold is the hinge zone or fold nose. The portions of layer that connect the hinge zones of adjacent folds are fold limbs. The inner arc of a fold is its core. The two types of fold are antiforms that fold up and over (A, Fig. 3), and synforms that fold down and under (S, Fig. 3). The axial surface is an imaginary surface that connects the hinge zones of the various folded layers and commonly bisects the angle between adjacent fold limbs. The shape of a fold is a function of the thickness of a layer with respect to a referent; for example a thickness measured at right angles to the surface of a layer (Fig. 3) or the thickness of the axial surface. If the thickness of a particular layer is constant normal (perpendicular) to the layer, the shape of the fold is parallel. If the thickness is constant parallel to the axial surface, the shape is similar.

We can now apply this vocabulary to Fig. 1 to see what can be gained. The metamorphic rocks from the Swiss Alps illustrated contain a series of asymmetric folds with limbs of differing length and axial surfaces tilted to the right. The asymmetry is an important interpretative tool. For example, these folds can be parasitic to larger unexposed folds. Because the antiforms close to the right and the axial surfaces tilt to the right, the asymmetry indicates that the host antiform hinge zone is to the right of the photograph and the host synform hinge zone is to the left. This information provides a key for a field geologist who is constructing a geological map. The map shows the surface distribution or outcrop of the rocks, and large map-scale folds strongly control outcrop patterns. The importance to non-geologists of determining rock outcrop in an area includes: locating weak rock types that favour mass wasting; finding sources of resistant rocks for road metal or building; locating the surface outcrops of aquifers and aquicludes that control the distribution of groundwater; and finding natural resources such as coal horizons or subsurface traps of hydrocarbons.

Another topic on which the fold asymmetry of Fig. 1 can shed some light is the regional deformation pattern. The asymmetry can indicate that during Alpine mountain building, rocks moved from left to right, as seen in the photograph. Small-scale data of this kind gathered from a large number of outcrops across the Alps, provide a basis for interpreting the overall horizontal and vertical displacements of the rocks that formed the mountains. In general, the basic pattern of rock motion in this region is one of shortening by rocks being displaced horizontally in one dominant direction. In the Alps, data from widely distributed outcrops of asymmetrical folds indicate that the mountains formed by Italy (part of the African plate) being driven north into Europe with concurrent vertical uplift. Another feature of Fig. 1 is the fold shapes of the layers. Light-coloured layers are mostly parallel in shape, whereas medium to dark grey layers are more similar in shape. This difference can indicate that the darker layers sustained more flow during fold formation.

A long narrow fold with parallel or isoclinal limbs closes to the right in Fig. 2. The fold is neither an antiform nor a synform; because it is lying on its side, it is called recumbent. Such folds are common in high-grade metamorphic rocks in the cores of mountain belts. They result from rocks travelling large horizontal distances by flow, including folding. The left-hand side of Fig. 2 illustrates another common feature in such settings, where two younger folds refold or overprint an older fold. They deform both the fold and its axial surface, producing a zigzag interference pattern. The key observation from this pattern is that the rocks were deformed more than once. The rule that younger structures deform older structures is the essence of interpreting the relative ages of multiple deformations. This is shown in Fig. 2, where younger folds deform older folds. The angular relationships between axial surfaces and hinge zones of different fold phases are used to classify the types of refolding and to provide insight into changes of regional shortening direction during mountain-building.

The various types of information listed above also have a role in determining how a fold developed. When one looks at rocks on the ground surface, it is very difficult to believe that anything so rigid could fold. However, almost all folds in rocks form in the subsurface region at depths in the crust of about 1 to 45 km. Buried rocks in the crust are subjected to elevated temperatures, pressures, and fluid pressures that trigger deformation mechanisms, allowing tectonic forces to drive folding processes.

Three processes produce folds through processes called buckling, bending, and passive folding (Fig. 4). During buckling, compression is applied parallel to the layers, which are of differing strengths (Fig. 4b). The compression causes shear stresses at a buckling instability, deflecting layers and hence initiating a fold. Most of the small and medium-sized folds seen in exposures of sedimentary and low-grade metamorphic rocks are buckle folds.

In contrast, bending folds develop when compression is normal to layering (Fig. 4b). The most common types of bending folds form above salt domes and as fault-bend folds. Salt rises into domes because it is much less dense and much weaker than most rocks. When a more dense rock buries salt, a gravitational instability exists because less dense materials are more stable when they are above more dense substances. So, the salt tends to escape vertically by doming and bending the layers above it as a tight, almost isoclinal fold of large proportions. Fault-bend folds occur when rocks bend during translation through a change in fault dip.

The third process, passive folding, unlike the first two, produces folds as an artefact because layering has no mechanical significance (Fig. 4c). The rock layers are not mechanically active; they merely record passively the deformation by changing shape. This type of process is particularly common in thick sequences of fine-grained sedimentary rocks and high-grade metamorphic rocks. These sequences lack the strength and density contrasts which aid buckling and bending.

The three folding processes can operate simultaneously in adjacent rock bodies, or sequentially in the same rock sequence if deformation conditions change. The processes do not always yield folds with distinctly different appearances. As a result we cannot make a simple correlation between appearance and process. The focus here will be on aspects of the development of buckle folds.

Two common kinematic behaviours, which are combinations of distortion, rotation, and translation, occur during buckling. One is flexural slip or flow, in which layers in the fold limbs move parallel to layering and each other. With flexural slip, this movement is restricted to slip on layer surfaces. An analogous behaviour is to flex (fold) a pack of cards. The cards arch by sliding over one another. If you have formed an antiform, the cards on top slide toward the hinge zone relative to those underneath. The same is true in a natural fold where layers slip from synforms to antiforms. Flexural flow distorts layers by layer-parallel shear, rather than by slip at layer boundaries as with flexural slip.

This ability to slip or flow parallel to the layering greatly weakens the rock, and hence makes folding much easier to initiate. Again, an analogy shows this result. Take two telephone books, cut off their bindings, remove their hard covers, and glue all the pages of one book together. If you flex (fold) these two ‘books’, the one with unglued pages will fold easily because the pages will slide on each other. The glued ‘book’, however, requires much more force because it is a single mechanical unit with no ability to slip.

The glued telephone ‘book’ would most probably fold by the process of tangential longitudinal strain. The behaviour is exactly the opposite of flexural slip because deformation is concentrated in the hinge zone as distortion and is not distributed through the fold limbs as slip. It is common in thick, strong rock layers or layers that are unable to slide. The hinge collapses by extending the outer arc of the layer and contracting the inner arc (synform S in Fig. 3). Veins, intragranular processes such as twinning, dislocation glide, and grain-boundary migration, or normal faults achieve the outer-arc extension. Inner-arc contraction can be by cleavage development, parasitic folding, reverse faulting, or intragranular processes. An interesting aspect of this behaviour is that the distortion pattern changes from the inner arc to the outer arc: the strain progresses from decreasing contraction to no deformation at a neutral point to increasing extension. The strain is thus positionally dependent within the hinge zone.

When examining an exposure of buckle folds, it is tempting to think that they form simultaneously. Yet, when rock layers compress parallel to layering, folding becomes a race between instabilities and later, fold amplification. Any rock sequence contains many imperfections that act as buckling instabilities, such as nonplanar layer surfaces, differentially cemented layer surfaces, or heterogeneously distributed minerals such as mica or clay minerals. Each instability generates a fold of a particular wavelength, which is a measure of fold width much like the wavelength of ocean waves. A layer has a dominant wavelength during buckling that grows more efficiently as a function of layer thickness and strength. Those instabilities that yield wavelengths closest to the dominant one grow much faster and becomes the visible folds. This selection process is enhanced by folds growing even faster when they amplify to a critical size, as would be the case for dominant folds.

Another feature of this growth process is that an amplifying fold will deflect adjacent layering and initiate two more folds (Fig. 3). Thus, folds grow serially along a layer from the initial buckling instability as they are also amplifying. Such a growth sequence is also possible during bending folds when one salt dome triggers an adjacent one, but it is absent during passive folding.

William M. Dunne

Bibliography

Price, N. J. and and Cosgrove, J. W. (1990) Analysis of geological structures. Cambridge University Press.
Ramsay, J. G. and and Huber, M. I. (1987) The techniques of modern structural geology, Vol. 2: Folds and fractures. Academic Press, London.