Folds and Folding

Chapter ................................ 11 Folds and folding Folds are eye-catching and visually attractive structures that can form in practically any rock type, tectonic setting and depth. For these reasons they have been recognized, admired and explored since long before geology became a science (Leonardo da Vinci discussed them some 500 years ago, and Nicholas Steno in 1669). Our understanding of folds and folding has changed over time, and the fundament of what is today called modern fold theory was more or less consolidated in the 1950s and 1960s. Folds, whether observed on the micro-, meso- or macroscale, are clearly some of our most important windows into local and regional deformation histories of the past. Their geometry and expression carry important information about the type of deformation, kinematics and tectonics of an area. Besides, they can be of great economic importance, both as oil traps and in the search for and exploitation of ores and other mineral resources. In this chapter we will first look at the geometric aspects of folds and then pay attention to the processes and mechanisms at work during folding of rock layers. 220 Folds and folding 11.1 Geometric description (a) Kink band a a It is fascinating to watch folds form and develop in the laboratory, and we can learn much about folds and folding by performing controlled physical experiments and numerical simulations. However, modeling must always be rooted in observations of naturally folded Trace of Axial trace rocks, so geometric analysis of folds formed in different bisecting surface settings and rock types is fundamental. Geometric analy- (b) Chevron folds sis is important not only in order to understand how various types of folds form, but also when considering such things as hydrocarbon traps and folded ores in the subsurface. There is a wealth of descriptive expressions in use, because folds come in all shapes and sizes. Hence we will start this chapter by going through the basic jargon related to folds and fold geometry. Shape and orientation (c) Concentric folds Folds are best studied in sections perpendicular to the folded layering, or perpendicular to what is defined as the axial surface, as shown in Figure 11.1. Unless indi- cated, we will assume that this is the section of observation in this chapter. In general, folds are made up of a hinge that connects two usually differently oriented limbs. The hinge may be sharp and abrupt, but more commonly the (d) Box fold Axial trace curvature of the hinge is gradual, and a hinge zone is Axial trace defined. A spectrum of hinge shapes exists, from the pointed hinges of kink bands and chevron folds (sharp and angular folds) to the well-rounded hinges of concentric folds (Figure 11.2). Classification of folds relative to hinge curvature is referred to as bluntness. Figure 11.2 (a) Kink band, where the bisecting surface, i.e. the The shape of folds can also be compared to mathemat- surface dividing the interlimb angle in two, is different from ical functions, in which case we can apply terms such as the axial surface. (b) Chevron folds (harmonic). (c) Concentric amplitude and wavelength. Folds do not necessarily folds, where the arcs are circular. (d) Box folds, showing two show the regularity of mathematical functions as we know sets of axial surfaces. Figure 11.1 Geometric aspects Axial surface Fold of folds. Hinge points axis Hinge line Hinge zone Axial trace Inflection point Limb Inflection line Limb Ampli- Hinge zone Non-cylindrical tude Wavelength fold Enveloping surface Interlimb angle Cylindrical fold 11.1 Geometric description 221 Shape categories (a) AB CDE F 1 2 Fold axis 3 Cylindrical fold Amplitude 4 (b) 5 Hinge line Figure 11.3 Fold classification based on shape. From Non-cylindrical fold Hudleston (1973). Figure 11.4 Cylindrical and non-cylindrical fold geometries. them from classes of elementary algebra. Nevertheless, simple harmonic analysis (Fourier transformation) has been somewhere, or transfer strain to neighboring folds (Box applied in the description of fold shape, where a math- 11.1), but the degree of cylindricity varies from fold to ematical function is fitted to a given folded surface. The fold. Hence, a portion of a fold may appear cylindrical as form of the Fourier transformation useful to geologists is observed at outcrop (Figure 11.5), even though some curvature of the axis must exist on a larger scale. fxðÞ¼b sin x þ b sin 3x þ b sin 5x ... ð11:1Þ 1 3 5 Cylindricity has important implications that can be This series converges rapidly, so it is sufficient to consider taken advantage of. The most important one is that the only the first coefficients, b1 and b3, in the description of poles to a cylindrically folded layer define a great circle, natural folds. Based on this method, Peter Hudleston and the pole (p-axis) to that great circle defines the fold prepared the visual classification system for fold shape axis (Figure 11.6a). When great circles are plotted instead shown in Figure 11.3. of poles, the great circles to a cylindrically folded layer In multilayered rocks, folds may be repeated with simi- will cross at a common point representing the fold axis, lar shape in the direction of the axial trace, as seen in Figure in this case referred to as the b-axis (Figure 11.6b). This 11.2a–c. Such folds are called harmonic. If the folds differ method can be very useful when mapping folded layers in wavelength and shape along the axial trace or die out in in the field, but it also works for other cylindrical struc- this direction they are said to be disharmonic. tures, such as corrugated fault surfaces. The point of maximum curvature of a folded layer is Another convenient property of cylindrical folds is that located in the center of the hinge zone and is called the they can be projected linearly, for example from the sur- hinge point (Figure 11.1). Hinge points are connected in face to a profile. Cylindricity is therefore commonly three dimensions by a hinge line. The hinge line is assumed when projecting mapped structures in an area commonly found to be curved, but where it appears as onto cross-sections, particularly in the early 1900 map- a straight line it is called the fold axis. ping of the Alps by Swiss geologists such as Emile Argand This takes us to an important element of fold geom- and Albert Heim. Since the validity of such projections etry called cylindricity. Folds with straight hinge lines are relies on the actual cylindricity of the projected structures, cylindrical. A cylindrical fold can be viewed as a partly the uncertainty increases with projection distance. unwrapped cylinder where the axis of the cylinder defines The axial surface,oraxial plane when approximately the fold axis (Figure 11.4a). At some scale all folds are planar, connects the hinge lines of two or more folded non-cylindrical, since they have to start and end surfaces. The axial trace of a fold is the line of intersection 222 Folds and folding BOX 11.1 FOLD OVERLAP STRUCTURES Individual folds can overlap and interfere. Just like faults, they initiate as small structures and interact through the formation of overlap or relay structures. Fold overlap structures were first mapped in thrust and fold belts, particularly in the Canadian Rocky Mountains, and many of the fundamental principles of fault overlap structures come from the study of fold populations. Fold overlaps are zones where strain is transferred from one fold to another. Rapid changes in fold amplitude characterize fold overlap structures. Overlap zone of the axial surface with the surface of observation, typic- ally the surface of an outcrop or a geologic section. The axial trace connects hinge points on this surface. Note Figure 11.5 Cylindrically folded granitic dike in that the axial surface does not necessarily bisect the limbs amphibolite. (Figure 11.2a). It is also possible to have two sets of axial surfaces developed, which is the case with so-called box folds, which are also called conjugate folds from the characteristic conjugate sets of axial surfaces (Figure (a) N (b) 11.2d). In other cases, folds show axial surfaces with variable orientations, and such folds are called polyclinal. The orientation of a fold is described by the orientation of its axial surface and hinge line. These two parameters can be plotted against each other, as done in Figure 11.7, and names have been assigned to different fold orienta- p b 15/106 tions. Commonly used terms are upright folds (vertical axial plane and horizontal hinge line) and recumbent folds (horizontal axial plane and hinge line). Most of the folds shown in Figure 11.7 are antiforms. Figure 11.6 Measurements of bedding around a folded An antiform is a structure where the limbs dip down conglomerate layer. (a) Poles to bedding plot along a great and away from the hinge zone, whereas a synform is the circle. The pole to this great circle (p-axis ¼ 15/106) represents opposite, trough-like shape (Figure 11.8b, c). Where a the fold axis. (b) The same data plotted as great circles. For a stratigraphy is given, an antiform is called an anticline perfectly cylindrical fold the great circles should intersect at the where the rock layers get younger away from the axial point (b-axis ¼ 15/106) representing the fold axis. Data from surface of the fold (Figures 11.8e and 1.6). Similarly, a the fold shown in Box 3.2. 11.1 Geometric description 223 Figure 11.7 Classification of folds based on the orientation of the hinge line and the axial surface.

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