In the nineteenth century, the predominant theory was that since its hot, molten formation, the Earth has been slowly cooling and thus shrinking and collapsing, like the skin of a drying apple, which produces mountain-building structures such as faults and folds. This theory has been refuted and is outdated. With the advent of plate tectonic theory and structural geologic research grounded in field mapping, mathematical analysis, analogue experiments and numerical simulations, we now have sophisticated and detailed explanations for rock folds. In Part I of this article (GEO ExPro, Vol. 10, No. 3 and in GEO ExPro App Edition No. 7, July 2013), we looked at the geometry and shape of folds. Part II discusses folding processes in relation to their form and development.
Rock folds come in various sizes, from micro-structure scale features observable on a thin-section of a rock sample to mountain-size folds. Nevertheless, folds in geology are what waves are in physics. Therefore, certain geometric measures can characterise rock folds, and these can be used to analyse them.
The size of a single fold is measured by its height (distance between crest and trough), width (distance between the inflection points bounding a fold), and amplitude (distance from crest to width, measured parallel to the axial plane). Another measure to note is wavelength or distance between two consecutive crests or troughs. The aspect ratio of a fold is the ratio of its amplitude to its width. Robert Twiss of University of California at Davis (Journal of Structural Geology, 1988) has suggested the following terms to describe a fold’s aspect ratio: wide (0.1 to <0.25); broad (0.25 to <0.63); equant (0.5 to 2.0); short (1.50 to <4); and tall (4 to <10).
Tightness of a fold is measured by its interlimb angle (the angle between the two limbs of a fold). In a classic paper published in 1964 (Proceedings of the Geologists’ Association) Michael Fleuty of Imperial College in London outlined several categories for fold tightness. The following is modified from his paper: gentle (interlimb angle <180° to 170°), broad (170° to 120°), open (120 to 70°), closed (70° or 30), tight (<30° to 10°), and isoclinal (<10° and the limbs have the same dip).
In a stack of sedimentary layers, each individual layer may deform independently due to its own or surrounding rock properties. Therefore, changes in fold wavelength and amplitude among layers result in disharmonic folds. In a sedimentary stack where folds keep the same shape across the layers they are called harmonic folds.
A fold is a kind of strain in the rock, and knowledge of the attitude and dimensions of folds helps us understand the direction and degree of stresses that have deformed the rock.
Bends, Buckles and Flows
Fred Donath and Ronald Parker in 1964 (GSA Bulletin) presented a totally different classification of folds based on the mechanism of their formation, for which they considered the ‘mean ductility’ and ‘ductility contrast’ in the folded strata. On this basis, folds are categorised into flexural folding in which layering and mechanical anisotropy between the layers play the dominant role (in other words, mean ductility is low to moderate); passive folding in which interlayer anisotropy is ineffective (mean ductility is high); and quasiflexural folding in which the geometry of the fold appears to be flexural but the overall behaviour of the folded sequence is passive (mean ductility is very high). The last category largely corresponds to disharmonic folding. The first two categories, flexural and passive, can be further subdivided into slip (between layers) and flow (within layers).
The Donath and Parker classification directs us to the genetic mechanisms of folding and the tectonic environments in which folds form. Three distinct mechanisms have been identified for the folding of rocks: bending, buckling, and passive folding.
Bending of rocks occurs when the deforming force is applied across (at high angle to) rock layers. For example, basement uplift along a fault, magma intrusion or salt diapirs all produce bends (folds) in the overlying sedimentary rocks. Bending often produces gentle or broad folds, especially in continental interiors (cratons) situated far from plate boundaries but subjected to some vertical stresses.
Buckling occurs when the deforming force is applied parallel to rock layers. This is usually caused by horizontal compressional tectonic forces and results in layer-parallel shortening of rocks and thickening (relief) of the rock body perpendicular to stress direction. Geologists have worked out mathematical relationships between wavelength of a buckle fold and thickness of the stiff layer embedded in a ductile rock mass. As a general rule, in a given stress field and ductility contrast between the layers, thicker stiff beds will have longer fold wavelengths and thinner stiff beds will have shorter wavelengths.
Bending and buckling may also be described as two modes of active folding in which rock layers with their inherent mechanical properties (notably stiffness or ductility) take part in the deformation process and control the fold shape. In contrast, in passive folding, rock layering itself does not play an active role in folding, and instead the rock mass as a whole is subjected to folding and is usually marked by penetrative cleavage developed in a direction nearly parallel to the axial surface of fold. Rock cleavage is a set of planar discontinues that develop as secondary features in the rock fabric; it also refers to the ability of a rock to split (cleave) along those planes. Well developed (continuous, spaced) cleavage occurs at temperatures of 200–350°C, corresponding to burial depths of 7–12 km.
Passive folding takes place in a mechanically isotropic rock mass and on a grain scale rather than a layer scale. Passive folds may be subdivided into passive-slip folds and passive-flow folds. In passive-slip folds there is minor but discrete displacement across (perpendicular to) rock layers and more conveniently along cleavage planes; they are also called shear folds. In passive-flow folds, there is across-layering material flow in a ductile environment and in the direction of folding.
Passive folding produces similar folds in which the fold shape is preserved throughout the layered sequence because of the lack of mechanical differences between layers. Examples of passive folding include folding of rocks in ductile shear zones and drag folds along brittle faults.
Flexural Slip and Flexural Flow
Bending and buckling produce flexural folds in which (as stated before) viscosity contrast between competent (stiff) and incompetent (ductile) rock layers plays an important role in the folding process. In flexural folds, competent layers do not change their thicknesses and incompetent layers are marked by cleavage sets nearly parallel to fold axial surface. Flexural folds are the most common folds in sedimentary basins.
Flexural folds are subdivided into flexural-slip folds and flexural-flow folds. In flexural-slip folds, there are displacements along bedding surfaces, much like the bending of a telephone directory book. These slips are greatest along the fold limbs and approach zero along the fold hinge. Flexural slip typically produces parallel or concentric folds in which the attitude and thickness of layers remain the same throughout the folded sequence. (For illustrations of similar, parallel and concentric folds, see Folds and Folding I, GEO ExPro, Vol. 10, No. 3.)
In flexural-flow folds, rock material in incompetent layers flows from fold limbs toward fold hinges, and therefore appreciable thickness changes occur in the rock layer. Obviously, flexural-flow requires more ductility contrast between layers than flexural slip. Flexural flow produces similar folds in the weak layers.
Free and forced folding
In free folding, rock layers are free to exert their mechanical properties on the development and shape of the folded stack and thus layer-parallel strain dominantly takes place. Buckling discussed above typically produces free folds.
In forced folding, the shape and geometric features of the folded stack are ‘forced on’ the layers usually by a fault that is the primary structure. In this case, to quote American geologist George Davis in his textbook Structural Geology (1996), the rock layers “just go along for a ride.” Notable examples of forced folding include drape fold (folding of sediments overlying a high-angle basement fault), faultbend fold (bending and slip of an anticlinal fold as a thrust block overrides the footwall block along a ramp), and fault propagation fold (asymmetric bending of rock strata along a thrust ramp). In these examples, folding depends on faults, and bending is the main process of folding.
Folds and petroleum fields
In summary, various forces produce rock folds. Horizontal compression resulting from the motion of tectonic plates is the most dominant force, which produces series of regional and basin-scale folds. But other forces can also create localised or even widely distributed fold structures. These include vertical stresses (such as magma intrusion or basement upwarping); slope instability (such as rollover anticlines on normal-fault surfaces and toe-thrust folds on continental slopes); and density instability (for example, salt diapirs below denser sediments).
Folds are important structures to study in petroleum fields for a variety of reasons. Large folds provide important petroleum traps, such as anticlines or fold-bend folds in foreland basins, rollover anticlines in extensional basins, and deepwater toe-thrust folds. In these kinds of traps, three-dimensional mapping of fold structures are thus necessary for reserve estimates. In addition folding creates natural fractures which provide crucial permeability for oil and gas production in tight reservoirs, which is why curvature analysis of rock strata is sometimes made on seismic images to gain an understanding of the distribution and relative population of fractures.
This is Part II in a two-part series by author Rasoul Sorkhabi. The first part - where we unfold rocks to better understand their geometry and genesis - is available here, Part I.