Geologic folds

Folds are the warping or bending of strata, foliation, or rock cleavage from an original horizontal or undeformed position into high and low areas. A fold is generally considered to be a product of deformation. Much of the folding of the earth's crust occurs near or along lithospheric plate boundaries and is considered a result of compressional stress.

Rock Deformation

Folding is a common type of deformation seen in crustal rocks. It is evident in the dipping (sloping) beds of mountains, where fold axes most often extend parallel to the length of the mountain chains. The folded beds may be observed in cliffs, roadcuts, and quarries, where the observer can see not only the steeply dipping beds but also some of the smaller anticlines (upward-arched folds) and synclines (downward-arched folds). When viewed from the air, mountain chains are often found to be composed of sinuous ridges of sedimentary strata, such as limestone and sandstone, which are more resistant to erosion. At the same time, valleys are underlain by rocks such as shale, which are more susceptible to erosion. This phenomenon can easily be seen from the highway west of Denver, Colorado, in the front ranges of the Rocky Mountains, or near Harrisburg, Pennsylvania, where Interstate Highway 81 follows a ridge north for miles toward Scranton.

In the North American midcontinent region, folds are less obvious, and the beds dip with inclinations of as little as 0.5 degrees and upward to about 5 degrees. The folds are usually only discernible by surveying techniques. Occasionally, there are very large folds in this region, hundreds of miles across and with very low dips. These folds are more circular than elongate in shape and are referred to as basins (downfolds) and arches, or domes (upfolds). Examples are the Michigan Basin, the Cincinnati Arch, and the Nashville Dome.

Rock Mechanics

Rock mechanics is the study of the mechanical behavior of rocks. One branch of this study is concerned with the response of rocks to force fields in their geologic environments. Associated with this field is the study of plate tectonics, which relates the large-scale structures of the earth to these forces. For example, in the crust of the earth there are differential forces caused by the interaction of the lithospheric plates, which are associated with the processes of plate tectonics as well as with hydrostatic (fluid) and lithostatic (gravity) pressures. Each may cause folding. The processes of plate tectonics form folded mountain chains at subduction zones. Differential lithostatic pressures associated with differences in specific gravity of different rocks and the effects of gravity form folds as in salt domes; lithostatic pressure, however, may act only as a confining pressure.

Rocks under stress behave in an elastic, plastic, or brittle manner, as do all solid materials. The sequence of occurrence relates these three properties during deformation. Solid materials, when first subjected to a force, behave elastically. That is, they change shape or volume, but if the force is removed they return to their original shape. If, however, the force is increased, permanent deformation occurs: First, the material changes shape plastically, and then it becomes brittle, breaking apart. Factors that affect the manner in which a rock behaves are time, temperature, confining pressure, and the pore pressure of water. Higher temperatures, water in pore space, lower confining pressures, or a longer length of time during which the rocks are subjected to a constant force tends to weaken rocks. Conversely, lower temperatures, lack of water, higher confining pressure, or a short period of time during which the force is applied tends to strengthen rocks.

Classification of fold types is a means by which structural geologists group the various kinds of folds in order to understand them and their origins. There are five major divisions in fold classification: descriptive or geometric, morphologic, by mechanics of origin and internal kinematics, by external kinematics and tectonic forces, and by position in the tectonic framework.

Descriptive Classifications

Descriptive or geometric classifications are based primarily on shape—that is, the attitude of the limbs, axial plane, and hinge line of the fold. The major use is for geologic mapping. The classification most commonly used is the attitude of the axial plane or the appearance of a fold in cross-section or vertical section normal to its axis. In this classification there are symmetrical folds, in which the axial plane bisects the fold; asymmetrical folds, in which the limbs have different dips and are therefore asymmetrically disposed about the axial plane; overturned folds, in which one limb has been rotated or tilted through the vertical so that the original bed is upside down; and recumbent folds, which have been rotated so that their axial planes are nearly horizontal. Plunging folds are those in which the axis of the fold is not horizontal. Upright folds have vertical axial planes, and inclined folds have inclined axial planes.

Another descriptive classification is based upon fold symmetry: Orthorhombic folds have the axial plane and the plane normal to the axis of the fold as symmetry planes (mirror planes); monoclinic folds have either the axial plane or the plane normal to the axis of the fold as a symmetry plane but not both; and triclinic folds are folds with no symmetry planes.

A third descriptive classification is based on the orientation of the axes of a fold. It includes cylindrical folds, which are described by the rotation of a line parallel to itself at a fixed distance from a central point and with parallel hinges (noncylindrical folds are folds with randomly oriented axes), and conical folds, which have axes divergent from an apex. A descriptive classification based upon flanks of folds includes isoclinal folds, with both flanks essentially parallel; open folds, which may be folded more tightly without rock flowage; and closed folds, which may not be folded more tightly without rock flowage. Monoclines are folds in which the strata dip or flex from the horizontal position in one direction only and are not a part of an anticline or syncline.

Morphological Classifications

Morphological classifications are based upon the shape of the fold in regard to depth, map view, and spatial relationships with adjacent folds. The changes in shapes and patterns formed by folds are not always apparent when an individual fold is viewed in the field; therefore, the distinction is made between descriptive and morphological classifications. Concentric or parallel folds maintain a constant thickness of beds, which means an anticline will decrease in size downward, whereas a syncline will decrease in size upward. Similar folds are bent into similar curves and do not increase or decrease in size downward, but rather maintain curves by thinning of flanks and thickening of crests and troughs. Disharmonic folds are different beds in a sequence of strata that have different amplitudes and wavelengths. Supratenuous folds die out downward. Nappes are large recumbent anticlines generally isolated by thrust faults.

Folds may also be considered in groups. Structural salients are observed when a sequence of folds is viewed in map view, and the curved fold axes are oriented convex toward the outer edge of the fold belt. Embayments are observed when a sequence of folds is seen in map view, and the curved fold axes are concave toward the outer edge of the fold belt. Homomorphic folding is the condition where folds cover an entire area, whereas idiomorphic folding is the condition where there is an intermittence of fold locality and often the folds are not linear in habit. Anticlinoriums are a series of anticlines and synclines forming a large arch, whereas synclinoriums are a series of anticlines and synclines forming a large trough. Both are generally tens of kilometers across. En echelon folds are a series of folds whose lengths are not extreme but that have axes that overlap in an oblique manner.

Classification by Mechanics of Origin and Internal Kinematics

Fold classifications based on mechanics of origin and internal kinematics describe the processes of folding in rocks. The mechanisms of folding reflect the elastic, plastic, or brittle manner of deformation dominant at different times during folding. Most folds appear, superficially, to be the result of plastic deformation. If, however, one takes a plastic material and pushes it from both sides, only the edges are deformed. In the case of mountain chains, there are often wide fold belts, which suggests that stress was transmitted equally across the width of the fold belt—the result of elastic deformation. The study of folds in thin sections (a thin slice of rock 0.03 millimeter thick prepared for viewing under the microscope), however, may show folding developing along minute shear planes and, therefore, must be the result of brittle deformation.

Flexure folding or flexural-slip folding, at times called true folding, is a form of elastic deformation. It can be demonstrated by taking a sheet of paper and folding it into 1-inch accordion pleats. When stretched out a little, then compressed slowly, the folds become tighter; when released, the folds will spread out again—elastic rebound. In folding the paper, permanent folds were induced, which is plastic deformation, but the entire “fold belt” behaves elastically. What happens vertically can be demonstrated by opening a book to the middle, flat on the tabletop. The pages will be bent or folded on each side of the binding into anticlines. Folding of the book has proceeded by each page sliding upward relative to the page below, resulting in a narrow edge of all pages being visible at the side. Here again, there is only elastic deformation because the pages of the book are not folded permanently. Shear folding—an example of brittle deformation—results from minute displacements along closely spaced fractures, which can be demonstrated with a stack of playing cards. Draw a vertical line in the middle of one side when the cards are neatly stacked and another line on the opposite side, then hold the deck of cards on end so that the line is horizontal. Next, indent the middle of the deck on the upper side, forming a U-shaped trough. The original line is now bent into a synclinal form by each card slipping relative to the adjacent card.

Classification by External Kinematics and Tectonic Forces

Fold classifications based on external kinematics and tectonic forces focus on external causes of folding. Block folding results from block uplift. Injection folding results from pluton emplacement. Folds caused by general crumpling can be attributed to two basic hypotheses: the tangential-compression hypothesis, or bench vise concept, and the vertical-tectonic concept, with material movement resulting from specific gravity differences. The first hypothesis assumes crustal shortening; the second assumes little or no difference in crustal length. Gravity gliding results when uplift occurs and rock masses move downslope along faults, producing folds and nappes. Rotational folding results from the rotation of blocks of the earth's crust, which are generally bounded by major faults. Regional coupling results from drag on blocks of the earth's crust sliding past one another. Differences in specific gravity produce folds in conjunction with the force of gravity. Two examples are salt anticlines and domes. Differential compaction produces folds where beds are draped over areas of less compaction. Ice shove may produce folds when a glacier encounters frozen, loosely consolidated rocks and sediments and shoves them into folds. Geosynclines are mobile downwarpings of the earth, generally elongate but also basinlike, measured in hundreds of kilometers, which subside as sedimentary and volcanic rocks accumulate to thicknesses of thousands of meters. This part of the tectonic cycle is often followed by orogeny, or mountain-building processes. Geosynclines are related to subduction zones of the plate tectonics theory. Geanticlines are mobile upwarpings of crustal material of regional extent; this term is particularly applied to anticlinal structures developed in a geosyncline.

Classification by Position in the Tectonic Framework

Classification of folds based on position in the tectonic framework is a way of describing components of a mountain chain. There have been few attempts to devise a formal classification, but there are fold types characteristic of particular tectonic regimes. A belt of crustal folds and metamorphism, generally centrally located within a mountain chain, is a region of shear folding vertical uplift. Examples include the Pennine Nappes of the Alps and the Piedmont and New England upland provinces of the Appalachian Mountains. The outer belt of shallow folding and thrusting of mountain chains is characterized by sedimentary rocks folded into anticlines and synclines with associated thrust faults and is often underlain by a plane of décollement. Examples of this portion of a mountain chain include the Valley and Ridge Province of the Appalachian Mountains, the Front Ranges of the Rocky Mountains, and the Jura Mountains of Europe.

Principal Terms

anticline: an arched upward fold of stratified rocks from whose central axis the strata slope downward; at the center, it contains stratigraphically older rocks

axial plane: a surface connecting all hinges of a fold that may or may not be planar; that is, the axial plane of a fold may vary from a flat plane to a complexly folded plane

axis: a line parallel to the hinges of a fold, also called the fold axis or hinge line

crestal plane: a plane or surface that goes through the highest points of all beds in a fold; it is coincident with the axial plane when the axial plane is vertical

flanks: a term describing the sides of a fold, also called limbs, legs, shanks, branches, or slopes; anticlines share syncline flanks, and synclines share anticline flanks

hinge: the line of maximum curvature or bending of a fold

plunge: the inclination and direction of inclination of the fold axis, measured in degrees from the horizontal

syncline: a downward bent fold of stratified rock from whose central axis the strata slope upward; at the center, it contains stratigraphically younger rocks

tectonics: the study of the form, pattern, and evolution of large-scale units of the earth's crust, such as basins, geosynclines, and mountain chains

trough: a line occupying the lowest points of a bed in a syncline; the trough plane connects the lowest points on all beds

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