Folds and Foliations

Folds and Foliations

Geometry of Folds Imagine a carpet lying flat on the floor. Push on one end of the carpet, and it will wrinkle or contort into a series of wavelike curves. Stresses developed during mountain building can similarly warp or bend bedding and foliation (or other planar features) in rock. The result a curve in the shape of a rock layer is called a fold.

Geometric characteristics of folds.

Not all folds look the same some look like arches, some look like troughs, and some have other shapes. To describe these shapes, we must first label the parts of a fold (figure above a). The hinge refers to a line along which the curvature is greatest, and the limbs are the sides of the fold that display less curvature. The axial surface is an imaginary plane that contains the hinges of successive layers and effectively divides the fold into two halves. With these terms in hand, we distinguish among the following: 

• Anticlines, synclines, and monoclines: Folds that have an arch-like shape in which the limbs dip away from the hinge are called anticlines (figure above a), whereas folds with a trough-like shape in which the limbs dip toward the hinge are called synclines (figure above b). A monocline has the shape of a carpet draped over a stair step (figure above c). 
• Non-plunging and plunging folds: If the hinge is horizontal, the fold is called a non-plunging fold, but if the hinge is tilted, the fold is called a plunging fold (figure above d). 
• Domes and basins: A fold with the shape of an overturned bowl is called a dome, whereas a fold shaped like an upright bowl is called a basin (figure above e, f). Domes and basins both display circular outcrop patterns that look like bull’s-eyes the oldest layer occurs in the centre of a dome, whereas the youngest layer is located in the centre of a basin. 

Characteristics of folds on outcrops and in the landscape.

Using these terms, now see if you can identify the various folds shown in figure above a–e.

Formation of Folds 

Fold development in flexural-slip and passive flow-folding.

Folds develop in two principal ways (figure above a, b). During formation of flexural-slip folds, a stack of layers bends, and slip occurs between the layers. The same phenomenon happens when you bend a deck of cards to accommodate the change in shape, the cards slide with respect to each other. Passive-flow folds form when the rock, overall, is so soft that it behaves like weak plastic and slowly flows; these folds develop simply because different parts of the rock body flow at different rates. 

Folding is caused by several different processes, as illustrated by the following cross sections.

Why do folds form? Some layers wrinkle up, or buckle, in response to end-on compression (figure above a–d). Others form where shear stress gradually shifts one part of a layer up and over another part. Still others develop where rock layers move up and over step-like bends in a fault and must curve to conform with the fault’s shape. Finally, some folds form when new slip on a fault causes a block of basement to move up so that the overlying sedimentary layers must warp.

Tectonic Foliation in Rocks 

In an undeformed sandstone, the grains of quartz are roughly spherical, and in an undeformed shale, clay flakes press  together into the plane of bedding so that shales tend to split parallel to the bedding. During ductile deformation, however, internal changes take place in a rock that gradually modify the original shape and arrangement of grains. For example, quartz grains may transform into cigar shapes, elongate ribbons, or tiny pancakes, and clay flakes may recrystallize or reorient so that they lie at an angle to the bedding. Overall, deformation can produce inequant grains and can cause them to align parallel to each other. We refer to layering developed by the alignment of grains in response to deformation as tectonic foliation. 

The development of tectonic foliation in rock.

We introduced foliation, such as slaty cleavage, schistosity, and gneissic layering, while discussing the effects of metamorphism. Here we add to the story by noting that such foliation forms in response to flattening and shearing in ductilely deforming rocks in other words, foliation indicates that the rock has developed a strain under metamorphic conditions (figure above a, b).
Credits: Stephen Marshak (Essentials of Geology)

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