The term fold is used in geology when one or a stack of originally flat and planar surfaces, such as sedimentary strata, are bent or curved as a result of plastic (i.e. permanent) deformation. Synsedimentary folds are those due to slumping of material before deformation. Folds in rocks vary in size from microscopic crinkles to mountain-sized folds. They occur singly as isolated folds and in extensive fold trains of different sizes, on a variety of scales. Folds form under varied conditions of stress, hydrostatic pressure, pore pressure, and temperature – hydrothermal gradient, as evidenced by their presence in soft sediments, the full spectrum of metamorphic rocks, and even as primary flow structures in some igneous rocks. A set of folds distributed on a regional scale constitutes a fold belt, a common feature of orogenic zones.
Anticline: linear, strata dip away from axial center, oldest strata in center.
Syncline: linear, strata dip toward axial center, youngest strata in center.
Dome: nonlinear, strata dip away from center in all directions, oldest strata in center.
Basin: nonlinear, strata dip toward center in all directions, youngest strata in center.
Monocline: linear, strata dip in one direction between horizontal layers on each side.
Chevron: angular fold with straight limbs and small hinges
Recumbent: linear, fold axial plane oriented at low angle resulting in overturned strata below the fold axis.
Slump: typically monoclinal, result of differential compaction or dissolution during sedimentation and lithification.
Ptygmatic: Folds are chaotic, random and disconnected. Typical of sedimentary slump folding, migmatites and decollement detachment zones.
Barstow syncline, a beautiful fold in Miocene shales and sandstones, Rainbow Basin, Mojave Desert, California. This fold formed at a bend in a strike-slip fault.
Folds near Calico ghost town, northeast of Barstow, Mojave Desert, California. Like the Barstow syncline, these folds formed at a bend in a strike-slip fault. Their axes strike east-west, indicating that the forces that deformed the rocks squeezed from the north and south.
Isoclinal folds in dolomitic marble, Mosaic Canyon, Death Valley. The student has his fingers on the same layer, which can be traced around a fold hinge to the left. A fold lying on its side like this is called “recumbent”.
Complicated folds near Calico ghost town, Mojave Desert. Field of view is about 3 m wide. Differential movement of layers above and below the chaotic zone crumpled the thinly bedded central layers. Just left of center a small anticline-syncline pair has broken along a thrust fault with a few cm displacement. Can you see the recumbent fold in the lower right?
A classic monocline near Mexican Hat, Utah. Mesozoic strata are bowed down along the fold. they are horizontal on the plateau at left and in the foreground but dip 45 degrees or more along the fold.
Folding of rocks must balance the deformation of layers with the conservation of volume in a rock mass. This occurs by several mechanisms.
Example of a large-scale crenulation, Glengarry Basin, W.A., an example of chevron-type flexural-slip folds.
Flexural slip allows folding by creating layer-parallel slip between the layers of the folded strata which, altogether, result in deformation. The best analog is bending a phone book, where volume preservation is accommodated by slip between the pages of the book.
Typically, folding is thought to occur by simple buckling of a planar surface and its confining volume. The volume change is accommodated by layer parallel shortening the volume, which grows in thickness. Folding under this mechanism is typically of the similar fold style, as thinned limbs are shortened horizontally and thickened hinges do so vertically.
If the folding deformation cannot be accommodated by flexural slip or volume-change shortening (buckling), the rocks are generally removed from the path of the stress. This is achieved by pressure dissolution, a form of metamorphic reaction, in which rocks shorten by dissolving constituents which move to areas of lower strain. Folds created in this way include examples in migmatites, and areas with a strong axial planar cleavage.