Fold occurs when one or a stack of originally flat and planar surfaces, such as sedimentary strata, are bent or curved as a result of permanent deformation. Synsedimentary folds are those due to slumping of sedimentary material before it is lithified. 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.
The Terminology of a Fold
Types of fold
|This anticline is in Alberta, Canadia in the Rocky Mountains|
Anticline is a fold that is convex up and has its oldest beds at its core. The term is not to be confused with antiform, which is a purely descriptive term for any fold that is convex up. Therefore if age relationships between various strata are unknown, the term antiform should be used.
|Syncline sidling hill|
A syncline is a fold with younger layers closer to the center of the structure. Synclines are typically a downward fold, termed a synformal syncline (i.e. a trough); but synclines that point upwards, or perched, can be found when strata have been overturned and folded (an antiformal syncline).
|Chevron folds with flat-lying axial planes, Millook Haven, North Cornwall, UK|
Chevron folds are a structural feature characterized by repeated well behaved folded beds with straight limbs and sharp hinges. Well developed, these folds develop repeated set of v-shaped beds. They develop in response to regional or local compressive stress. Inter-limb angles are generally 60 degrees or less. Chevron folding preferentially occurs when the bedding regularly alternates between contrasting competences.
|Recumbent fold Bahrain|
|Plunging chevron folds|
A fold whose axis plane is not horizontal (not Parallel to sea level). Direction of plunge - the direction in which the axis is inclined nose - indicate the direction of plunge. In anticline, plunge is directed towards nose and in syncline it is directed away from nose.
Dome and Basin
|Desert of Mauritania. dome.|
Folds are chaotic, random and disconnected. Typical of sedimentary slump folding, migmatites and decollement detachment zones. Ptygmatic folds generally represent conditions where the folded material is of a much greater viscosity than the surrounding medium.