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 oldest rock layers form the core of the fold, and outward from the core progressively younger rocks occur.
10 Amazing Geological Folds You Should See
Folds are classified on the basis of several geometric factors:
Tightness of folding
The tighness of folds can be described as open (limbs dip gently), tight (limbs dip steeply) or isoclinal (limbs are parallel).Orientation of axial plane
The orientation of the axial plane relative to the horizontal together with the orientation of fold limbs allow subdivision into upright (axial plane vertical, limbs symmetric), overturned (axial plane moderately inclined, one limb overturned), or recumbent (axial plane near horizontal, one limb inverted).
Thickness of folded beds
Thickly-bedded, brittle units tend to form concentric folds with the bed thickness preserved normal to bedding surfaces. Thinly-bedded, clay-rich units have a tendency to develop a foliation parallel to the axial plane and form similar folds with the vertical distance between top and bottom of the unit preserved through the deformation.
Types of folds
|This anticline is in Alberta, Canada in the Rocky Mountains
An anticline fold is a type of fold that is convex upward, meaning that it looks like an "A" in cross-section. The oldest rocks are at the core of the fold, and the youngest rocks are on the flanks. Anticlines are often found in mountain ranges, where they are formed by the compression of the Earth's crust.
Anticlines can be symmetrical or asymmetrical. A symmetrical anticline is one in which the axial plane, which is the plane that divides the fold into two equal halves, is vertical. An asymmetrical anticline is one in which the axial plane is tilted.
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).
A monocline fold is a step-like fold in rock strata consisting of a zone of steeper dip within an otherwise horizontal or gently dipping sequence. Monoclines can be either symmetrical or asymmetrical, and the dip of the fold can range from a few degrees to nearly vertical.
local warping in horizontal strata. Rock beds lying at two level separated by steep inclined limbs. It is form by vertical movement and generally found fault below monocline. a step-like fold in rock strata consisting of a zone of steeper dip within an otherwise horizontal or gently-dipping sequence.
|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. Chevron folds are typically found in mountain belts, where they can be used to infer the direction of compressive stress. They can also be found in sedimentary basins, where they can be used to map out the distribution of different rock types.
|Recumbent fold in Oman
A recumbent fold is a fold in which the axial plane is essentially horizontal, meaning that the two limbs of the fold are parallel to the ground. Recumbent folds are often isoclinal, meaning that the limbs of the fold are also parallel to each other.
Recumbent folds can form in a variety of ways, but they are most commonly associated with mountain building events. When two tectonic plates collide, the crust can be thickened and compressed, causing the rocks to fold. Recumbent folds can also form in response to extensional forces, such as those that occur in rift valleys.
An isoclinal fold is a type of fold in which the limbs are parallel or nearly parallel to each other. This is in contrast to other types of folds, such as symmetrical folds, in which the limbs are at an angle to each other.
Isoclinal folds are formed by intense compressional forces. The forces are so strong that the limbs of the fold are forced to become parallel. Isoclinal folds are often found in mountain belts, where they have been formed by the collision of tectonic plates. Isoclinal folds are similar to symmetrical folds, but these folds both have the same angle and are parallel to each other. 'Iso' means 'the same' (symmetrical), and 'cline' means 'angle,' so this name literally means 'same angle.' So isoclinal folds are both symmetrical and aligned in a parallel fashion.
|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.
A dome and basin fold is a type of geological structure that consists of a central dome-shaped uplift surrounded by a basin-shaped depression. Dome and basin folds can be found in all types of tectonic settings, but they are particularly common in areas of compressional deformation, such as mountain belts.
Dome and basin folds can form in a variety of ways, but the most common mechanism is the interference of two or more sets of folds. When two sets of folds interfere, they can produce a variety of complex structures, including dome and basin folds. 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.
Folds are chaotic, random and disconnected. Typical of sedimentary slump folding, migmatites and decollement detachment zones. A ptygmatic fold is a type of fold that is characterized by its irregular and lobate shape. Ptygmatic folds are often found in migmatites, which are rocks that are composed of both metamorphic and igneous rocks. Ptygmatic folds are thought to form as a result of the partial melting of rocks during metamorphism.
Slump folds are formed when a mass of sediment becomes unstable and slides down a slope. This can happen due to a variety of factors, such as heavy rain, earthquake shaking, or oversteepening of the slope. As the sediment slides down the slope, it can become folded and deformed.
Slump folds are often found in areas where there is a steep slope and where the sediment is poorly consolidated. This can include coastal cliffs, river banks, and mountain slopes. Slump folds can also be found in areas where there has been recent earthquake activity.