Fold (geology)

Folds form under varied conditions of stress, pore pressure, and temperature 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.

They reflect the same shape and style, the direction in which the closures of the major folds lie, and their cleavage indicates the attitude of the axial planes of the major folds and their direction of overturning [5] A fold can be shaped like a chevron, with planar limbs meeting at an angular axis, as cuspate with curved limbs, as circular with a curved axis, or as elliptical with unequal wavelength.

Asymmetrical folds generally have an axis at an angle to the original unfolded surface they formed on.

Similar folds tend to display thinning of the limbs and thickening of the hinge zone.

Concentric folds are caused by warping from active buckling of the layers, whereas similar folds usually form by some form of shear flow where the layers are not mechanically active.

Isolated thick competent layers in a less competent matrix control the folding and typically generate classic rounded buckle folds accommodated by deformation in the matrix.

In the case of regular alternations of layers of contrasting properties, such as sandstone-shale sequences, kink-bands, box-folds and chevron folds are normally produced.

In extension, listric faults form rollover anticlines in their hanging walls.

In both reverse and normal faults this leads to folding of the overlying sequence, often in the form of a monocline.

[15] Recently deposited sediments are normally mechanically weak and prone to remobilization before they become lithified, leading to folding.

To distinguish them from folds of tectonic origin, such structures are called synsedimentary (formed during sedimentation).

The asymmetry of the slump folds can be used to determine paleoslope directions in sequences of sedimentary rocks.

[18] The emplacement of igneous intrusions tends to deform the surrounding country rock.

Typically, folding is thought to occur by simple buckling of a planar surface and its confining volume.

If the folding deformation cannot be accommodated by a 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 process, in which rocks shorten by dissolving constituents in areas of high strain and redepositing them in areas of lower strain.

Folds generated in this way include examples in migmatites and areas with a strong axial planar cleavage.

Rocks that deform more easily form many short-wavelength, high-amplitude folds.

Most anticlinal traps are produced as a result of sideways pressure, folding the layers of rock, but can also occur from sediments being compacted.