Folds appear on all scales, in all
rock types, at all levels in the
crust. They arise from a variety of causes.
Layer-parallel shortening ,
Morro Solar, Peru When a sequence of layered rocks is shortened parallel to its layering, this deformation may be accommodated in a number of ways, homogeneous shortening, reverse faulting or folding. The response depends on the thickness of the mechanical layering and the contrast in properties between the layers. If the layering does begin to fold, the fold style is also dependent on these properties. 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.
Fault-related folding Many folds are directly related to faults, associated with their propagation, displacement and the accommodation of strains between neighboring faults.
Fault bend folding Fault-bend folds are caused by displacement along a non-planar fault. In non-vertical faults, the hanging-wall deforms to accommodate the mismatch across the fault as displacement progresses. Fault bend folds occur in both extensional and thrust faulting. In extension, listric faults form
rollover anticlines in their hanging walls. In thrusting,
ramp anticlines form whenever a thrust fault cuts up section from one detachment level to another. Displacement over this higher-angle ramp generates the folding.
Fault propagation folding Fault propagation folds or
tip-line folds are caused when displacement occurs on an existing fault without further propagation. In both reverse and normal faults this leads to folding of the overlying sequence, often in the form of a
monocline.
Detachment folding When a thrust fault continues to displace above a planar detachment without further fault propagation,
detachment folds may form, typically of box-fold style. These generally occur above a good detachment such as in the
Jura Mountains, where the detachment occurs on middle
Triassic evaporites.
Folding in shear zones s within a
shear zone,
Cap de Creus Shear zones that approximate to
simple shear typically contain minor asymmetric folds, with the direction of overturning consistent with the overall shear sense. Some of these folds have highly curved hinge-lines and are referred to as
sheath folds. Folds in shear zones can be inherited, formed due to the orientation of pre-shearing layering or formed due to instability within the shear flow.
Folding in sediments 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). Slump folding: When
slumps form in poorly consolidated sediments, they commonly undergo folding, particularly at their leading edges, during their emplacement. The asymmetry of the slump folds can be used to determine paleoslope directions in sequences of sedimentary rocks. Dewatering: Rapid dewatering of sandy sediments, possibly triggered by seismic activity, can cause convolute bedding. Compaction: Folds can be generated in a younger sequence by differential compaction over older structures such as fault blocks and
reefs.
Igneous intrusion The emplacement of igneous intrusions tends to deform the surrounding
country rock. In the case of high-level intrusions, near the Earth's surface, this deformation is concentrated above the intrusion and often takes the form of folding, as with the upper surface of a
laccolith.
Flow folding The compliance of rock layers is referred to as
competence: a competent layer or bed of rock can withstand an applied load without collapsing and is relatively strong, while an incompetent layer is relatively weak. When rock behaves as a fluid, as in the case of very weak rock such as rock salt, or any rock that is buried deeply enough, it typically shows
flow folding (also called
passive folding, because little resistance is offered): the strata appear shifted undistorted, assuming any shape impressed upon them by surrounding more rigid rocks. The strata simply serve as markers of the folding. Such folding is also a feature of many igneous intrusions and
glacier ice.{{cite journal ==Folding mechanisms==