Fold (geology)

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Folds of alternate layers of limestone and chert occur in Greece. The limestone and chert were originally deposited as flat layers on the floor of a deep sea basin. These folds were created by Alpine deformation. Folding of alternate layers of limestone layers with chert layers.jpg
Folds of alternate layers of limestone and chert occur in Greece. The limestone and chert were originally deposited as flat layers on the floor of a deep sea basin. These folds were created by Alpine deformation.

In structural geology, a fold is a stack of originally planar surfaces, such as sedimentary strata, that are bent or curved during permanent deformation. Folds in rocks vary in size from microscopic crinkles to mountain-sized folds. They occur as single isolated folds or in periodic sets (known as fold trains). Synsedimentary folds are those formed during sedimentary deposition.


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. A set of folds distributed on a regional scale constitutes a fold belt, a common feature of orogenic zones. Folds are commonly formed by shortening of existing layers, but may also be formed as a result of displacement on a non-planar fault (fault bend fold), at the tip of a propagating fault (fault propagation fold), by differential compaction or due to the effects of a high-level igneous intrusion e.g. above a laccolith.

Kink band folds in the Permian of New Mexico, USA Folded gyprock.jpg
Kink band folds in the Permian of New Mexico, USA
Rainbow Basin syncline in the Barstow Formation near Barstow, California Rainbow Basin.JPG
Rainbow Basin syncline in the Barstow Formation near Barstow, California

Fold terminology

Fold sketch 3D model Fold sketch 3D model .tif
Fold sketch 3D model

The fold hinge is the line joining points of maximum curvature on a folded surface. This line may be either straight or curved. The term hinge line has also been used for this feature. [1]

A fold surface seen perpendicular to its shortening direction can be divided into hinge and limb portions, the limbs are the flanks of the fold and the hinge zone is where the limbs converge. Within the hinge zone lies the hinge point, which is the point of minimum radius of curvature (maximum curvature) of the fold. The crest of the fold represents the highest point of the fold surface whereas the trough is the lowest point. The inflection point of a fold is the point on a limb at which the concavity reverses; on regular folds, this is the midpoint of the limb.

Flank & hinge Flank & hinge.PNG
Flank & hinge

The axial surface is defined as a plane connecting all the hinge lines of stacked folded surfaces. If the axial surface is planar then it is called an axial plane and can be described in terms of strike and dip.

Folds can have a fold axis. A fold axis, “is the closest approximation to a straight line that when moved parallel to itself, generates the form of the fold.” (Davis and Reynolds, 1996 after Donath and Parker, 1964; Ramsay 1967). A fold that can be generated by a fold axis is called a cylindrical fold. This term has been broadened to include near-cylindrical folds. Often, the fold axis is the same as the hinge line. [2] [3]

Descriptive features

Fold size

Minor folds are quite frequently seen in outcrop; major folds seldom are except in the more arid countries. Minor folds can, however, often provide the key to the major folds they are related to. 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 [4]

Fold shape

Chevron folds, Ireland Chevron folds, Ireland.jpg
Chevron folds, Ireland

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.

Fold tightness

Interlimb angles Interlimb angles.jpg
Interlimb angles

Fold tightness is defined by the size of the angle between the fold's limbs (as measured tangential to the folded surface at the inflection line of each limb), called the interlimb angle. Gentle folds have an interlimb angle of between 180° and 120°, open folds range from 120° to 70°, close folds from 70° to 30°, and tight folds from 30° to 0°. [5] Isoclines, or isoclinal folds, have an interlimb angle of between 10° and zero, with essentially parallel limbs.

Fold symmetry

Not all folds are equal on both sides of the axis of the fold. Those with limbs of relatively equal length are termed symmetrical, and those with highly unequal limbs are asymmetrical. Asymmetrical folds generally have an axis at an angle to the original unfolded surface they formed on.

Facing and vergence

Vergence is calculated in a direction perpendicular to the fold axis.

Deformation style classes

Folds that maintain uniform layer thickness are classed as concentric folds. Those that do not are called similar folds. 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. Ramsay has proposed a classification scheme for folds that often is used to describe folds in profile based upon the curvature of the inner and outer lines of a fold and the behavior of dip isogons. that is, lines connecting points of equal dip on adjacent folded surfaces: [6]

Ramsay classification of folds by convergence of dip isogons (red lines). Ramsay Classification.PNG
Ramsay classification of folds by convergence of dip isogons (red lines).
Ramsay classification scheme for folds
ClassCurvature CComment
1Cinner > CouterDip isogons converge
1AOrthogonal thickness at hinge narrower than at limbs
1BParallel folds
1COrthogonal thickness at limbs narrower than at hinge
2Cinner = CouterDip isogons are parallel: similar folds
3Cinner < CouterDip isogons diverge

Types of fold

An anticline in New Jersey NJ Route 23 anticline.jpg
An anticline in New Jersey
A monocline at Colorado National Monument Monocline.JPG
A monocline at Colorado National Monument
Recumbent fold, King Oscar Fjord Caledonian orogeny fold in King Oscar Fjord.jpg
Recumbent fold, King Oscar Fjord



(A homocline involves strata dipping in the same direction, though not necessarily any folding.)

Causes of folding

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

Box fold in La Herradura Formation, Morro Solar, Peru Pliegue en cofre.jpg
Box fold in La Herradura Formation, 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. [9]

Rollover anticline Rollover.png
Rollover anticline
Ramp anticline Faultbendfold.png
Ramp anticline
Fault-propagation fold Thrust with fault propagation fold.svg
Fault-propagation fold

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. [10] 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. [11]

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. [12]

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. [13]

Folding in shear zones

Dextral sense shear folds in mylonites within a shear zone, Cap de Creus Dextral shear folds.JPG
Dextral sense shear folds in mylonites 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. [14]

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. [15]

Dewatering: Rapid dewatering of sandy sediments, possibly triggered by seismic activity, can cause convolute bedding. [16]

Compaction: Folds can be generated in a younger sequence by differential compaction over older structures such as fault blocks and reefs. [17]

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. [18]

Flow folding

Flow folding: depiction of the effect of an advancing ramp of rigid rock into compliant layers. Top: low drag by a ramp: layers are not altered in thickness; Bottom: high drag: lowest layers tend to crumple. Advancing ramp in incompetent layers.PNG
Flow folding: depiction of the effect of an advancing ramp of rigid rock into compliant layers. Top: low drag by a ramp: layers are not altered in thickness; Bottom: high drag: lowest layers tend to crumple.

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. [20] Such folding is also a feature of many igneous intrusions and glacier ice. [21]

Folding mechanisms

Folding of rocks must balance the deformation of layers with the conservation of volume in a rock mass. This occurs by several mechanisms.

Flexural slip

Flexural slip allows folding by creating layer-parallel slip between the layers of the folded strata, which, altogether, result in deformation. A good analogy is bending a phone book, where volume preservation is accommodated by slip between the pages of the book.

The fold formed by the compression of competent rock beds is called "flexure fold".


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 typical of a similar fold style, as thinned limbs are shortened horizontally and thickened hinges do so vertically.

Mass displacement

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

Mechanics of folding

Folds in the rock are formed about the stress field in which the rocks are located and the rheology, or method of response to stress, of the rock at the time at which the stress is applied.

The rheology of the layers being folded determines characteristic features of the folds that are measured in the field. Rocks that deform more easily form many short-wavelength, high-amplitude folds. Rocks that do not deform as easily form long-wavelength, low-amplitude folds.

Economic Implication

Mining industry

anticline oil trap Anticlinal Oil trap.png
anticline oil trap

Layers of rock that fold into a hinge need to accommodate large deformations in the hinge zone. This results in voids between the layers. These voids, and especially the fact that the water pressure is lower in the voids than outside of them, act as triggers for the deposition of minerals. Over millions of years, this process is capable of gathering large quantities of trace minerals from large expanses of rock and depositing them at very concentrated sites. This may be a mechanism that is responsible for the veins. To summarize, when searching for veins of valuable minerals, it might be wise to look for highly folded rock, and this is the reason why the mining industry is very interested in the theory of geological folding. [22]

Oil industry

Anticlinal traps are formed by folding of rock. For example, if a porous sandstone unit covered with low permeability shale is folded into an anticline, it may contain hydrocarbons trapped in the crest of the fold. Most anticlinal traps are created as a result of sideways pressure, folding the layers of rock, but can also occur from sediments being compacted. [23]

See also


  1. M.J. Fleury, The description of folds, Proceedings of the Geologists' Association, Volume 75, Issue 4, 1964, Pages 461-492, ISSN 0016-7878,
  2. Sudipta Sengupta; Subir Kumar Ghosh; Kshitindramohan Naha (1997). Evolution of geological structures in micro- to macro-scales. Springer. p. 222. ISBN   0-412-75030-9.
  3. RG Park (2004). "Fold axis and axial plane". Foundations of structural geology (3rd ed.). Routledge. p. 26. ISBN   0-7487-5802-X.
  4. Barnes, J. W., & Lisle, R. J. (2013). "5 Field Measurements and Techniques". Basic geological mapping: 4th Edition. John Wiley & Sons. p. 79. ISBN   978-1-118-68542-6.CS1 maint: multiple names: authors list (link)
  5. Lisle, Richard J (2004). "Folding". Geological Structures and Maps: 3rd Edition . Elsevier. pp.  33. ISBN   0-7506-5780-4.
  6. See, for example, R. G. Park (2004). "Figure 3.12: Fold classification based upon dip diagrams". Foundations of structural geology (3rd ed.). Routledge. p. 31 ff. ISBN   0-7487-5802-X.
  7. Neville J. Price; John W. Cosgrove (1990). "Figure 10.14: Classification of fold profiles using dip isogon patterns". Analysis of geological structures. Cambridge University Press. p. 246. ISBN   0-521-31958-7.
  8. 1 2 Park, R.G. (2004). Foundation of Structural Geology (3 ed.). Routledge. p. 33. ISBN   978-0-7487-5802-9.
  9. Ramsay, J.G.; Huber M.I. (1987). The techniques of modern structural geology. 2 (3 ed.). Academic Press. p. 392. ISBN   978-0-12-576922-8 . Retrieved 2009-11-01.
  10. Withjack, M.O.; Schlische (2006). "Geometric and experimental models of extensional fault-bend folds". In Buiter S.J.H. & Schreurs G. (ed.). Analogue and numerical modelling of crustal-scale processes. Special Publications 253. R.W. Geological Society, London. pp. 285–305. ISBN   978-1-86239-191-8 . Retrieved 2009-10-31.
  11. Rowland, S.M.; Duebendorfer E.M.; Schieflebein I.M. (2007). Structural analysis and synthesis: a laboratory course in structural geology (3 ed.). Wiley-Blackwell. p. 301. ISBN   978-1-4051-1652-7 . Retrieved 2009-11-01.
  12. Jackson, C.A.L.; Gawthorpe R.L.; Sharp I.R. (2006). "Style and sequence of deformation during extensional fault-propagation" (PDF). Journal of Structural Geology. 28 (3): 519–535. Bibcode:2006JSG....28..519J. doi:10.1016/j.jsg.2005.11.009 . Retrieved 2009-11-01.
  13. Reicherter, K., Froitzheim, N., Jarosinki, M., Badura, J., Franzke, H.-J., Hansen, M., Hübscher, C., Müller, R., Poprawa, P., Reinecker, J., Stackebrandt, W, Voigt,T., von Eynatten, H. & Zuchiewicz, W. (2008). "19. Alpine Tectonics north of the Alps". In McCann, T. (ed.). The Geology of Central Europe. Geological Society, London. pp. 1233–1285. ISBN   978-1-86239-264-9 . Retrieved 2009-10-31.CS1 maint: uses authors parameter (link)
  14. Carreras, J.; Druguet E.; Griera A. (2005). "Shear zone-related folds". Journal of Structural Geology. 27 (7): 1229–1251. Bibcode:2005JSG....27.1229C. doi:10.1016/j.jsg.2004.08.004 . Retrieved 2009-10-31.
  15. Bradley, D.; Hanson L. (1998). "Paleoslope Analysis of Slump Folds in the Devonian Flysch of Maine" (PDF). Journal of Geology. 106 (3): 305–318. Bibcode:1998JG....106..305B. doi:10.1086/516024. S2CID   129086677 . Retrieved 2009-10-31.
  16. Nichols, G. (1999). "17. Sediments into rocks: post-depositional processes". Sedimentology and stratigraphy. Wiley-Blackwell. p. 355. ISBN   978-0-632-03578-6 . Retrieved 2009-10-31.
  17. Hyne, N.J. (2001). Nontechnical guide to petroleum geology, exploration, drilling, and production. PennWell Books. p. 598. ISBN   978-0-87814-823-3 . Retrieved 2009-11-01.
  18. Orchuela, I.; Lara M.E.; Suarez M. (2003). "Productive Large Scale Folding Associated with Igneous Intrusions: El Trapial Field, Neuquen Basin, Argentina" (PDF). AAPG Abstracts. Retrieved 2009-10-31.
  19. Arvid M. Johnson; Raymond C. Fletcher (1994). "Figure 2.6". Folding of viscous layers: mechanical analysis and interpretation of structures in deformed rock. Columbia University Press. p. 87. ISBN   0-231-08484-6.
  20. Park, R.G. (1997). Foundations of structural geology (3rd ed.). Routledge. p. 109. ISBN   0-7487-5802-X.; RJ Twiss; EM Moores (1992). "Figure 12.8: Passive shear folding". Structural geology (2nd ed.). Macmillan. pp. 241–242. ISBN   0-7167-2252-6.
  21. Hudleston, P.J. (1977). "Similar folds, recumbent folds and gravity tectonics in ice and rocks". Journal of Geology. 85 (1): 113–122. Bibcode:1977JG.....85..113H. doi:10.1086/628272. JSTOR   30068680. S2CID   129424734.

Further reading

Related Research Articles

Structural geology Science of the description and interpretation of deformation in the Earths crust

Structural geology is the study of the three-dimensional distribution of rock units with respect to their deformational histories. The primary goal of structural geology is to use measurements of present-day rock geometries to uncover information about the history of deformation (strain) in the rocks, and ultimately, to understand the stress field that resulted in the observed strain and geometries. This understanding of the dynamics of the stress field can be linked to important events in the geologic past; a common goal is to understand the structural evolution of a particular area with respect to regionally widespread patterns of rock deformation due to plate tectonics.

Fault (geology) Fracture or discontinuity in rock across which there has been displacement

In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within the Earth's crust result from the action of plate tectonic forces, with the largest forming the boundaries between the plates, such as subduction zones or transform faults. Energy release associated with rapid movement on active faults is the cause of most earthquakes. Faults may also displace slowly, by aseismic creep.


In structural geology, an anticline is a type of fold that is an arch-like shape and has its oldest beds at its core, whereas a syncline is the inverse of a anticline. A typical anticline is convex up in which the hinge or crest is the location where the curvature is greatest, and the limbs are the sides of the fold that dip away from the hinge. Anticlines can be recognized and differentiated from antiforms by a sequence of rock layers that become progressively older toward the center of the fold. Therefore, if age relationships between various rock strata are unknown, the term antiform should be used.

Shear zone

A shear zone is a very important structural discontinuity surface in the Earth's crust and upper mantle. It forms as a response to inhomogeneous deformation partitioning strain into planar or curviplanar high-strain zones. Intervening (crustal) blocks stay relatively unaffected by the deformation. Due to the shearing motion of the surrounding more rigid medium, a rotational, non co-axial component can be induced in the shear zone. Because the discontinuity surface usually passes through a wide depth-range, a great variety of different rock types with their characteristic structures are produced.


Mylonite is a fine-grained, compact metamorphic rock produced by dynamic recrystallization of the constituent minerals resulting in a reduction of the grain size of the rock. Mylonites can have many different mineralogical compositions; it is a classification based on the textural appearance of the rock.

Vein (geology)

In geology, a vein is a distinct sheetlike body of crystallized minerals within a rock. Veins form when mineral constituents carried by an aqueous solution within the rock mass are deposited through precipitation. The hydraulic flow involved is usually due to hydrothermal circulation.

Shear (geology)

In geology, shear is the response of a rock to deformation usually by compressive stress and forms particular textures. Shear can be homogeneous or non-homogeneous, and may be pure shear or simple shear. Study of geological shear is related to the study of structural geology, rock microstructure or rock texture and fault mechanics.

Foliation (geology)

Foliation in geology refers to repetitive layering in metamorphic rocks. Each layer can be as thin as a sheet of paper, or over a meter in thickness. The word comes from the Latin folium, meaning "leaf", and refers to the sheet-like planar structure. It is caused by shearing forces, or differential pressure. The layers form parallel to the direction of the shear, or perpendicular to the direction of higher pressure. Nonfoliated metamorphic rocks are typically formed in the absence of significant differential pressure or shear. Foliation is common in rocks affected by the regional metamorphic compression typical of areas of mountain belt formation.

Joint (geology)

A joint is a break (fracture) of natural origin in the continuity of either a layer or body of rock that lacks any visible or measurable movement parallel to the surface (plane) of the fracture. Although they can occur singly, they most frequently occur as joint sets and systems. A joint set is a family of parallel, evenly spaced joints that can be identified through mapping and analysis of the orientations, spacing, and physical properties. A joint system consists of two or more intersecting joint sets.

Lineations in structural geology are linear structural features within rocks. There are several types of lineations, intersection lineations, crenulation lineations, mineral lineations and stretching lineations being the most common. Lineation field measurements are recorded as map lines with a plunge angle and azimuth.

Fracture (geology) Geologic structure

A fracture is any separation in a geologic formation, such as a joint or a fault that divides the rock into two or more pieces. A fracture will sometimes form a deep fissure or crevice in the rock. Fractures are commonly caused by stress exceeding the rock strength, causing the rock to lose cohesion along its weakest plane. Fractures can provide permeability for fluid movement, such as water or hydrocarbons. Highly fractured rocks can make good aquifers or hydrocarbon reservoirs, since they may possess both significant permeability and fracture porosity.

Strike-slip tectonics is concerned with the structures formed by, and the tectonic processes associated with, zones of lateral displacement within the Earth's crust or lithosphere. It is one of the three main types of tectonic regime, the others being extensional tectonics and thrust tectonics. These match the three types of plate boundary, transform (strike-slip), divergent (extensional) and convergent (thrust). Areas of strike-slip tectonics are associated with particular deformation styles including Riedel shears, flower structures and strike-slip duplexes. This type of tectonics is characteristic of several geological environments, including oceanic and continental transform faults, zones of oblique collision and the deforming foreland of a zone of continental collision.

This glossary of geology is a list of definitions of terms and concepts relevant to geology, its sub-disciplines, and related fields. For other terms related to the Earth sciences, see Glossary of geography terms.

Section restoration

In structural geology section restoration or palinspastic restoration is a technique used to progressively undeform a geological section in an attempt to validate the interpretation used to build the section. It is also used to provide insights into the geometry of earlier stages of the geological development of an area. A section that can be successfully undeformed to a geologically reasonable geometry, without change in area, is known as a balanced section.

Detachment fold

A detachment fold, in geology, occurs as layer parallel thrusting along a decollement develops without upward propagation of a fault; the accommodation of the strain produced by continued displacement along the underlying thrust results in the folding of the overlying rock units. As a visual aid, picture a rug on the floor. By placing your left foot on one end and pushing towards the other end of the rug, the rug slides across the floor (decollement) and folds upward. Figure 1, is a generalized representation of the geometry assumed by a detachment fault.

Zagros fold and thrust belt Geologic zone

The Zagros fold and thrust belt is an approximately 1,800-kilometre (1,100 mi) long zone of deformed crustal rocks, formed in the foreland of the collision between the Arabian Plate and the Eurasian Plate. It is host to one of the world's largest petroleum provinces, containing about 49% of the established hydrocarbon reserves in fold and thrust belts (FTBs) and about 7% of all reserves globally.

Strain partitioning is commonly referred to as a deformation process in which the total strain experienced on a rock, area, or region, is heterogeneously distributed in terms of the strain intensity and strain type. This process is observed on a range of scales spanning from the grain – crystal scale to the plate – lithospheric scale, and occurs in both the brittle and plastic deformation regimes. The manner and intensity by which strain is distributed are controlled by a number of factors listed below.

A geological contact is a boundary which separates one rock body from another. A contact can be formed during deposition, by the intrusion of magma, or through faulting or other deformation of rock beds that brings distinct rock bodies into contact.

3D fold evolution

In geology, 3D fold evolution is the study of the full three dimensional structure of a fold as it changes in time. A fold is a common three-dimensional geological structure that is associated with strain deformation under stress. Fold evolution in three dimensions can be broadly divided into two stages, namely fold growth and fold linkage. The evolution depends on fold kinematics, causes of folding, as well as alignment and interaction of the each structure with respect to each other. There are several ways to reconstruct the evolution progress of folds, notably by using depositional evidence, geomorphological evidence and balanced restoration. Understanding the evolution of folds is important because it helps petroleum geologists to gain a better understanding on the distribution of structural traps of hydrocarbon.

Southland Syncline

The Southland Syncline is a major geological structure located in the Southland Region of New Zealand's South Island. The syncline folds the Mesozoic greywackes of the Murihiku Terrane. The northern limb of the fold is steep to overturned, while the southern limb dips shallowly to the northeast. The axial plan dips to the northeast and the axis plunges to the southeast.