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Fold (geology)
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.
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.
Contents
Fold terminology
Descriptive features
Fold size
Fold shape
Fold tightness
Fold symmetry
Facing and vergence
Deformation style classes
Kink band folds in the Permian of
New Mexico, USA
Types of fold
Causes of folding
Layer-parallel shortening
Fault-related folding
Fault bend folding
Fault propagation folding
Detachment folding
Folding in shear zones
Folding in sediments
Igneous intrusion
Flow folding
Rainbow Basin syncline in the
Barstow Formation near Barstow,
California
Folding mechanisms
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Flexural slip
Buckling
Mass displacement
Mechanics of folding
Economic Implication
Mining industry
Oil industry
See also
Notes
Further reading
Fold terminology
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 sketch 3D model
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.
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]
Flank & hinge
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]
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Fold shape
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
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.
Chevron folds, Ireland
Interlimb angles
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 scheme for folds
Class
Curvature C
Comment
1
Cinner > Couter
Dip isogons converge
1A
Orthogonal thickness at hinge narrower than at limbs
1B
Parallel folds
1C
Orthogonal thickness at limbs narrower than at hinge
2
Cinner = Couter
Dip isogons are parallel: similar folds
3
Cinner < Couter
Dip isogons diverge
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Types of fold
Anticline: linear, strata normally dip away from the axial center,
oldest strata in center irrespective of orientation.
Syncline: linear, strata normally dip toward the axial center,
youngest strata in center irrespective of orientation.
Antiform: linear, strata dip away from the axial center, age
unknown, or inverted.
Synform: linear, strata dip toward the axial center, age unknown,
or inverted.
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.
Monocline: linear, strata dip in one direction between horizontal
layers on each side.
Chevron: angular fold with straight limbs and small hinges
Recumbent: linear, fold axial plane oriented at a low angle
resulting in overturned strata in one limb of the fold.
Slump: typically monoclinal, the result of differential compaction
or dissolution during sedimentation and lithification.
Ptygmatic: Folds are chaotic, random and disconnected. Typical
of sedimentary slump folding, migmatites and decollement
detachment zones.
Parasitic: short-wavelength folds formed within a larger
wavelength fold structure - normally associated with differences
in bed thickness[8]
Disharmonic: Folds in adjacent layers with different wavelengths
and shapes[8]
Ramsay classification of folds by
convergence of dip isogons (red
lines).[7]
An anticline in New Jersey
(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
A monocline at Colorado National
When a sequence of layered rocks is shortened parallel to its
Monument
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
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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]
Recumbent fold, King Oscar Fjord
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.[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]
Box fold in La Herradura
Formation, Morro Solar,
Peru
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]
Rollover anticline
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
Ramp anticline
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]
Fault-propagation fold
Folding in sediments
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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]
Dextral sense shear folds in
mylonites within a shear zone, Cap
de Creus
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
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
https://en.wikipedia.org/wiki/Fold_(geology)
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.[19]
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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".
Buckling
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
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]
anticline oil trap
Oil industry
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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
3D fold evolution
Orogeny
Mountain building
Rock mechanics
Thrust fault
Notes
1. M.J. Fleury, The description of folds, Proceedings of the Geologists' Association, Volume 75, Issue 4,
1964, Pages 461-492, ISSN 0016-7878, https://doi.org/10.1016/S0016-7878(64)80023-7.
2. Sudipta Sengupta; Subir Kumar Ghosh; Kshitindramohan Naha (1997). Evolution of geological
structures in micro- to macro-scales (https://books.google.com/books?id=sqS2hxRe-nQC&pg=PA22
2). Springer. p. 222. ISBN 0-412-75030-9.
3. RG Park (2004). "Fold axis and axial plane" (https://books.google.com/books?id=ycASqdxSG3YC&p
g=PA26). 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.
5. Lisle, Richard J (2004). "Folding". Geological Structures and Maps: 3rd Edition (https://archive.org/de
tails/geologicalstruct00lisl). Elsevier. pp. 33 (https://archive.org/details/geologicalstruct00lisl/page/n4
1). ISBN 0-7506-5780-4.
6. See, for example, R. G. Park (2004). "Figure 3.12: Fold classification based upon dip diagrams" (http
s://books.google.com/books?id=ycASqdxSG3YC&pg=PA32). 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" (https://books.google.com/books?id=80gYS1IzUWsC&pg=PA246). Analysis of
geological structures. Cambridge University Press. p. 246. ISBN 0-521-31958-7.
8. Park, R.G. (2004). Foundation of Structural Geology (https://books.google.com/books?id=ycASqdxS
G3YC&q=parasitic+folds&pg=PA33) (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 (https://books.google.
com/books?id=_hXzNILoSnAC&q=layer+parallel+shortening+anisotropy+chevron+folds+buckling&p
g=PA430). 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" (https://books.google.com/books?id=oOMfgdykbAkC&q=%22fault+bend+fold%22+extensional
+rollover&pg=PA298). 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 (https://books.google.com/books?id=IWnmBEtmg2MC&q=%2
2ramp+anticline%22) (3 ed.). Wiley-Blackwell. p. 301. ISBN 978-1-4051-1652-7. Retrieved
2009-11-01.
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12. Jackson, C.A.L.; Gawthorpe R.L.; Sharp I.R. (2006). "Style and sequence of deformation during
extensional fault-propagation" (http://www3.imperial.ac.uk/pls/portallive/docs/1/4205901.PDF) (PDF).
Journal of Structural Geology. 28 (3): 519–535. Bibcode:2006JSG....28..519J (https://ui.adsabs.harv
ard.edu/abs/2006JSG....28..519J). doi:10.1016/j.jsg.2005.11.009 (https://doi.org/10.1016%2Fj.jsg.20
05.11.009). 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" (https://books.google.com/books?id=0KXptTYvKv8C
&q=%22detachment+fold%22+jura&pg=PA1235). 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.
14. Carreras, J.; Druguet E.; Griera A. (2005). "Shear zone-related folds" (http://cat.inist.fr/?aModele=affi
cheN&cpsidt=17083207). Journal of Structural Geology. 27 (7): 1229–1251.
Bibcode:2005JSG....27.1229C (https://ui.adsabs.harvard.edu/abs/2005JSG....27.1229C).
doi:10.1016/j.jsg.2004.08.004 (https://doi.org/10.1016%2Fj.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" (https://alaska.usgs.gov/staff/geology/bradley/bradley_pubs/Paleoslope_Maine_1998.pdf)
(PDF). Journal of Geology. 106 (3): 305–318. Bibcode:1998JG....106..305B (https://ui.adsabs.harvar
d.edu/abs/1998JG....106..305B). doi:10.1086/516024 (https://doi.org/10.1086%2F516024).
S2CID 129086677 (https://api.semanticscholar.org/CorpusID:129086677). Retrieved 2009-10-31.
16. Nichols, G. (1999). "17. Sediments into rocks: post-depositional processes" (https://books.google.co
m/books?id=F31jOYy-79EC&q=%22convolute+bedding%22+dewatering&pg=PA215).
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 (htt
ps://books.google.com/books?id=3A8YIW3uuX0C&q=compaction+anticline&pg=PA184). 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" (http://www.searchanddiscovery.com/docume
nts/abstracts/2003barcelona/short/83257.pdf) (PDF). AAPG Abstracts. Retrieved 2009-10-31.
19. Arvid M. Johnson; Raymond C. Fletcher (1994). "Figure 2.6" (https://books.google.com/books?id=IO
JKgOcjCGQC&pg=PA87). 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 (https://books.google.com/books?id=ycASqdxS
G3YC&pg=PA109) (3rd ed.). Routledge. p. 109. ISBN 0-7487-5802-X.; RJ Twiss; EM Moores (1992).
"Figure 12.8: Passive shear folding" (https://books.google.com/books?id=14fn03iJ2r8C&pg=PA241).
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 (https://ui.adsabs.harvard.edu/ab
s/1977JG.....85..113H). doi:10.1086/628272 (https://doi.org/10.1086%2F628272). JSTOR 30068680
(https://www.jstor.org/stable/30068680). S2CID 129424734 (https://api.semanticscholar.org/CorpusI
D:129424734).
22. https://www.win.tue.nl/~mpeletie/Research/minerals.shtml
23. https://energyeducation.ca/encyclopedia/Oil_and_gas_traps
Further reading
Barnes, J. W.; Lisle, R. J. (2013). "Field Measurements and Techniques". Basic Geological Mapping
(4th ed.). John Wiley & Sons. p. 79. ISBN 978-1-118-68542-6.
https://en.wikipedia.org/wiki/Fold_(geology)
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Davis, George H.; Reynolds, Stephen J. (1996). "Folds". Structural Geology of Rocks and Regions.
New York: John Wiley & Sons. pp. 372–424. ISBN 0-471-52621-5.
Donath, F.A.; Parker, R.B. (1964). "Folds and Folding". Geological Society of America Bulletin. 75:
45–62. doi:10.1130/0016-7606(1964)75[45:FAF]2.0.CO;2 (https://doi.org/10.1130%2F0016-7606%2
81964%2975%5B45%3AFAF%5D2.0.CO%3B2). ISSN 0016-7606 (https://www.worldcat.org/issn/00
16-7606).
Energy Education, page=[1] (https://energyeducation.ca/encyclopedia/Oil_and_gas_traps)
Lisle, Richard J. (2004). "Folding". Geological Structures and Maps (https://archive.org/details/geolog
icalstruct00lisl) (3rd ed.). Elsevier. pp. 33 (https://archive.org/details/geologicalstruct00lisl/page/n41).
ISBN 0-7506-5780-4 – via Archive Foundation.
McKnight, Tom L.; Hess, Darrel (2000). "The Internal Processes: Folding" (https://archive.org/details/
physicalgeographmckn/page/409). Physical Geography: A Landscape Appreciation. Upper Saddle
River, NJ: Prentice Hall. pp. 409–14 (https://archive.org/details/physicalgeographmckn/page/409).
ISBN 0-13-020263-0 – via Archive Foundation.
Mark Peletier, p. [2] (https://www.win.tue.nl/~mpeletie/Research/minerals.shtml)
Pollard, David D.; Fletcher, Raymond C. (2005). Fundamentals of Structural Geology (https://archive.
org/details/trent_0116405531629). Cambridge University Press. ISBN 0-521-83927-0 – via Archive
Foundation.
Ramsay, J.G., 1967, Folding and fracturing of rocks: McGraw-Hill Book Company, New York, 560pp.
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