Detachment fold

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Topographic map showing detachment folds in the eastern Sichuan Basin, China. Three-gorges-map.png
Topographic map showing detachment folds in the eastern Sichuan Basin, China.

A detachment fold, in geology, occurs as layer parallel thrusting along a decollement (or detachment) 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 (with your left foot) towards the other end of the rug, the rug slides across the floor (decollement) and folds upward (detachment fold). Figure 1, is a generalized representation of the geometry assumed by a detachment fault.

Contents

Figure 1. The general geometry of a detachment fold illustrating the shortening above a layer parallel decollement and the resulting geometry of a detachment fold in a compressional environment. Detachment fold.png
Figure 1. The general geometry of a detachment fold illustrating the shortening above a layer parallel decollement and the resulting geometry of a detachment fold in a compressional environment.

Definitions

Detachment folding occurs as strain imposed on a mechanically weak layer or incompetent unit, such as shale or salt, or at the boundary between an incompetent and more competent unit, induces resistance from the units resulting in folding typically observed in the competent unit. [1] [2] [3] Once the resistance of these units is overcome with strain or the variation in strain between the units becomes great enough, a shearing motion known as a detachment fault may occur. Defined, a detachment fault may be located within an incompetent unit or at the boundary of an incompetent and a competent unit, which accommodates for strain differences between the units and allows displacement to occur in a planar field. [1] [2] [3] [4] [5] Detachment folding occurs in regions of thick-skinned deformation, where the basement is involved in deformation and thin-skinned deformation, where deformation occurs at relatively shallow depth in the crust.

Modes of detachment folding

One of the principal ideas that should be recognized in each model is the law of conservation of volume, as conservation is a fundamental law in physics; it should also apply to geology. Two ways to maintain volume conservation are thickening of units and synclinal deflection of incompetent material; it is likely that both may occur.

Figure 2. A model of the law of conservation of volume by synclinal deflection; that is the area (given by A then a subscript) of the anticline should be equal to the area of shortening plus the synclinal deflection A1 = A2+A3+A4. Synclinal deflection below the origin (dotted line) is shown marked by the A3 and A4. Synclinal Deflection.GIF
Figure 2. A model of the law of conservation of volume by synclinal deflection; that is the area (given by A then a subscript) of the anticline should be equal to the area of shortening plus the synclinal deflection A1 = A2+A3+A4. Synclinal deflection below the origin (dotted line) is shown marked by the A3 and A4.

J. Contreras (2010) developed a model for low amplitude detachments using the conservation of mass equation. The results suggest the occurrence of layer thickening as an initial response to shortening and volume conservation. [6] Hayes and Hanks (2008) confirm layer thickening during the onset of folding, specifically their field data places the thickening at the hinges of folds rather than the limbs. [2] When defining the geometry of detachment folding it may be necessary to define layer thickening as it has been recorded to affect the overall geometry. [7] Though variable limb thickness is assumed; over time, limb rotation and limb length become the dominant mechanisms for deformation, leading to an increase in fold amplitude. [6]

Synclinal deflection, figure 2, is the result of folding where synclines, adjacent to an anticline in fold geometry, extend into the lower incompetent unit; these typically occur in regions of high wavelength and low amplitude. [3] The occupancy of this area causes displacement above the detachment in the form of material migration to the anticlinal core. [3] [8] Withdrawal from the regional position is dependent on thickness and viscosity differences between the competent and incompetent units as well as the ductile nature of the incompetent unit, [3] like Contreras, [6] recognized a transition from unit deflection and material migration, to limb rotation and limb lengthening.

Detachment fold evolution

Though many models have been developed to help explain the kinematic evolution of single layer detachment faulting; [7] [9] [10] [11] [12] many models do not account for multiple layers, complex fold geometries [12] or differential strain through fold geometries or mechanically dissimilar stratigraphic units. [13] These models may not be good indicators of detachment folding on a large scale and are better suited to assist in interpreting fold geometries of detachment folds as their kinematic evolution is generally associated with single fold, single unit deformations. The definition of disharmonic folds (below) however, incorporates many types of symmetric folds over a larger area encompassing many geometries and attributes of the basic models and may be better suited to the application of these models.

Figure 3. Disharmonic folding of a detachment fold using a symmetric geometric fold as a model. As compression develops, space issues arise in the anticlinal core. To accommodate these space issues folding becomes tighter within the inner units, creating a disharmonic fold geometry. Disharmonic Folding.png
Figure 3. Disharmonic folding of a detachment fold using a symmetric geometric fold as a model. As compression develops, space issues arise in the anticlinal core. To accommodate these space issues folding becomes tighter within the inner units, creating a disharmonic fold geometry.

By incorporating elementary fold geometries [7] [9] [10] [11] [12] under the term disharmonic fold detachment folds may then be classified into one of two categories; disharmonic folds or lift-off folds. Disharmonic folds, figure 3, are defined as detachment folds characterized by parallel geometries at the outer limbs and non-parallel interlimb geometries at stratigraphically distinct and lower units; caused by differential strain as a result of strain dissipation or change in mechanical stratigraphy, where the termination of folding typically results in a detachment. [2] [3] [12] Lift-off detachment folds are characterized by isoclinal folding in all units, with a tight isoclinal folded weak unit in the anticline and parallel geometries sometimes existing along the outer units. [3] [14] Present day examples detachment folding can be found in the Jura Mountains of Central Europe. This region complements the idea of detachment fold evolution put forth by Mitra [3] in that it encompasses many of the basic fold geometries and comprises both disharmonic and lift-off geometries.

Disharmonic and lift-off detachment folds are commonly assumed to form by separate modes of deformation; however, [3] Mitra (2003) in a unified kinematic model challenged these ideas by suggesting an evolution of detachment folding wherein progressive deformation yields a fold transition from disharmonic geometry to lift-off detachment folding. While most kinematic models are developed to yield the most simplistic geometries by placing boundary conditions within the model and limiting variables; the unified model incorporates: mechanical stratigraphy parameter [2] limb lengthening, limb rotation, [6] [8] [14] area balancing and anticlinal and synclinal deflection, to develop a system that uniformly demonstrates the evolution of detachment folding.

The evolution of detachment folding begins with the model assumption of a low-amplitude and short compressional environment with a mechanically dissimilar incompetent and competent unit. Folding initiates by shortening; limb lengthening and rotation and hinge migration, cause synclinal deflection below its original position accompanied by the flow of ductile material beneath the synclinal trough to the anticlinal core; resulting in increased amplitude of the anticlinal fold. [3] [4] [5] [6] [15]

Further compression dominated by hinge migration, yields tightening of folds and space accommodation issues within the anticlinal core; leading to the formation of disharmonic folds . [16] [17] Epard and Groshong, (1994) recognize a similar pattern to disharmonic folding they label it second-order shortening. [18] Basic models and experiments [4] [6] [12] [19] as well as concentric fold models [9] [20] fail to recognize disharmonic folds as they focus on single layer detachment folding, lack the resolution in experimental methods or, though the assumption of multiple units is made, restrict unit parameters which may cause disharmony through deformation. Continued shortening and excess material within the anticlinal core not only results in increased amplitude and disharmonic folds, but may lead to the onset of thrusts out of the folded synclinal or anticlinal regions. Through further deformation by limb rotation and through hinge migration, isoclinal folds eventually assume lift-off geometries. [4] [3] Thrust faults in the synclinal fold, if any formed, may also be rotated to assist in the formation of detached lift-off folds upon further tightening and rotation (figure 4). [3]

Detachment faulting

Figure 4. Schematic depicting faulting of a symmetric detachment fold. The result of continued limb rotation and compression is the formation of faults in the forelimb and backlimb of the fold. Eventually these faults reconnect with the detachment and a pop-up may occur. Symmetric Detachment Faulting.GIF
Figure 4. Schematic depicting faulting of a symmetric detachment fold. The result of continued limb rotation and compression is the formation of faults in the forelimb and backlimb of the fold. Eventually these faults reconnect with the detachment and a pop-up may occur.

It is documented in many cases that faulting may develop from detachment folding or through the kinematic mechanisms involved with folding. [4] [3] [6] [7] [15] [19] [21] In general, faulting may occur during fault slip and detachment folding in two ways. Firstly, faulting may be induced when progressive folding or tightening of a folded limb reaches its maximum fold geometry resulting in a transition from folding to shearing. [4] [12] Secondly, it has been suggested a fault may propagate into the anticlinal core if material flow and accommodation space are not at equilibrium. [4] The idea of insufficient material flow may not be as well addressed as faulting due to continued folding and rotation, but the grounds for such an argument lay within a strongly held belief of area conservation; without conservation faulting will likely compensate. The basic geometries of detachment faulting of a symmetric detachment fold are shown in figure 4. Refer to Mitra [4] [15] for an evolutionary model of faulted detachment folds in the asymmetric and symmetric settings.

Faulting may occur in a symmetric or asymmetric fold, yielding fault geometries that are both alike and dissimilar. Faulting in either setting is reliant on the lock-up and strain accumulation of a fold typically at its critical angle. Asymmetric folding develops in the forelimb (the limb furthest from the source of thrust) of the fold and may either absorb strain into or transmit strain through the stratigraphic units composing the fold. [15] A system that absorbs strain is recognized as a trishear zone [22] being triangular in shape; while a parallel deformation zone transmits shear across the units of the fold [15] and typically takes on the form of a parallelogram or is rectangular in geometry. These two deformation patterns may exist in a single fold and at some time during continued deformation may reconnect with the detachment. It is also the case that a backthrust may occur in an asymmetric fold geometry as shear across the forelimb due to rotation and migration of beds.

Symmetric faults were essentially covered previously under the name ‘lift-off’ folds, see figure 4. Progressive limb rotation and lock-up in a symmetric fold induces shear at both the forelimb and backlimb of the fold which may then result in faults on both limbs causing lift-off. Like the asymmetric fold faulting, as progressive slip along the basal detachment occurs, either the forelimb or backlimb (the limb closest to the source of thrust) thrust may reconnect with the basal detachment. [15] For a more robust definition of faulting reference Mitra 2002. [4] [15]

Related Research Articles

Structural geology The 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.

Thrust fault

A thrust fault is a break in the Earth's crust, across which older rocks are pushed above younger rocks.

Tectonics The processes that control the structure and properties of the Earths crust and its evolution through time

Tectonics are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of mountain building, the growth and behavior of the strong, old cores of continents known as cratons, and the ways in which the relatively rigid plates that constitute the Earth's outer shell interact with each other. Tectonics also provide a framework for understanding the earthquake and volcanic belts that directly affect much of the global population.

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. Synsedimentary folds are those formed during sedimentary deposition.

Anticline

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

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.

The Lewis Overthrust is a geologic thrust fault structure of the Rocky Mountains found within the bordering national parks of Glacier in Montana, United States and Waterton Lakes in Alberta, Canada. The structure was created due to the collision of tectonic plates about 170 million years ago that drove a several mile thick wedge of rock 50 mi (80 km) eastwards, causing it to overlie softer Cretaceous age rock that is 400 to 500 million years younger.

Décollement

Décollement is a gliding plane between two rock masses, also known as a basal detachment fault. Décollements are a deformational structure, resulting in independent styles of deformation in the rocks above and below the fault. They are associated with both compressional settings and extensional settings.

Thrust tectonics Study of the structures formed by, and the tectonic processes associated with, the shortening and thickening of the crust

Thrust tectonics or contractional tectonics is concerned with the structures formed by, and the tectonic processes associated with, the shortening and thickening of the crust or lithosphere.

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.

Salt tectonics

Salt tectonics, or halokinesis, or halotectonics, is concerned with the geometries and processes associated with the presence of significant thicknesses of evaporites containing rock salt within a stratigraphic sequence of rocks. This is due both to the low density of salt, which does not increase with burial, and its low strength.

Chevron (geology)

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. Turbidites, characterized by alternating high-competence sandstones and low-competence shales, provide the typical geological setting for chevron folds to occurs.

Ovda Regio

Ovda Regio is a Venusian crustal plateau located near the equator in the western highland region of Aphrodite Terra that stretches from 10°N to 15°S and 50°E to 110°E. Known as the largest crustal plateau in Venus, the regio covers an area of approximately 15,000,000 square kilometres (5,800,000 sq mi) and is bounded by regional plains to the north, Salus Tessera to the west, Thetis Regio to the east, and Kuanja as well as Ix Chel chasmata to the south. The crustal plateau serves as a place to hold the localized tessera terrains in the planet, which makes up roughly 8% of Venus' surface area. The kinematic evolution of crustal plateaus on Venus has been a debated topic in the planetary science community. Understanding its complex evolution is expected to contribute to a better knowledge of the geodynamic history of Venus. It is named after a Marijian forest spirit that can appear as both male and female.

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.

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.

Thick-skinned deformation is a geological term which refers to crustal shortening that involves basement rocks and deep-seated faults as opposed to only the upper units of cover rocks above the basement which is known as thin-skinned deformation. While thin-skinned deformation is common in many different localities, thick-skinned deformation requires much more strain to occur and is a rarer type of deformation.

Main Central Thrust

The Main Central Thrust is a major geological fault where the Indian Plate has pushed under the Eurasian Plate along the Himalaya. The fault slopes down to the north and is exposed on the surface in a NW-SE direction (strike). It is a thrust fault that continues along 2200 km of the Himalaya mountain belt.

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.

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.

References

  1. 1 2 Homza, T. and W. Wallace (1995) Geometric and kinematic models for detachment folds with fixed and variable detachment depths, Journal of Structural Geology, 17/4: 575-588
  2. 1 2 3 4 5 Hayes, M. and C. Hanks (2008) Evolving mechanical stratigraphy during detachment folding, Journal of Structural Geology, 30: 548-564
  3. 1 2 3 4 5 6 7 8 9 10 11 12 13 Mitra, S. (2003) A unified kinematic model for the evolution of detachment folds, Journal of Structural Geology, 25: 1659-1673
  4. 1 2 3 4 5 6 7 8 9 Mitra, S. (2002) Structural models of faulted detachment folds, American Association of Petroleum Geologist Bulletin, 86/9: 1673-1694
  5. 1 2 Stewart, S. (1996) Influence of detachment layer thickness on style of thin-skinned shortening, Journal of Structural Geology, 18/10: 1271-1274
  6. 1 2 3 4 5 6 7 Contreras, J. (2010) A model for low amplitude detachment folding and syntectonic stratigraphy based on the conservation of mass equation, Journal of Structural Geology, 32, 566-579
  7. 1 2 3 4 Hardy, S. and Poblet, J. (1994) Geometric and numerical model of progressive limb rotation in detachment folds, Geology, 22, 371-374
  8. 1 2 Wiltschko, D.V. and Chapple, W. M. (1977) Flow of weak rocks in Appalachian Plateau folds, American Association of Petroleum Geologists Bulletin, 61, 5, 653-669
  9. 1 2 3 Dalstrom, C. D. C. (1990) Geometric constraints derived from the law of conservation of volume and applied to evolutionary models for detachment folding, American Association of Petroleum Geologists Bulletin, 75, 3, 336-344
  10. 1 2 Suppe, J. (1983) Geometry and kinematics of fault-bend folding Archived 2016-03-03 at the Wayback Machine , American Journal of Science, 283, 684-721
  11. 1 2 Mitchel, M. M. and Woodward, N. B. (1988) Kink detachment fold in the southwest Montana fold and thrust belt, Geology, 16, 162-165
  12. 1 2 3 4 5 6 Poblet, J. and McClay, Ken. (1996) Geometry and kinematics of single-layer detachment folds, American Association of Petroleum Geologists Bulletin, 80, 7, 1085-1109
  13. ↑ Fischer, M. and Jackson, P. (1999) Stratigraphic controls on deformation patterns in fault-related folds: a detachment fold example from the Sierra Madre Oriental, northeast Mexico, Journal of Structural Geology, 21, 613-633
  14. 1 2 Hardy, S. and Finch, E. (2005) Discrete-element modeling of detachment folding, Basin Research, 17, 507-520
  15. 1 2 3 4 5 6 7 Mitra, S. (2002) Fold-accommodation faults, American Association of Petroleum Geologists Bulletin, 86, 4, 671-693
  16. ↑ Hardy, S. and Finch, E. (2005). Discrete-element modeling of detachment folding. Basin Research, 17, 507-520
  17. ↑ Mitra, S. and Namson, J. (1989) Equal-area balancing, American Journal of Science, 289, 563-599
  18. ↑ Epard, J. L. and Groshong, R. H., Jr. (1994) Kinematic model of detachment folding including limb rotation, fixed hinges and layer-parallel strain [ permanent dead link ], Tectonophysics 247, 85-103
  19. 1 2 Storti, F., Salvini, F., and McClay, K. (1997). Fault-related folding in sandbox analogue models of thrust wedges . Journal of Structural Geology, 19, 3-4, 583-602
  20. ↑ Dahlstrom, C. D. A. (1969) Balanced cross sections, Canadian Journal of Earth Sciences, 6, 743-757
  21. ↑ Bowsworth, W. (1983) Foreland deformation in the Appalachian Plateau, central New York: the role of small-scale detachment structures in regional overthrusting, Journal of Structural Geology, 6, 1-2, 73-81
  22. ↑ Zehnder, A. T. and Allmendinger, R. W. (2000) Velocity field for the trishear model, Journal of structural Geology, 22, 1009-1014
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