Section restoration

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Example of restored and balanced section from National Wildlife Refuge 1002 Area, Alaska USGS Balanced Section.png
Example of restored and balanced section from National Wildlife Refuge 1002 Area, Alaska

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. [1]

Contents

Comparably a palinspastic map is a map view of geological features, often also including present-day coastlines to aid the reader in recognising the area, representing the state before deformation.

2D restoration

Development of technique

The earliest attempts to produce restored sections were on foreland fold and thrust belts. [2] This technique assumed a stratigraphic template with unit thicknesses either constant or smoothly varying across the section. Line lengths were measured on the present-day deformed section and transferred to the template, to rebuild the section as it was before deformation started. This method does not guarantee that area is conserved, only line length. The technique was applied to areas of extensional tectonics initially using vertical simple shear. [3] [4] Over the next decade several types of commercial restoration software became available, allowing the technique to be routinely applied.

Deformation algorithms

SimpleshearRestoration.png

In order to calculate the change in shape of an element within the section, various deformation algorithms are used. Initially many of these were applied manually, but are now available in specialist software packages. It is worth mentioning that these deformation algorithms are approximations and idealizations of actual strain paths and deviate from reality (Ramsey and Huber, 1987). Geologic media are typically not continuum materials; that is, they are not isotropic media as is implicitly assumed in all strain algorithms used for cross-section balancing. That said, balanced cross sections maintain material balance, which is important for conceptualizing kinematic histories of deformed regions.

Vertical/inclined shear

This mechanism deforms an element to accommodate a change in shape by movement on closely spaced parallel planes of slip. The commonest assumption is vertical shear although comparisons with well understood examples suggest that antithetic inclined shear (i.e. in the opposite sense of dip to the controlling fault) at about 60°–70° is the best approximation to the behaviour of real rocks under extension. [5] [6] These algorithms preserve area but do not, in general, preserve line length. Restoration using this type of algorithm can be carried out by hand, but is normally done using specialist software. This algorithm is not generally thought to represent the actual mechanism by which deformation occurs, just to represent a reasonable approximation.

Flexural slip

In a flexural slip algorithm deformation occurs by unfolding the deformed fault bounded horse by slip along bedding planes. [1] This modelling mechanism does represent a real geological mechanism, as shown by slickensides along folded bedding planes. [7] The shape of the unfolded horse is further constrained either by using the restored fault boundary to the previous horse in the restored section of by using an internal pin within the block itself, assuming this was unsheared during the deformation. This algorithm is normally only used in software based restoration. It preserves both area and line length.

Trishear

A trishear algorithm is used to model and restore fault-propagation folds as other algorithms fail to explain thickness changes and strain variations associate with such folds. The deformation within the tip-zone of the propagating fault is idealised to heterogeneous shear within a triangular zone starting at the fault tip. [8]

Compaction

In most section restorations there is an element of backstripping and decompaction. This is necessary to adjust the geometry of the section for the compactional effects of later sediment loading. [9]

Forward modelling

Section restoration involves undeforming a natural example, a form of inverse modelling. [10] In many cases carrying out forward modelling helps to test out concepts for all or part of the section.

3D restoration

A basic assumption of 2D restoration is that the displacement on all faults is within the plane of the section. It also assumes that no material enters or leaves the section plane. In areas of complex multi-phase or strike slip deformation or where salt is present, this is rarely the case. 3D restoration can only be carried out using specialist software, such as Midland Valley's Move3D, Paradigm's Kine3D or Schlumberger's Dynel3D. The results of such restoration can be used to study the migration of hydrocarbons at an earlier stage. [11]

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 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.

Fold (geology) Stack of originally planar surfaces

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.

Shear zone Structural discontinuity surface in the Earths crust and upper mantle

In geology, a shear zone is a thin zone within the Earth's crust or upper mantle that has been strongly deformed, due to the walls of rock on either side of the zone slipping past each other. In the upper crust, where rock is brittle, the shear zone takes the form of a fracture called a fault. In the lower crust and mantle, the extreme conditions of pressure and temperature make the rock ductile. That is, the rock is capable of slowly deforming without fracture, like hot metal being worked by a blacksmith. Here the shear zone is a wider zone, in which the ductile rock has slowly flowed to accommodate the relative motion of the rock walls on either side.

Mylonite Metamorphic rock

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.

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.

Moab Fault

The Moab Fault, near Moab, Utah, United States, is an extensional fault that runs approximately NW-SE, passing to the west of the Arches National Park. It is about 45 km (28 mi) long and has a maximum displacement of about 960 m (3,150 ft). The fault connects with the Tenmile Graben in the north and extends through the Moab-Spanish Valley to the south. The fault outcrop has a well-defined fault zone bordered by a damage zone of minor faults and fractures.

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. It is one of the three main types of tectonic regime, the others being extensional tectonics and strike-slip tectonics. These match the three types of plate boundary, convergent (thrust), divergent (extensional) and transform (strike-slip). There are two main types of thrust tectonics, thin-skinned and thick-skinned, depending on whether or not basement rocks are involved in the deformation. The principle geological environments where thrust tectonics is observed are zones of continental collision, restraining bends on strike-slip faults and as part of detached fault systems on some passive margins.

Fracture (geology) Geologic discontinuity feature, often a joint or fault

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.

A deformation mechanism, in geotechnical engineering, is a process occurring at a microscopic scale that is responsible for changes in a material's internal structure, shape and volume. The process involves planar discontinuity and/or displacement of atoms from their original position within a crystal lattice structure. These small changes are preserved in various microstructures of materials such as rocks, metals and plastics, and can be studied in depth using optical or digital microscopy.

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.

The salt tectonics off the Louisiana gulf coast can be explained through two possible methods. The first method attributes spreading of the salt because of sedimentary loading while the second method points to slope instability as the primary cause of gliding of the salt. The first method results in the formation of growth faults in the overlying sediment. Growth faults are normal faults that occur simultaneously with sedimentation, causing them to have thicker sediment layers on the downthrown sides of the faults. In the second method both the salt and the sediment are moving, making it more likely to migrate.

El Tigre Fault

The El Tigre Fault is a 120 km long, roughly north-south trending, major strike-slip fault located in the Western Precordillera in Argentina. The Precordillera lies just to the east of the Andes mountain range in South America. The northern boundary of the fault is the Jáchal River and its southern boundary is the San Juan River. The fault is divided into three sections based on fault trace geometry, Northern extending between 41–46 km in length, Central extending between 48–53 km in length, and Southern extending 26 km in length. The fault displays a right-lateral (horizontal) motion and has formed in response to stresses from the Nazca Plate subducting under the South American Plate. It is a major fault with crustal significance. The Andes Mountain belt trends with respect to the Nazca Plate/South American Plate convergence zone, and deformation is divided between the Precordilleran thrust faults and the El Tigre strike-slip motion. The El Tigre Fault is currently seismically active.

Karakoram fault system Fault system in the Himalayan region across India and Asia

The Karakoram fault is an oblique-slip fault system in the Himalayan region across India and Asia. The slip along the fault accommodates radial expansion of the Himalayan arc, northward indentation of the Pamir Mountains, and eastward lateral extrusion of the Tibetan plateau. Current plate motions suggest that the convergence between the Indian Plate and the Eurasian Plate is around 44±5 mm per year in the western Himalaya-Pamir region and approximately 50±2 mm per year in the eastern Himalayan region.

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.

Paleostress inversion refers to the determination of paleostress history from evidence found in rocks, based on the principle that past tectonic stress should have left traces in the rocks. Such relationships have been discovered from field studies for years: qualitative and quantitative analyses of deformation structures are useful for understanding the distribution and transformation of paleostress fields controlled by sequential tectonic events. Deformation ranges from microscopic to regional scale, and from brittle to ductile behaviour, depending on the rheology of the rock, orientation and magnitude of the stress etc. Therefore, detailed observations in outcrops, as well as in thin sections, are important in reconstructing the paleostress trajectories.

This is a compilation of the properties of different analog materials used to simulate deformational processes in structural geology. Such experiments are often called analog or analogue models. The organization of this page follows the review of rock analog materials in structural geology and tectonics of Reber et al. 2020.

References

  1. 1 2 Groshong, R. (2006). "Structural Validation, Restoration and Prediction". 3-D structural geology: a practical guide to quantitative surface and subsurface map interpretation. Birkhäuser. pp. 305–372. ISBN   978-3-540-31054-9 . Retrieved 2010-02-20.
  2. ↑ Bally, A.W.; Gordy P.L.; Stewart G.A. (1966). "Structure, seismic data and orogenic evolution of Southern Canadian Rocky Mountains". Bulletin of Canadian Petroleum Geology. 14: 337–381.
  3. ↑ Gibbs, A.D. (1983). "Balanced cross-section construction from seismic sections in areas of extensional tectonics". Journal of Structural Geology. 5 (2): 153–160. Bibcode:1983JSG.....5..153G. doi:10.1016/0191-8141(83)90040-8.
  4. ↑ Williams, G.; Vann I. (1987). "The geometry of listric normal faults and deformation in their hanging walls". Journal of Structural Geology. 9 (7): 789–795. Bibcode:1987JSG.....9..789W. doi:10.1016/0191-8141(87)90080-0.
  5. ↑ Hauge, T.A.; Gray G.G. (1996). "A critique of techniques for modelling normal-fault and rollover geometries". In Buchanan P.G. & Nieuwland D.A. (ed.). Modern Developments in Structural Interpretation, Validation and Modelling. Special Publications. Vol. 99. Geological Society, London. pp. 89–97. Retrieved 2010-02-09.
  6. ↑ Withjack, M.O.; Schlische R.W. (2006). "Geometric and experimental models of extensional fault-bend folds" (PDF). In Buiter S.J.H. & Schreurs G. (ed.). Analogue and Numerical Modelling of Crustal-Scale Processes. Special Publications. Vol. 253. Geological Society, London. pp. 285–305. Retrieved 2010-02-09.
  7. ↑ Faill, R.T. (1973). "Kink-Band Folding, Valley and Ridge Province, Pennsylvania". Bulletin of the Geological Society of America. 84 (4): 1289–1314. Bibcode:1973GSAB...84.1289F. doi:10.1130/0016-7606(1973)84<1289:KFVARP>2.0.CO;2 . Retrieved 2010-02-20.
  8. ↑ Erslev, E.A. (1991). "Trishear fault-propagation folding". Geology. 19 (6): 617–620. Bibcode:1991Geo....19..617E. doi:10.1130/0091-7613(1991)019<0617:TFPF>2.3.CO;2 . Retrieved 2010-02-20.
  9. ↑ Skuce, A.G. (1996). "Forward modelling of compaction above normal faults: an example from the Sirte Basin, Libya". In Buchanan P.G. and Nieuwland D.A. (ed.). Modern Developments in Structural Interpretation, Validation and Modelling. Special Publications. Vol. 99. London: Geological Society. pp. 135–146. Retrieved 2010-02-20.
  10. ↑ Poblet, J.; Bulnes M. (2007). "Predicting strain using forward modelling of restored cross-sections: Application to rollover anticlines over listric normal faults". Journal of Structural Geology. 29 (12): 1960–1970. Bibcode:2007JSG....29.1960P. doi:10.1016/j.jsg.2007.08.003.
  11. ↑ Clarke, S.M.; Burley S.D.; Williams G.D.; Richards A.J.; Meredith D.J.; Egan S.S. (2006). "Integrated four-dimensional modelling of sedimentary basin architecture and hydrocarbon migration". In Buiter S.J.H. & Schreurs G. (ed.). Analogue and Numerical Modelling of Crustal-Scale Processes. Special Publications. Vol. 253. Geological Society, London. pp. 185–211. ISBN   978-1-86239-191-8 . Retrieved 2010-02-20.
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