
Structural geology is the study of the three-dimensional distribution of
rock
Rock most often refers to:
* Rock (geology), a naturally occurring solid aggregate of minerals or mineraloids
* Rock music, a genre of popular music
Rock or Rocks may also refer to:
Places United Kingdom
* Rock, Caerphilly, a location in Wales ...
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 (e.g.,
mountain building
Mountain formation refers to the geological processes that underlie the formation of mountains. These processes are associated with large-scale movements of the Earth's crust (tectonic plates). Folding, faulting, volcanic activity, igneous intr ...
,
rifting) due to
plate tectonics.
Use and importance
The study of geologic structures has been of prime importance in
economic geology
Economic geology is concerned with earth materials that can be used for economic and/or industrial purposes. These materials include precious and base metals, nonmetallic minerals and construction-grade stone. Economic geology is a subdisciplin ...
, both
petroleum geology
Petroleum geology is the study of origin, occurrence, movement, accumulation, and exploration of hydrocarbon fuels. It refers to the specific set of geological disciplines that are applied to the search for hydrocarbons (oil exploration).
Sedime ...
and
mining geology
Mining geology is an applied science which combines the principles of economic geology and mining engineering to the development of a defined mineral resource. Mining geologists and engineers work to develop an identified ore deposit to economica ...
. Folded and faulted rock
strata
In geology and related fields, a stratum ( : strata) is a layer of rock or sediment characterized by certain lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as ei ...
commonly form traps that accumulate and concentrate fluids such as
petroleum and
natural gas. Similarly, faulted and structurally complex areas are notable as permeable zones for
hydrothermal fluids, resulting in concentrated areas of base and precious metal
ore deposits. Veins of minerals containing various metals commonly occupy faults and fractures in structurally complex areas. These structurally fractured and faulted zones often occur in association with
intrusive igneous rocks. They often also occur around geologic
reef complexes and collapse features such as ancient
sinkholes. Deposits of
gold,
silver,
copper,
lead,
zinc, and other metals, are commonly located in structurally complex areas.
Structural geology is a critical part of
engineering geology, which is concerned with the physical and mechanical properties of natural rocks. Structural fabrics and defects such as faults, folds, foliations and
joints are internal weaknesses of rocks which may affect the stability of human engineered structures such as
dams, road cuts,
open pit mines and
underground mines or road
tunnels.
Geotechnical risk, including
earthquake risk can only be investigated by inspecting a combination of structural geology and
geomorphology
Geomorphology (from Ancient Greek: , ', "earth"; , ', "form"; and , ', "study") is the scientific study of the origin and evolution of topographic and bathymetric features created by physical, chemical or biological processes operating at or n ...
. In addition, areas of
karst
Karst is a topography formed from the dissolution of soluble rocks such as limestone, dolomite, and gypsum. It is characterized by underground drainage systems with sinkholes and caves. It has also been documented for more weathering-resistant ro ...
landscapes which reside atop caverns, potential sinkholes, or other collapse features are of particular importance for these scientists. In addition, areas of steep slopes are potential collapse or landslide hazards.
Environmental geologists and
hydrogeologists need to apply the tenets of structural geology to understand how geologic sites impact (or are impacted by)
groundwater flow and penetration. For instance, a hydrogeologist may need to determine if seepage of toxic substances from waste dumps is occurring in a residential area or if salty water is seeping into an
aquifer.
Plate tectonic
Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large te ...
s is a theory developed during the 1960s which describes the movement of continents by way of the separation and collision of crustal plates. It is in a sense structural geology on a planet scale, and is used throughout structural geology as a framework to analyze and understand global, regional, and local scale features.
Methods
Structural geologists use a variety of methods to (first) measure rock geometries, (second) reconstruct their deformational histories, and (third) estimate the stress field that resulted in that deformation.
Geometries
Primary data sets for structural geology are collected in the field. Structural geologists measure a variety of planar features (
bedding planes
In geology, a bed is a layer of sediment, sedimentary rock, or pyroclastic material "bounded above and below by more or less well-defined bedding surfaces".Neuendorf, K.K.E., J.P. Mehl, Jr., and J.A. Jackson, eds., 2005. ''Glossary of Geology'' ...
,
foliation planes, fold axial planes,
fault planes, and joints), and
linear features (stretching lineations, in which minerals are ductilely extended; fold axes; and intersection lineations, the trace of a planar feature on another planar surface).
upright=1.3, Illustration of measurement conventions for planar and linear structures
Measurement conventions
The inclination of a planar structure in geology is measured by ''
strike and dip
Strike and dip is a measurement convention used to describe the orientation, or attitude, of a planar geologic feature. A feature's strike is the azimuth of an imagined horizontal line across the plane, and its dip is the angle of inclination m ...
''. The strike is the line of intersection between the planar feature and a horizontal plane, taken according to the right hand convention, and the dip is the magnitude of the inclination, below horizontal, at right angles to strike. For example; striking 25 degrees East of North, dipping 45 degrees Southeast, recorded as N25E,45SE.
Alternatively, dip and dip direction may be used as this is absolute. Dip direction is measured in 360 degrees, generally clockwise from North. For example, a dip of 45 degrees towards 115 degrees azimuth, recorded as 45/115. Note that this is the same as above.
The term ''hade'' is occasionally used and is the deviation of a plane from vertical i.e. (90°-dip).
Fold axis plunge is measured in dip and dip direction (strictly, plunge and azimuth of plunge). The orientation of a fold axial plane is measured in strike and dip or dip and dip direction.
Lineations are measured in terms of dip and dip direction, if possible. Often lineations occur expressed on a planar surface and can be difficult to measure directly. In this case, the lineation may be measured from the horizontal as a ''rake'' or ''pitch'' upon the surface.
Rake is measured by placing a protractor flat on the planar surface, with the flat edge horizontal and measuring the angle of the lineation clockwise from horizontal. The orientation of the lineation can then be calculated from the rake and strike-dip information of the plane it was measured from, using a
stereographic projection
In mathematics, a stereographic projection is a perspective projection of the sphere, through a specific point on the sphere (the ''pole'' or ''center of projection''), onto a plane (geometry), plane (the ''projection plane'') perpendicular to ...
.
If a fault has lineations formed by movement on the plane, e.g.;
slickensides, this is recorded as a lineation, with a rake, and annotated as to the indication of throw on the fault.
Generally it is easier to record strike and dip information of planar structures in dip/dip direction format as this will match all the other structural information you may be recording about folds, lineations, etc., although there is an advantage to using different formats that discriminate between planar and linear data.
Plane, fabric, fold and deformation conventions
The convention for analysing structural geology is to identify the planar structures, often called ''planar fabrics'' because this implies a
textural formation, the linear structures and, from analysis of these, unravel deformations.
Planar structures are named according to their order of formation, with original sedimentary layering the lowest at S0. Often it is impossible to identify S0 in highly deformed rocks, so numbering may be started at an arbitrary number or given a letter (S
A, for instance). In cases where there is a
bedding-plane
In geology, a bed is a layer of sediment, sedimentary rock, or pyroclastic material "bounded above and below by more or less well-defined bedding surfaces".Neuendorf, K.K.E., J.P. Mehl, Jr., and J.A. Jackson, eds., 2005. ''Glossary of Geology'' ...
foliation caused by burial metamorphism or
diagenesis this may be enumerated as S0a.
If there are folds, these are numbered as F
1, F
2, etc. Generally the axial plane foliation or
cleavage of a fold is created during folding, and the number convention should match. For example, an F
2 fold should have an S
2 axial foliation.
Deformations are numbered according to their order of formation with the letter D denoting a deformation event. For example, D
1, D
2, D
3. Folds and foliations, because they are formed by deformation events, should correlate with these events. For example, an F
2 fold, with an S
2 axial plane foliation would be the result of a D
2 deformation.
Metamorphic events may span multiple deformations. Sometimes it is useful to identify them similarly to the structural features for which they are responsible, e.g.; M
2. This may be possible by observing
porphyroblast formation in cleavages of known deformation age, by identifying metamorphic mineral assemblages created by different events, or via
geochronology.
Intersection lineations in rocks, as they are the product of the intersection of two planar structures, are named according to the two planar structures from which they are formed. For instance, the intersection lineation of a S
1 cleavage and bedding is the L
1-0 intersection lineation (also known as the cleavage-bedding lineation).
Stretching lineations may be difficult to quantify, especially in highly stretched ductile rocks where minimal foliation information is preserved. Where possible, when correlated with deformations (as few are formed in folds, and many are not strictly associated with planar foliations), they may be identified similar to planar surfaces and folds, e.g.; L
1, L
2. For convenience some geologists prefer to annotate them with a subscript S, for example L
s1 to differentiate them from intersection lineations, though this is generally redundant.
Stereographic projections
Stereographic projection
In mathematics, a stereographic projection is a perspective projection of the sphere, through a specific point on the sphere (the ''pole'' or ''center of projection''), onto a plane (geometry), plane (the ''projection plane'') perpendicular to ...
is a method for analyzing the nature and orientation of deformation stresses, lithological units and penetrative fabrics wherein linear and planar features (structural strike and dip readings, typically taken using a
compass clinometer) passing through an imagined sphere are plotted on a two-dimensional grid projection, facilitating more holistic analysis of a set of measurements. Stereonet developed by
Richard W. Allmendinger
Richard W. (Rick) Allmendinger is a structural geologist and Professor Emeritus of Earth and Atmospheric Sciences at Cornell University.
His work is focused on energy and climate change as well as earthquakes and the regional tectonics of South ...
is widely used in the structural geology community.
Rock macro-structures
On a large scale, structural geology is the study of the three-dimensional interaction and relationships of stratigraphic units within
terranes of rock or geological regions.
This branch of structural geology deals mainly with the orientation, deformation and relationships of stratigraphy (bedding), which may have been faulted,
folded or given a foliation by some tectonic event. This is mainly a geometric science, from which ''
cross sections'' and three-dimensional ''block models'' of rocks, regions, terranes and parts of the Earth's crust can be generated.
Study of regional structure is important in understanding
orogeny
Orogeny is a mountain building process. An orogeny is an event that takes place at a convergent plate margin when plate motion compresses the margin. An ''orogenic belt'' or ''orogen'' develops as the compressed plate crumples and is uplifted t ...
,
plate tectonics and more specifically in the oil,
gas and
mineral exploration industries as structures such as faults, folds and
unconformities are primary controls on ore mineralisation and oil traps.
Modern regional structure is being investigated using
seismic tomography
Seismic tomography or seismotomography is a technique for imaging the subsurface of the Earth with seismic waves produced by earthquakes or explosions. P-, S-, and surface waves can be used for tomographic models of different resolutions based on ...
and
seismic reflection in three dimensions, providing unrivaled images of the Earth's interior, its faults and the deep crust. Further information from
geophysics such as
gravity and airborne magnetics can provide information on the nature of rocks imaged to be in the deep crust.
Rock microstructures
Rock microstructure or ''texture'' of rocks is studied by structural geologists on a small scale to provide detailed information mainly about
metamorphic rocks and some features of
sedimentary rocks
Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles t ...
, most often if they have been folded.
Textural study involves measurement and characterisation of
foliations
Foliation may refer to:
* Foliation, a geometric device used to study manifolds
* Foliation (geology), a property of certain rocks
* A pagination system in book production
* Vernation, the growth and arrangement of leaves
* In architecture, an orn ...
,
crenulations, metamorphic minerals, and timing relationships between these structural features and mineralogical features.
Usually this involves collection of hand specimens, which may be cut to provide
petrographic thin sections which are analysed under a
petrographic microscope.
Microstructural analysis finds application also in multi-scale statistical analysis, aimed to analyze some rock features showing scale invariance.
[
]
Kinematics
Geologists use rock geometry measurements to understand the history of strain in rocks. Strain can take the form of brittle faulting and ductile folding and shearing. Brittle deformation takes place in the shallow crust, and ductile deformation takes place in the deeper crust, where temperatures and pressures are higher.
Stress fields
By understanding the constitutive relationships between stress and strain in rocks, geologists can translate the observed patterns of rock deformation into a stress field during the geologic past. The following list of features are typically used to determine stress fields from deformational structures.
*In perfectly brittle rocks, faulting occurs at 30° to the greatest compressional stress. (Byerlee's Law)
*The greatest compressive stress is normal to fold axial planes.
Characterization of the mechanical properties of rock
The mechanical properties of rock play a vital role in the structures that form during deformation deep below the earth's crust. The conditions in which a rock is present will result in different structures that geologists observe above ground in the field. The field of structural geology tries to relate the formations that humans see to the changes the rock went through to get to that final structure. Knowing the conditions of deformation that lead to such structures can illuminate the history of the deformation of the rock.
Temperature and pressure play a huge role in the deformation of rock. At the conditions under the earth's crust of extreme high temperature and pressure, rocks are
ductile. They can bend, fold or break. Other vital conditions that contribute to the formation of structure of rock under the earth are the
stress and strain fields.
Stress-strain curve
Stress is a pressure, defined as a directional force over area. When a rock is subjected to stresses, it changes shape. When the stress is released, the rock may or may not return to its original shape. That change in shape is quantified by strain, the change in length over the original length of the material in one dimension. Stress induces strain which ultimately results in a changed structure.
Elastic deformation refers to a reversible deformation. In other words, when stress on the rock is released, the rock returns to its original shape. Reversible, linear, elasticity involves the stretching, compressing, or distortion of atomic bonds. Because there is no breaking of bonds, the material springs back when the force is released. This type of deformation is modeled using a linear relationship between stress and strain, i.e. a
Hookean
In physics, Hooke's law is an empirical law which states that the force () needed to extend or compress a spring by some distance () scales linearly with respect to that distance—that is, where is a constant factor characteristic of ...
relationship.
:
Where σ denotes stress,
denotes strain, and E is the
elastic modulus, which is material dependent. The elastic modulus is, in effect, a measure of the strength of atomic bonds.
Plastic deformation refers to non-reversible deformation. The relationship between stress and strain for permanent deformation is nonlinear. Stress has caused permanent change of shape in the material by involving the breaking of bonds.
One mechanism of plastic deformation is the movement of dislocations by an applied stress. Because rocks are essentially aggregates of minerals, we can think of them as poly-crystalline materials. Dislocations are a type of crystallographic defect which consists of an extra or missing half plane of atoms in the periodic array of atoms that make up a crystal lattice. Dislocations are present in all real crystallographic materials.
Hardness
Hardness is difficult to quantify. It is a measure of resistance to deformation, specifically permanent deformation. There is precedent for hardness as a surface quality, a measure of the abrasiveness or surface-scratching resistance of a material. If the material being tested, however, is uniform in composition and structure, then the surface of the material is only a few atomic layers thick, and measurements are of the bulk material. Thus, simple surface measurements yield information about the bulk properties. Ways to measure hardness include:
*
Mohs Scale
The Mohs scale of mineral hardness () is a qualitative ordinal scale, from 1 to 10, characterizing scratch resistance of various minerals through the ability of harder material to scratch softer material.
The scale was introduced in 1812 by th ...
*
Dorry abrasion test
A dory is a small, shallow-draft boat, about long. It is usually a lightweight boat with high sides, a flat bottom and sharp bows. It is easy to build because of its simple lines. For centuries, the dory has been used as a traditional fishin ...
*
Deval abrasion test Deval is a given name, a surname, Gotra of varied origins (Indian and French). It may refer to:
Surname
*Chandra Prakash Deval (born 1949), Indian poet and translator
* Charles Deval (1806–1862), French ophthalmologist
*Govind Ballal Deval (185 ...
*
Indentation hardness
Indentation hardness is used often in metallurgy and materials science and can be thought of as resistance to penetration by an indenter.
Toughness
Toughness can be described best by a material's resistance to cracking. During plastic deformation, a material absorbs energy until fracture occurs. The area under the stress-strain curve is the work required to fracture the material. The toughness modulus is defined as:
:
Where
is the ultimate tensile strength, and
is the strain at failure. The modulus is the maximum amount of energy per unit volume a material can absorb without fracturing. From the equation for modulus, for large toughness, high strength and high ductility are needed. These two properties are usually mutually exclusive. Brittle materials have low toughness because low plastic deformation decreases the strain (low ductility). Ways to measure toughness include:
Page impact machine
Page most commonly refers to:
* Page (paper), one side of a leaf of paper, as in a book
Page, PAGE, pages, or paging may also refer to:
Roles
* Page (assistance occupation), a professional occupation
* Page (servant), traditionally a young mal ...
and
Charpy impact test
In materials science, the Charpy impact test, also known as the Charpy V-notch test, is a standardized high strain rate test which determines the amount of energy absorbed by a material during fracture. Absorbed energy is a measure of the mate ...
Resilience
Resilience is a measure of the elastic energy absorbed of a material under stress. In other words, the external work performed on a material during deformation. The area under the elastic portion of the stress-strain curve is the strain energy absorbed per unit volume. The resilience modulus is defined as:
:
where
is the yield strength of the material and E is the elastic modulus of the material. To increase resilience, one needs increased elastic yield strength and decreased modulus of elasticity.
See also
*
Crenulation
*
List of rock textures
*
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 int ...
*
Stereographic projection
In mathematics, a stereographic projection is a perspective projection of the sphere, through a specific point on the sphere (the ''pole'' or ''center of projection''), onto a plane (geometry), plane (the ''projection plane'') perpendicular to ...
*
Tectonophysics
*
Vergence (geology)
*
Hydrogeology
References
Further reading
*
*
*
*
*
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