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Key of Structures


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natural: structures developed in nature without human influence.
experimental: structures developed under known conditions in experiments.


primary / secondary
primary: developed during genesis of the rock.
secondary: secondary fabrics reflect deformation.

rock type


The word fold is used when one or stacks of originally flat and planar surfaces such as sedimentary beds become bent or curved as a result of plastic (i.e. permanent) deformation. Folds may result from a primary deformation, which means that folding occurred during the formation of the rock, or a consequence of a secondary, i.e. tectonic deformation. Slumps in soft sediments and flow folds in lavas are examples of primary folds. In structural geology we are mainly concerned by the tectonic folds which are, in general, produced by a shortening parallel to the layering of the rocks. Folds in rocks vary in size from microscopic crinkles to mountain-size folds. They occur singly as isolated folds and in extensive fold trains of different sizes, in a variety of scales. A set of folds distributed on a regional scale constitutes a fold belt.
Folds form under varied conditions of stress, hydrostatic pressure, pore pressure, and temperature as evidenced by their presence in sediments, sedimentary rocks, the full spectrum of metamorphic rocks, and in some igneous rocks. Their spectacular presence indicates that plastic (i.e. permanent) deformation has resulted in gradual and continuous changes in a rock layer both in its attitudes and internally. However, it must be remembered that the absence of folds does not indicate the absence of a pervasive deformation.

Fault (brittle):
A fault is a fracture between blocks of rock that have been displaced past each other, in a direction parallel to the fracture plane. A fault zone is a centimetre to kilometres wide region fractured by a group of related faults, often parallel or anastomosing.

Shear zone:
A shear zone is a zone across which blocks of rock have been displaced without prominent development of visible fractures. Shear zones are thus regions of localised ductile displacement, in contrast to fault zones that are regions of localised brittle deformation.

Foliation is a general term covering secondary, parallel and often closely spaced planar elements found in deformed rocks.
Foliation planes are referred to as S-surfaces. Where S-surfaces of different generations are present in the same fabric, they are given numerical suffixes according to relative age: S0 is the primary surface, generally bedding, and S1, S2, Sn for secondary foliations in order of determined superposition. A foliation is essentially penetrative, that is it appears everywhere throughout the entire rock mass and tends to obliterate earlier structures.

Lineation is a general term to describe any repeated, penetrative linear structure that occurs in a rock. A lineation may be a primary igneous or sedimentary structure, such as an array of elongate K-feldspar porphyroblasts, inclusions or pebbles, oriented with their long dimensions mutually parallel. The primary alignment of markers is correlated with the direction of flow of magma or paleacurrent and form what are known as flow lines. But a lineation can be a secondary structure produced as a result of deformation. In structural geology we are particularly concerned with the latter.
Lineations are common in deformed rocks. A single deformation may produce several sets of lineations with different orientations within a given foliation plane. Many lineations are associated with a foliation and actually lie in that surface. This implies that a lineation, is as penetrative as the foliation,
Lineations are referred to as L-elements of the rock fabric. Where L-lines of different generations are present in the same fabric, they are given numerical suffixes according to relative age: L0 is the primary line and L1, L2, Ln for secondary lineations in order of determined superposition.

Because most rocks are brittle at low temperature and low lithostatic pressure virtually every rock at or near the Earth’s surface contain evidence of brittle failure. Planar discontinuities along which rocks loose their cohesion result from brittle behaviour of rocks. They are called joints if there is no component of displacement parallel to the plane. These geological parting planes have their economic importance. From the viewpoint of hydrology and petroleum exploitation they provide the necessary permeability for migration and accumulation. They often control the deposition of ore bodies. They divide the rocks in blocks that must be taken into consideration for engineering, quarrying, mining and geomorphology. Most joints form by development of cracks that are planes across which the rock cohesion is lost, a process called fracturing. Many joints may be partially or wholly healed by introduction of secondary minerals, often giving rise to ore deposits, or by recrystallisation of the original minerals. Fractures with secondary crystallisation are called veins.

overprinted structure

One of the above mentioned structures overprinting an older structure.


Geological area
Name for the geological area.


search description

any word


structure subtype 1

Interlimb angles
In profile, the smaller angle made by the limbs of a fold is termed the inter-limb angle, a measure of the tightness of the fold. It is the angle subtended by the tangents at two adjacent inflection points, which may reflect the intensity of compression. A qualitative classification is based on the minimum angle between the limbs, separating five tightness classes:
gentle (180 to ca. 120°)
open (120-70°),
close (70-30°),
tight (less than 30°) and
isoclinal (0°, i.e. parallel limbs).

The degree of localisation of foliation planes may vary between two broad categories: penetrative foliation and spaced foliation. In penetrative foliation, all platy grains have a statistically preferred planar orientation. Intervals between the foliation planes (or cleavage) are seen on a microscopic scale. Spaced foliation planes are discrete, tabular cleavage domains separated by thin lamellae of uncleaved rock or rock with a differently oriented, older, primary or secondary fabric. Spacing between the foliation planes is significantly larger than a few grains size. These intervening rock lamellae are called microlithons.
Fault (brittle) + Shear zone
Faults are classified according to the direction of the relative movement between fault blocks, which is related to the type of stress causing the fault. Three basic types of faults are recognised:

Strike slip faults
A fault with dominant horizontal movement parallel to the fault plane is called a strike slip fault. A fault involving relative displacement parallel to the dip of the fault plane is a dip slip fault. Strike slip faults usually have very steep or vertical dips and are then referred to as transcurrent faults or wrench faults. A large transcurrent fault that terminates in another large structure is called a transfer fault.
The sense of the strike slip displacement on a fault is described by the terms sinistral and dextral. A fault is sinistral if, to an observer standing on one block and facing the other, the opposite block appears to have been displaced to his left. Conversely, if the movement is to the right the fault is dextral.
A transform fault is a strike slip fault that forms at plate boundaries. They are fracture zones commonly striking at right angles to the mid-oceanic ridges and they appear to offset the ridges. But they differ from transcurrent faults in that the direction of horizontal movements is in the opposite direction to that required if the faults were strike slip faults responsible for offsetting the ridges after they were formed. These faults are active between the ridges and dead beyond the offsets.

Normal faults
A normal fault is a high angle, dip slip fault on which the hanging wall has moved down relative to the footwall. Because of the separation of geological horizons that results from normal faulting, such faults are also termed extension faults.

Reverse faults
A reverse fault is a dip slip fault on which the hanging wall has moved up and over the footwall. Such faults exhibit a repetition or overlap of a geological horizon, and are accordingly also termed compression fault. A thrust fault is a low-angle reverse fault along which the hanging wall forms thrust-sheets (nappes) of allochthonous rocks emplaced over the autochthonous footwall.
A window (or fenster) is an exposure of the rock below a thrust fault that is completely surrounded by rocks above the thrust. A klippe is an exposure of the hanging wall completely surrounded by rock of the footwall.

The terms normal fault and reverse fault, while strictly defined for faults with zero strike slip displacement, can also be used for faults with small strike slip displacements accompanying much larger dip slip displacements. Where the strike slip and dip slip displacements are similar in magnitude, the fault can be called an oblique slip fault.

Slickenside striae
Slickenside striae are the direct result of frictional sliding and are a common linear structure on fault surfaces. Grooves and ridges or striations are precisely parallel linear channels made by shear abrasion of one wall of a fracture on the other. They indicate the direction of the fault slip vector.
Slickenside striae may be found on bedding surfaces involved in flexural slip folding. They indicate that successive layers have slip over one another as the folds tightened. These lineations usually make a large angle with the fold axis and consistently show that the upper beds move upward towards the anticline axes. Slickenside striae are not penetrative (they are confined to surfaces) and therefore are not a fabric element.
Intersection lineations
Since any two planar surfaces intersect in a line, most rocks that are folded with concomitant development of an axial plane foliation display the intersection lineation between bedding and the axial plane foliation. The trace of bedding on a foliation plane commonly appears as colour stripes generally parallel to local fold hinges. Similar lineations can also be due to the intersection of two foliations, for example, the intersection of a crenulation cleavage and the earlier secondary foliation. The more planar surfaces there are in an exposure, the more potential lineations there will be. The trace of any plane on a random joint surface produces a linear structure that is insignificant in structural analysis; direct measurement of a lineation must be done on the foliation plane that belongs to the same episode of deformation.
When an axial plane cleavage and a bedding fissility are both prominent, the rock has a tendency to break up along elongate fragments sub-parallel to the fold axis. The resulting structure is known as pencil structure.
Intersection lineations and pencil structures are often utilised to determine the orientation of the fold axis where they are not exposed.
Axes of folds as lineations
Fold hinges are linear structures. Intense, small scale folding or crenulation of an earlier foliation produces a prominent linear structure due to the parallel, closely spaced and regular fold hinges. The crenulation lineation is a fabric element parallel to the associated hinge directions. It generally is a good indication of superposed deformation. Two or more sets of such lineations may intersect one another, sometimes in a conjugate manner, forming all sort of small-scale interference patterns.
Many schists exhibit spectacular examples of this type of lineation. Rolling of some minerals during deformation may produce a wrinkling of an existing part of the foliation. If carried out far enough, this may produce some kind of chevron folds parallel to the rotation axis of the minerals.
Mullions and Rods
Mullions are coarse structures formed in the original rock material as opposed to segregated or introduced material. The mullion is a columnar corrugation of the surface of a competent layer, at any size. These long features are remarkably cylindrical. They have a ribbed or grooved appearance, often cuspate in shape with broad smoothly curved convex surfaces separated by narrow, sharp, inward-closing hinges. The individual surface features are very persistent along the length of the mullion. Characteristically, micas coat mullions, but polished or longitudinally striated surfaces have been described. They often are lobate synforms separated by cusps on a surface that separates a competent and an incompetent layer.
Rod is a morphological term for elongate, cylindrical and monomineralic bodies of some segregated mineral (quartz, calcite, pyrite, etc.) enclosed in metamorphic rocks of all grades. In profile, rods may have any outline, from elliptical to irregular to that of a dismembered fold.
Both mullions and rods, they are generally parallel to related fold hinges and some descriptions may well comprise detached fold hinges. They are generally believed to be elongate parallel to the local fold axes.
Stretching lineations
An important type of lineation is formed by the parallel alignment of a set of objects that have acquired an elongate shape as a result of deformation. Individual detrital grains or fragments of any size may be deformed and/or rotated, to define a lineation. Ooids and spherulites are generally approximately spherical before deformation and they therefore must be deformed, rather than simply rotated, before their ellipsoidal shape can define an elongation (also extension or stretching) lineation. Such lineations also occur in pebbles or boulder beds. Extension lineations are generally inclined to the related fold axes at an angle close to 90° (transverse lineations) but locally may be parallel to the fold axis. In a given area it is commonly one or the other. However, there are examples where pebbles oriented with their short dimension perpendicular to bedding, have their long dimension perpendicular to the fold axis in limb areas and parallel to the fold axis in hinge areas.
Mineral Lineations
Metamorphic minerals often grow with a preferred crystallographic and dimensional orientation, i.e. with their long axes in parallel alignment. Mineral lineations are delineated by the long axes of individual, elongate or platy crystals (for example amphibole crystals, sillimanite needles) or mineral aggregates aligned and sub-parallel within a foliation plane. They are a penetrative element of the rock fabric, commonly parallel to other types of lineation, and serve to reinforce them. Mineral lineations may be parallel or inclined to the axes of related folds.
"Pressure shadow" or ''pressure fringe'' structures generally comprise spindle-shaped aggregates of new grains growing on opposed sides of a single host porphyroblast or competent, detrital grain. They form when material dissolved by pressure solution is re-pricipitated as fibres in the pressure shadow behind rigid grains. The fibres form long tails. The central grain along with both tails produce an elongate structure that is generally aligned parallel to a foliation and may define a lineation.
Boudinage depicts the periodical segmentation of pre-existing bodies, generally more competent than the rock surrounding them, when they are inhomogeneously stretched during deformation. Typically, a strong layer or dyke is broken up into a series of elongate and aligned blocks (whose cylinder-like shape motivated the name boudin). Boudin profiles are variable, with rectangular, rhomboidal and lozenge shapes being common. In low-grade rocks, boudins are usually separated and form a pull-apart structure or gap, which is generally mineralised. The zone of separation is referred to as a scar. The ductile layers surrounding separate boudins often flow in the space between boudins, forming scar folds (or neck folds). At higher grades, and in unconsolidated rocks, the competent layers have generally not broken through; narrow, thinned necks separate and alternate with boudins of relatively still, thick layers and the resulting structure is known as pinch-and-swell. Pinch-and-swell and pull-apart structures may be combined at any level since they really depend on the ductility contrast between the strong bed and its matrix. Objects such as fossils, pebbles and minerals are also deformed into small boudinage like structures (linear streaking of minerals).
Boudins are separated by material that originally lay on either side of the boudinaged layer or by mineral aggregates that have grown in situ as individual boudins moved apart. The shape of boudins as seen in profile varies and there is a complete spectrum of shapes between isolated, rectangular or rhomboid blocks to regular, elliptical pinch and swells.
Boudins are commonly linear and aligned parallel to the axes of related folds. However, stretching may take place in two directions in the plane of layering. Segmentation in these two directions produces nearly equidimensional boudins rather than the elongate forms. This process is referred to as chocolate-tablet boudinage.
Structures similar to boudins and pinch-and-swells may occur in certain zones of homogeneous strongly foliated rocks with no apparent lithological contrast between the boudins and the host rocks. These generally long lens-shaped structures are described as foliation boudinage.
Boudins (and mullions) tend to be large in size and are commonly restricted to certain layers or, in the case of mullions, are restricted even to certain surfaces in a deformed sequence. Thus at outcrop scale they are a non-penetrative feature.
A boudin axis can be measured, like a fold axis, as the nearest approximation to a line that, if moved parallel to itself, generates the boudin form. The neckline connects points of minimum layer-thickness. The length of a boudin is measured parallel to the boudin axis. The width and the thickness are dimensions orthogonal to this axis.

Joint sets and systems
Joints occur in almost every exposure as families of fractures with a rather regular spacing in a given rock type. A set is a group of joints approximately parallel to one another and sharing a common origin. However, it has been shown that they are not necessarily of the same age. Systematic joints are characterised by a roughly planar geometry, non-systematic joints are curved and irregular in geometry.

structure subtype 2

Upright, inclined and recumbent folds
Folds with approximately vertical axial surfaces are termed upright. Folds with dipping axial planes are termed inclined (80° < steeply < 60° < moderately < 30° < gently < 10°). Those with sub-horizontal axial planes are termed recumbent. Plunging folds have axial planes rotated by more than 90°. Large recumbent folds with several kilometre long inverted limbs are sometimes called fold nappes.

Fault (brittle):
Faults dipping more or less than 45° are called, respectively, high angle faults and low angle faults. The rock immediately above and below a non-vertical fault is referred to, respectively, as the hanging wall and the footwall of the fault.
The shape of fault surfaces is important. In general, fault surfaces are curved, and this gives rise to space problems if the adjacent blocks are rigid. This is because when the opposing blocks are displaced, they cannot remain uniformly in contact and voids must develop. The voids may be filled by broken rocks of the fault walls, or may provide sites at which minerals are subsequently deposited from circulating fluids. A listric fault is a curved, concave upward fault, that is, it gradually flattens with depth.

Shear zone:
Faults dipping more or less than 45° are called, respectively, high angle faults  and low angle faults. The rock immediately above and below a non-vertical fault is referred to, respectively, as the hanging wall and the footwall of the fault.
The shape of fault surfaces is important. In general, fault surfaces are curved, and this gives rise to space problems if the adjacent blocks are rigid. This is because when the opposing blocks are displaced, they cannot remain uniformly in contact and voids must develop. The voids may be filled by broken rocks of the fault walls, or may provide sites at which minerals are subsequently deposited from circulating fluids. A listric fault is a curved, concave upward fault, that is, it gradually flattens with depth.

Bedding foliation
It is common to observe a single set of foliation planes parallel to the bedding. It is termed as bedding foliation. This may arise through tight folding with a resulting very small angle between the axial plane foliation and the sedimentary layers. However, most examples of bedding foliation have occurred where the layering is relatively undeformed. The inference is that it is due to vertical compaction of the sediments under the static load of overlying strata. In the latter case, foliation is not axial planar to any phase of folding. Instead, foliation results essentially from oriented crystallisation during diagenesis and is, therefore, a primary structure.
Shear foliation
When a ductile shear zone develops in massive rocks like granites, the foliation that initially develops from the massive rock is not associated with a synchronous system of folds. The foliation typically shows a progressive rotation along with an increase in intensity (depicted by foliation planes that come progressively closer) from the undeformed massive rock towards a strongly foliated, planar zone. Typically also, this rotation is symmetrical on both sides of the planar zone, conferring to the shear foliation a sigmoidal shape, which is a most reliable structure to readily define the relative movements involved.
Extensional cleavage; C and C’ planes
Extensional clavage describes a type of secondary planar feature that is not parallel to the axial plane of folds. For example, some cleavage planes of crenulation cleavage may have undergone a large shear strain relative to adjacent microlithons, implying some movement origin (strain slip).
These narrow structures across which markers are displaced as by faulting should be referred to as micro-shear -zones, that formed with or without a component of shortening perpendicular to the zone. The spaced foliation formed parallel to a plane of high resolved shear stress and not parallel to the 12 plane of the finite strain ellipsoid.
This is particularly clear where a succession of shear-zones is found in an isotropic igneous rock so that there is no earlier foliation that could be simply rotated. Each individual shear zone actually develops its own, typically curved foliation pattern on a small scale. The resulting feature in rocks where non-ruptural shear zones are sufficiently abundant to develop a penetrative fabric is commonly called S-C structure. In this case, parting of the rock is easier along the shear planes C. S stands for the foliation (schistosity) planes.
In other rocks, this situation meets two explanations:
- During non-coaxial strain history, expected during folding, there is sometimes shear on flattening planes of the finite strain ellipsoid. Thus, foliation develops parallel to a principal plane of the strain ellipsoid and shearing occurs parallel to this foliation because resistance to shear on the foliation is low.
- Foliations may be a phenomenon related to high shear strain and are parallel to surfaces of no finite strain shear strain. In foliated rocks, one or more sets of secondary, spaced foliations may appear systematically oblique to the early foliation. They are called extensional crenulation cleavage or C’ planes. They are essentially small-scale shear zones oblique to a pre-existing foliation, such that the displacement on the so-called “cleavage” results in net extension parallel to the earlier planar anisotropy. Such structures may indicate intense, non-coaxial and partitioned flow. Conjugate sets may arise to accommodate extreme flattening.
Shear foliations are common in high-grade gneisses and deformed igneous rocks that have suffered intense shear deformation.
Joints in relations to other structures
Tabular regions
In layered rocks that have been subjected to little or no deformation the most prominent joints usually intersect the layering at high angles and exhibit a marked consistency in their orientation patterns. The interpretation of these vertical joint systems rarely leads to unequivocal conclusions regarding the stress or strain history of the area surveyed.
Folded regions
Where the layers are folded, joints normally are also nearly perpendicular to bedding and may be referred to the hinge trends. Longitudinal joints are roughly parallel to fold axes. Cross-joints are approximately perpendicular to fold axes. Diagonal joints generally occur in paired, conjugate sets arranged more or less symmetrically with reference to the longitudinal and cross-joints.
Faulted regions
Joints associated with faults may predate the faults and therefore have not necessarily a genetic relation to the faults apart from an eventual control on the orientation of the fault planes. The pinnate joints form an en échelon array that occurs preferentially in the immediate vicinity of the fault plane and intersect the fault in an acute angle pointing in the direction of relative movement of the block containing the fractures. En échelon means that these joints are n an overlapping or staggered arrangement. Each is relatively short but collectively they form a linear zone, in which the strike of the individual of the individual joint is oblique to that of the linear zone as a whole. In other words, it is like if you draw a ladder in perspective. The uprights would be the linear zones, therungs all together define the en échelon arrangement. Pinnate fractures can form both before and during slip on the associated fault. They have been observed in experiments on a wide variety of materials.
Intrusive bodies
Joint systems in igneous rock bodies may be quite different from joint systems in the surrounding rocks. They are often symmetrically related to the contacts of the body, suggesting an origin during emplacement and cooling. One prominent joint set is commonly seen at a high angle to the nearest contact and may display a special configuration known as columnar jointing, in which the joints isolate elongate prisms with more or less regular hexagonal cross sections.


structure subtype 3

First and second order folds
On the limbs and in the hinge zones of large folds there are often small folds whose attitude is parallel to that of major folds. The small folds are called parasitic or subsidiary folds with respect to the larger ones. They may also be referred to as second-order folds while the large folds are referred to as first order. In widely folded regions, the largest folds are called first-order folds, the next largest are called second-order folds and so forth.
The axes of parasitic folds are habitually closely parallel to the axis of the major fold with which they are associated. They are said to be congruous, by contrast with incongruous parasitic folds whose axes deviate appreciably from the attitude of the major fold axis.
First-order folds may be symmetrical whereas the congruous, second-order folds are asymmetrical and their sense of asymmetry, referred to as local vergence, varies systematically across the axial surfaces of the first-order folds. This systematic variation is such that, looking down the axial direction, all parasitic folds in a limb have a clockwise vergence or are described as Z folds, whereas those in the other limb have anticlockwise vergence or are S folds. Symmetrical M folds generally occur in the hinge zone. In the field, the asymmetry is used to locate hinges of the next larger order folds if they were both generated together.

Fault (brittle):
Fault rocks
Fault zones are commonly filled with fragmental, crushed material due to fragmentation of the fault walls as a consequence of the fault displacement.. They are known as fault breccia or cataclasites, microbreccia if the fragments are microscopic, and gouge when the material consists mainly of a clay-like powder. They are aggregates of angular, broken fragments of the rocks that compose the fault walls. The fragments range in size and may be held together by some cementing material generally formed by precipitation between fragments of infilling minerals from circulating fluids. Many rich veins of metalliferous ores occur in this setting.
Thin black films or massive rock on some faults are made of a glassy material called pseudotachylyte, which typically intrude also the adjacent fractured rock. Pseudotachylites formed by melting as a consequence of frictional heating caused by rapid movement (> 10 cm/s) along the plane.
Shear zone:
Fault rocks
In metamorphic rocks, hard rocks characterised by a platy or streaky structure in thin section mark fault zones. Such fault rocks are called mylonites. Their fine-grain size and distinctive microstructure may be due entirely to ductile deformation accompanied by recrystallisation. They may contain larger fragments or minerals derived from the initial rock; these fragments are called porphyroclasts.
The original meaning of the term mylonite can be broadened to include any fine-grained metamorphic rock with well-developed "flow" structure or the special term blastomylonite can be used for such rocks that have been extensively recrystallised after mylonitisation. Extreme grain size reduction and dynamic recrystallisation may produce a hard, dark fault filling of ultramicroscopic grains containing larger fragments. This material is known as ultramylonites. Conversely, a protomylonite is a rock in the early stages of mylonitisation

Fracture cleavage
Fracture cleavage consists of regularly spaced fractures (microfaults or joints) that sharply divide the rock into a series of tabular microlithons; there is essentially no internal deformation within the microlithons. Fracture cleavage can be envisioned as a dense population of joints or discrete shear planes. It is generally formed in low metamorphic grade competent rocks such as sandstone and limestone beds, where fracture cleavage may coexist with and grade into slaty cleavage in interlayered pelites. Such sets of foliation-like, closely spaced yet non-penetrative fractures may eventually be confused with dense sets of joints, from microscopic to metre scale. The latter sort of fracture cleavage is misleading because it is not true foliation in terms of finite strain: it is a false cleavage.
Solution cleavage
Solution cleavage consists of regularly spaced dissolution surfaces (e.g. stylolitic joints) that divide the rock into a series of microlithons without internal deformation. Dissolution surfaces often contain dark seams of insoluble material that may impart a prominent striping to the rock. Solution cleavage is generally formed in low metamorphic grade rocks rich in fluids and is common within limestone.
Slaty Cleavage
The word slate originated as a quarryman's term for fine-grained rocks that were sufficiently fissile to be split into thin, planar slabs suitable for roofing tiles and blackboards. The term slaty cleavage describes the fabric responsible for the planar parting of rocks whose individual grains are not obvious without a microscope.
Slaty cleavage is defined by the parallel alignment of inequant phyllosilicate grains (illite, chlorite, micas) too small to be visible to the naked eye. Under the microscope, slates have a domainal structure, which means that the cleavage is marked by spaced bands of well-oriented, recrystallised platy minerals, with intervening domains of unaltered and unstrained rock. Cleaved and uncleaved domains are distinguished by their differences of composition and fabric. The uncleaved domains (termed microlithons) are generally lenticular and surrounded by the film like cleavage domain. The long dimensions of the microlithons are usually parallel to the cleavage and in sections cut perpendicular to cleavage the film like domains appear as an anastomosing network. Spacing of slaty-cleavage planes range from a fraction of a millimetre to one or two millimetres.
The microlithons are rich in the major constituents of the rock other than layer silicates (usually quartz) and any crystals (including layer silicates) they contain generally show little or no preferred orientation. The microlithons vary in size and in the number of grains that they contain.
Seams or accumulation of insoluble residues (often oxides) commonly accentuates the cleavage surfaces. They are rich in minute, flaky or tabular minerals (mica, chlorite, and talc) whose parallel, planar arrangement defines the planar domain.
Slaty cleavage is best shown in fine-grained shales that have been deformed under low metamorphic grade. The juxtaposition of quartz-rich domains and cleavage domains with insoluble minerals suggests that preferential solution by pressure solution be involved in the formation of slaty cleavages.
At higher metamorphic grades fabrics similar to slaty cleavage are coarser because crystallisation is more important. Schistosity refers to foliation planes with phyllosilicates coarse enough to be discerned with the unaided eye. The fabric of rocks called phyllites, which are intermediate in grain size between slates and schists, can be adequately described as slaty cleavage or schistosity.
Schistosity is most common in high-grade metamorphic rocks but also occurs in low-grade rocks, particularly in greenschist facies rocks of retrograde origin.
Crenulation Cleavage
Crenulation cleavage is a spaced foliation created when an earlier foliation is folded (crenulated) on a microscale. The small crinkle folds may be symmetric or asymmetric, the latter being the most common. The crenulation cleavage is defined by the parallel alignment of grains in the limbs of the tight to isoclinal microfolds or by microfaults developed parallel to the fold limbs. Thus, this type of cleavage is also referred to as strain slip cleavage, but it is safer to prefer the non-genetic name crenulation cleavage.
In a few areas, two crenulation surfaces, which intersect at angles generally between 60° and 90°, formed approximately simultaneously. These surfaces are described as conjugate crenulation cleavages and may be associated with conjugate folds or kinks. The cleavage surfaces are generally less regularly spaced than their counterpart in areas where only one family of surfaces exists and tend to be more localised in their distribution. Usually, one direction is more commonly developed than the other is, but either set may be prominent from place to place, and there is no consistency about which accommodated the last deformation.
Crenulation cleavage is found in all metamorphic grades.
Differentiated layering
Foliation may be defined by alternating layers of different composition because metamorphism reorganised the chemical components of a rock and produced new minerals with new orientations. The compositional layering, which is visible in hand specimen, is referred to as differentiated layering. Slaty cleavage, crenulation cleavage, and schistosity can all be differentiated.
Differentiated layering is found in coarse-grained, granular metamorphic rocks of all grades. In high-grade rocks it is customarily described as gneissic layering, most commonly defined by alternating mafic and felsic layers parallel with the lithological banding. Gneissic layering may be more or less modified bedding, and thus reflects initial sedimentary compositional differences, or a foliation due entirely to differentiation during deformation.
Transposed layering
Transposition layering is largely defined by a rotated pre-deformation layering, whether it is bedding or an older foliation.
In a laminated sequence of rocks, successive layers tend to have different competence. Under intense deformation, appressed to isoclinal folds are produced whose limbs may become parallel and thinned until they coincide with the foliation. Fold hinges are sharp and folds are intrafolial folds. Hinges may be teared apart along the stretched limbs. Where hinges are completely detached from the limbs, the folds are rootless intrafolial folds. In areas where deformation is intense, the fold hinges are obscured. There is then practically no variation in the orientation of the transposed bedding. The transposed sequence may be mistaken for a normal sedimentary succession. Nevertheless, the pseudo-bedding has no real stratigraphic significance; inferences concerning the stratigraphic sequence, the gross disposition of the stratigraphic units and direction of younging may be entirely misleading. Transposition is found on all sizes ranging from hand specimens to major structure extending for several kilometres. The development of a transposed foliation has been described here with specific reference to bedding, but any foliation may be transposed.


Joints that have dimensions ranging from tens of centimetres to hundreds of meters and repeat distances of several centimetres to tens of meters are called masterjoints. In addition, most rocks contain numerous inconspicuous joints of smaller size and closer spacing, some of them, the microjoints or microfractures, visible only in thin section under the microscope.


structure subtype 4

There commonly are folds geometrically associated with faults. They are in general controlled by the fault geometry.
Drag folds
Due to some resistance to slip along fault planes, bedding adjacent to faults may curve in the direction of movement of the opposite block. These minor flexures are known as drag folds. This interpretation suggests that faulting is initiated first and that folding occurs adjacent to the fault as one block is dragged along the other. Trains of drag folds are common in incompetent layers between two competent layers of in the proximity of thrust faults. Their use can be misleading because curvature of opposite sense to the displacement, termed reverse drag, are common. Reverse drag is a common feature of listric normal faults where hanging wall folds are concave towards the slip direction. It is clearly independent of true drag effects but hardly distinguished from true drag when they appear separately. In addition, the orientation of such folds is often not controlled by the movement direction but rather the intersection between bedding and the fault plane. Drags to decipher fault movements should be used with extreme care.

En échelon folds
In some non-cylindrically folded surfaces, doubly plunging, relatively short folds with steeply dipping axial planes are arranged spatially such that culminations and depressions in successive folds lie along lines that make an acute angle with the approximately parallel fold axes. Such folds are stepped, consistently overlapping, and said to be arranged en échelon. Taking the axial planes as roughly orthogonal to the shortening direction, their distribution permits to decipher the potential fault they are related to. Such folds are common above strike slip faults in the basement, that have not broken the cover. The en échelon folding reveals the relative sense of movement

Drape folds - Forced folds
Drape folds are curvatures in a sedimentary layer that conforms passively to the configuration of underlying structures. A fold formed by differential compaction is an example of a drape fold. Sedimentary layers may also drape the fault scars developed at the buried basement / cover interface.
Folds whose overall shape and trend are dominated by the shape and trend of an underlying forcing member are called forced folds. Drape- and forced folding include both compression- and extension-induced relative movements of block basement along essentially high angle faults.

Thrust related folds
Thrust movement at depth generates geometrically necessary folds in the allochthonous hanging-wall as it moves over topographic irregularities of the thrust faults. Kink-like, box folds result. Two types of ramp-related folds are common in thrusts belts.
Fault bend folds form where a thrust steps up from a lower décollement to a higher one. As slip occurs a syncline-anticline pair is formed at the base and top of the ramp. The ramp anticline terminates downward into the upper-flat.
In a fault-propagation fold the ramp does not continue to an upper flat. Fault slip decreases to zero in the up-section direction and the fault dies out into the axial surface of a syncline. Folding accommodates shortening above and in front of the tip line of a thrust.
Growth folds develop in sedimentary strata at the same time as they are being deposited. Antiformal structures commonly grow over a ramp or a duplex zone to build antiformal stacks.
The common association of folds and thrusts at a regional scale defines a fold and thrust belt. The fold shape is determined by the shape of an advancing ramp that does not tie into an upper flat. The ramp fault is replaced upward by an asymmetric fold, which is overturned in the direction of transport. There is a systematic and predictable geometric relation between a fold and the thrust that generated it, and we can therefore use fold geometry to infer fault position and geometry at depth.

Normal fault related folds
As in thrust systems, geometrically necessary folds are generated above topographic irregularities of extensional faults. Tilting of the hanging wall of a normal listric fault toward the main fault produces a half-antiform called rollover anticline. The half antiform reflects the deformation necessary to accommodate the hanging wall to a curved fault surface.

Fault (brittle):
Rocks in contact at a fault plane often have shiny or polished surfaces of easy parting known as slickensides. Slickensides may be featureless, but sometimes feel smoother in the direction of slip.
Slickensides commonly display a prominent striation believed to be scratches parallel to the direction of relative movement of the fault. Some striae may be grooves scored on one side of a fault by hard particles drawn over it by the other side. The striating object can be found pinned in one side and the sense of relative movement can be defined. Most striations are thought to be defined by mineral streaks in the fine-grained material along fault planes.
Using surface features related to striations (such as small steps, fibrous crystals, tensile fractures or tension gashes and drag folds) structural geologists are able to define the relative sense of movement.

Shear zone:
Movement criteria
refracted foliation
bended passive marker
SC structures
delta - clast
sigma – clast
sheared clast
asymmetric boudin
bookshelf structure

Preferred Dimensional and Crystallographic Orientation
The four different mechanisms to achieve preferred dimensional orientation of grains, other than layer silicates, can be classified as follows:
Shape-controlled rotation
Rigid rotation of grains is demonstrated in deformed sediments where detrital grains, showing no evidence of strain and preserving their clastic appearance, are lying in the axial plane foliation. Rotation as the rock is compacted is mostly a secondary process that can be combined with strain or growth of the rotating body.
Within-crystal slip or diffusion
A grain may be flattened in the plane of the foliation as a result of plastic deformation: the change in shape results from lattice distortion through movement of dislocations or twinning. A purely diffusion-controlled mass transfer can flatten grains: The change in shape results from ionic diffusion within grains (Herring-Nabarro creep) or along grain boundaries in the absence of a fluid phase (Coble creep). These processes are important in producing a crystallographic preferred orientation.
The shape of the grain may be changed by solution on sides parallel to the foliation (surfaces of high normal stress) with or without concomitant addition to the grain on boundaries inclined to the foliation (supporting lower normal stresses). The process involves fluids flowing along grain boundaries and can result in significant volume loss if the dissolved material is transported out of the system.
Preferred dimensional growth
Metamorphic reactions consume some minerals and produce others that modify the rock fabric. For example, fibres grow as elongate grains in the foliation plane, a mechanism that also produces a crystallographic preferred orientation. Along with metamorphic crystallisation, strained crystals tend to recrystallise into unstrained, larger crystals via dynamic recrystallisation processes (governed by the release of internal strain and surface energies).

Origin of Joints
Extension and Shear Joints.
It is convenient to relate fractures to the three principal stress axes of a region. If the total displacement is normal to the fracture surface, it is an extension joint. If the shear component has some finite, yet negligible value, the structure sometimes called a shear joint is really a fault. Let us keep the words joint and fault to separate extension from shear joints.
Joints filled with secondary mineral coatings are better termed veins. Extension joints in isotropic rocks form normal to one of the principal directions of stress, otherwise there would exist a shearing stress on the potential joint plane and a corresponding finite shear displacement would occur. In the laboratory tests and presumably in most natural conditions, joints form normal to 3. It is not impossible, however, to have joints normal to 2 or even 1 given suitable anisotropy of the tensile strength. A potential shear fracture may be segmented in parallel and obliquely aligned extension joints or veins, often with an en échelon arrangement, the alignment being parallel to the potential fault.
Unloading joints.
Rocks possess elastic properties closely related to those measured in the laboratory. In non-orogenic environments, uplift and exhumation give rise to changes in the horizontal and vertical strains and stresses. In particular, confining pressure is released during decompression and the buried rocks tend to expand. Near the surface, upward extension is easier than horizontal extension because the normal stress on horizontal planes must approach one atmosphere, which is significantly less than for the normal stress on vertical planes. The state of stress therefore becomes non-hydrostatic with 3 approximately perpendicular to the earth's surface. Extension joints formed in this situation are about parallel to the topography, and this results in sets of flat-lying, curved and large joints referred to as sheeting or sheet structure. The amount of expansion to be expected from release of stored stress consequent on burial is indicated by the values of the compressibility of rocks, the ratio of volume change to pressure change. Pressure changes of 2 kbars, corresponding with depth changes of about 6 km, lead to volume changes ranging from a few percent to a few tenths of a percent. If such volume changes are accomplished mainly by vertical extension, and if this extension takes place fast enough, horizontal extension joints may form. Parting of bedding parallel joints is also related to unloading.
Joints due to volume changes.
Most rock bodies consist of several rock types juxtaposed in layers or other configurations. When such bodies are decompressed or cooled from conditions of hydrostatic stress, local deviatoric stresses will be set up within them because of the differences in compressibility or thermal contraction coefficients between adjacent but different units. Local deviatoric stresses will also be set up on a granular scale, where adjacent mineral grains of different orientation or composition will tend to undergo slightly different strains during decompression or cooling. Local non-hydrostatic stress generated by decompression or cooling may be important in joint formation even where the original state of stress is itself non-hydrostatic or where the total state of stress is governed by regional deformation as well as by the internal makeup of a rock body.
Stylolitic joints are characterised by interlocking ‘teeth’ oriented normal or oblique to the joint surface. These joints have accommodated shortening in the peak directions by pressure solution. The stylolites are particularly common in limestones. The stylolitic planes are planes of removed material.
Columnar jointing in sills and lava flows is a spectacular example of joints due to volume change accompanying thermal contraction. Hot igneous rocks contract more than the cooler country rock on either side. Downward movement of the overlying country rock accommodates vertical contraction. If the boundary between the two rock types is to remain coherent, then compressional structures must develop in the country rock or extensional structures must develop in the cooling magma to accommodate horizontal contraction. Polygonal (typically hexagonal or pentagonal), columnar arrangement of these cooling joints is enhanced if contraction is equal in all horizontal directions and if thermal and mechanical properties are identical in all horizontal directions. Columnar jointing is similar in many ways to desiccation-related mud cracks in sediments, which may penetrate dip in the layers, thus opening a fracture that will be filled by an other sediment, making a Neptunian dyke.


Lithology of the rock displaying the structure.






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