Joints are fractures where the two walls remain in contact without shear displacement. They form due to regional tectonics, folding, faulting, or stress release during uplift or cooling. Joint spacing depends on factors like lithology, bed thickness, structural position, and strain. Regularly spaced joints may form due to pore pressure variations, stress shadows, or inter-layer forces controlling where new joints can form. Joint orientation provides information about a reservoir's permeability anisotropy.
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joints
1. 129
JOINTS
Joints (also termed extensional fractures) are planes of separation on which no shear displacement
has taken place. The two walls of the resulting opening typically remain in tight (matching) contact.
Joints may result from regional tectonics (i.e. the compressive stresses in front of a mountain belt),
folding (due to curvature of bedding), faulting, or internal stress release during uplift or cooling. They
often form under high fluid pressure (i.e. low effective stress), perpendicular to the smallest principal
stress.
Joint growth is controlled by the mechanical layer thickness of the deforming rock. The aperture
of a joint is the space between its two walls measured perpendicularly to the mean plane. Apertures
can be open (resulting in permeability enhancement) or occluded by mineral cement (resulting in
permeability reduction). A joint with a large aperture (> few mm) is a fissure. If present in sufficient
number, open joints may provide adequate porosity and permeability such that an otherwise
impermeable rock may become a productive fractured reservoir. In quarrying, the largest block size
depends on joint frequency; abundant fractures are desirable for quarrying crushed rock and gravel.
Joint sets and systems
Joints are ubiquitous features of rock exposures and often form families of straight to curviplanar
fractures typically perpendicular to the layer boundaries in sedimentary rocks. A set is a group of
joints with similar orientation and morphology. Several sets usually occur at the same place with no
apparent interaction, giving exposures a blocky or fragmented appearance. Two or more sets of joints
present together in an exposure compose a joint system. Joint sets in systems commonly intersect at
constant dihedral angles. They are conjugate for dihedral angles from 30 to 60°, orthogonal when
the dihedral angle is nearly 90°
Geometry
The geometry of joint systems refers to the orientation (plotted on stereonets and rose-diagrams), the
scale, the shapes and trajectories, the spacing, the aperture, the intersections and terminations of the
studied joints. The mean orientation and orientation distribution, spacing and relative chronology are
general characters used to define joint sets. In this respect, a three-dimensional observation is essential
to avoid skewed sampling measurements due to simple geometrical reasons.
- Bedding-contained joints terminate at the top and bottom of beds.
- Systematic joints are characterised by a roughly planar geometry; they have relatively long traces
and typically form sets of approximately parallel and almost equally spaced joints.
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- Non-systematic joints are usually short, curved and irregularly spaced. They usually terminate
against systematic joints.
Spacing
The sizes and spacing (the average orthogonal distance between neighbouring fracture planes) are
essential characteristics of joint sets. In isotropic rocks (e.g. granite) joint spacing follows an
approximately log-normal frequency (the number of joints occurring within a unit length)
distribution. In anisotropic (layered) rocks, joint spacing differs according to several parameters.
Bed thickness
For the same lithology, joints are more closely spaced in thinner beds. This is because the formation
of joints relieves tensile stress in the layer over a lateral distance proportional to the joint length. Since
joints end at layer boundaries, which are rock discontinuities, the longer joints in thicker layers need
to be spaced less frequently.
Spacing scaled with layer thickness is used in mapping lithological contacts, particularly in air-photo
interpretation or in surface mapping of heavily weathered or inaccessible exposures. Spacing may
also reveal differences in the joint systems at limb and hinge positions on large folds, or at different
distances from large faults.
Much work has documented linear relationships between average joint spacing, D, and bed thickness,
T:
D T= α
The slopes α are a function of lithology and, by inference, of mechanical properties. However, this
linear relationship might be valid for beds less than 1.5m thick. The slope may change for thicker
beds. A continuous curve with a positive slope and a negative second derivative:
2 2
d D dT 0<
reasonably fits all data.
Lithology
Stronger, more brittle rocks have more closely spaced joints than weaker rocks. Similarly, rocks with
low tensile strength show more joints than stiffer lithologies, because strain is the same along layers
of different types. But higher stresses are required to achieve the same amount of strain in the stronger
layers. Therefore, strong layers fracture more frequently. However, this response is particularly
sensitive to local pore fluid pressure.
Structural position and strain
The structural position (particularly within folds) and the magnitude of extensional strain also control
joint spacing.
Why evenly spaced joints?
The regular spacing of joints has met several explanations, none of which having been established as
proven mechanism. Yet, nearly all infer that joints from in sequence. Among these hypoheses:
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Pore pressure / porosity interaction
When a joint forms, fluids flow into the fracture and the pore pressure in the adjoining rock
diminishes. The local Mohr circle moves away from the failure envelope, and no fracture is possible
near the initial one. Another fracture can only form beyond the volume of rock with reduced pore
pressure. The minimum spacing thus depends on the permeability of the rock. This minimum distance
would be the measured finite spacing of joint sets.
Exercise
Explain with a Mohr diagram and any given permeability level how pore pressure
variations may control joint spacing.
Stress shadow
When a joint forms, it relieves the tensile stress on either side of the fracture plane that becomes a
zero-stress surface. Stress builds up gradually away from the fracture until it reaches the remote stress
level. The next fracture can only form beyond the volume of rock with reduced tensile stress (the
stress shadow). The minimum spacing thus depends on the width of stress shadows, hence on elastic
properties of the rock. The uniform spacing is ruled by adjacent stress shadows with joints in the
middle. Stress shadows are larger for longer joints.
Inter-layer forces
Each layer is submitted to forces transmitted by adjacent layers. Differential strain between layers
exerts tensile stresses in the more competent ones (a process invoked for boudinage). Spacing
between joints is determined by the length of layer necessary to build up stresses to the tensile strength
level of the concerned lithology.
Joint patterns
There are five main arrangements:
- Parallel sets are curved or straight
- Fans sets along fold or intrusion crests
- Radiate sets around intrusion centers
- Concentric sets around intrusion and collapse centers (cone, ring or cylindrical)
- Polygonal sets as columnar or prismatic.
Master joints
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 master joints. 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.
Joint orientation
The permeability of a fractured reservoir is often highly anisotropic because joints generally form in
sub-parallel sets. However, more than one fracture set may be present, resulting in a complex fracture
network. Knowledge of the spatial distribution and orientation of joints is therefore required to
optimise the development of fractured reservoirs.
Joints are measured on a sampling station. The technique aims at measuring their orientation and the
fracture density.
- A circle is drawn with a piece of chalk attached to a string of determined length (r = radius) on a
perfectly exposed surface.
- One measures the orientation (strike and dip) and length L of each fracture within the circle,
marking with the chalk each measured fracture to avoid data duplication.
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- The fracture density is the cumulative fracture length over the circle’s area:
2
f L .rρ= π∑
Where dips are not obtained, such as on photographs, orientation data can be presented on a rose
diagram: Joints in a given orientation sector (e.g. within 10°) are counted. A radial line is drawn in
the median direction of each sector. The length of the line indicates the number of joints occurring in
each sector.
Relative timing of joint formation
Joints of the same generation have likely the same orientation. However, rocks experience different
stress regimes during their history with the result that several fracture sets are superimposed on each
other to produce a fracture network. Cross-cutting relationships of different joint sets allow
determining their relative age. Some rules have been established to help in this task.
- Early joints tend to be long and relatively continuous.
- Early joints arrest the propagation and modify the orientation of later ones.
- As more joint sets develop in the rock, modification of the stress orientation by the pre-existing
fractures may result in a poor correlation between the orientation of late joints and the regional stress
field responsible for their formation.
Younger joints must terminate against older joints because an extension fracture cannot propagate
across another, older extension fracture. The principal stresses are reoriented near an early joint,
which is a free surface unable to support a shear stress within the rock and, therefore, is a principal
plane of the stress ellipsoid. Consequently, later joints curve into an orientation at right angle to the
earlier ones as they approach them and abut against these (abutting relation); younger joints are
therefore shorter.
Joint anatomy
Joints normally are barren cracks or empty fissures but some may contain coatings. Narrow veins
with infilling minerals, commonly quartz or calcite, are also extension fractures treated as joints.
Joint surfaces
Barren joints are characterised by clean, granular and jagged breaks. They are conchoidal structures,
meaning that they are uneven surfaces with low relief convexities and concavities (like those of a
clam shell) that do not follow any natural plane of separation. Such structures are typically seen when
an amorphous material (glass, flint, obsidian, etc) is fractured. Likewise, some joint surfaces display
delicate ornaments falling into two groups: plumose-marks, the most common type, and rib-marks.
Preservation of these delicate features specifies that the joint is not a shear fracture.
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Plumose structures
Plumose structures are aggregates of gentle, curvilinear undulations (the hackle marks) that radiate
from the point where the joint originated and fan outward from a generally straight, more rarely
curved axial line, then resembling the shape and imprint of a feather. The origin commonly is some
rock heterogeneity such as ripples on bedding planes or inclusions (concretion, nodule, clast, fossil,
etc) in beds. Hackles are often very fine near the joint origin, while the differential relief may amplify
lengthwise towards the joint margin (the fringe). Hackles diverge sharply at angles of about 30° from
the central axis, gradually curving to angles of about 70° near the margins of the joint surface. The
scale of plumose patterns seems to depend on the grain size of the rock.
Markings similar to plumose structures are seen on fracture surfaces in glass and other brittle
materials. They are interpreted as surface irregularities that develop as a consequence of local
variations in propagation of the fracture front in terms of velocity and heterogeneities in the rock.
Experiments show that the diverging rays of the plumose structures always remain parallel to the
direction of propagation of the fracture. Thus, constructing lines at right angles to these rays yields
the position and shape of the fracture front at different times of its evolution. The fracture fronts form
a series of concentric ellipses, the centre of which marks the site of fracture initiation.
Rib-marks
Rib-marks form a series of regular, concentric and arcuate changes or ramps in the orientation of the
joint surface, giving cuspate, waveforms or rounded ridges or furrows. The central zone of rib- marks
(the mirror) is often circular or elliptical. Wallner lines are similar to ribs but they occur as one or
two sets oblique to the hackles.
Rib-marks represent changes in fracturing direction as the stress field changes. Experiments have
shown that those that are sinusoidal in profile and smooth on their crests record the location at which
the velocity of propagation of relatively fast-propagating fractures cutting through a solid material
diminishes (the stress field is vibrating). Strongly asymmetric rib marks, with sharp crests and
occasionally deviating from parallelism (arrest marks) are associated with slow crack propagation.
They are old joint terminations, mapping successive temporary arrests of the crack front during
repeated crack growth under recurring loading/unloading conditions.
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Interpretation
Plumose and rib marks can be superposed on a joint plane and generally are orthogonal to each other.
Such delicate features interlock on opposed faces of joints, and this precludes shear movement (hence
joints are mode 1 fractures parallel to the ( )1 2;σ σ plane). Plumose and rib marks are a direct
expression of the joint path because the edges of the fracture constantly twist and tilt as they advance.
Plume axes develop parallel to the main propagation direction, commonly parallel to bedding.
Considerations in linear fracture mechanics suggest that these surface structures are controlled by
fracture velocity and /or stress intensity. The average propagation velocity has been measured to be
half speed of sound. Experiments further suggest that propagation velocities of cracks with plumose
ornamentation may exceed half the speed of sound.
Joint content - Secondary deposits
The secondary deposits along joints are dilational, if the vein material occupies space between the
two original fracture surfaces, or non-dilational if the vein material occupies space made available
by replacement of the original rock outside the two original fracture surfaces.
In engineering practice, the mineralogy and texture of veins is important because joints with different
fillings can have different mechanical properties governing storage and flow of fluids. Then the
fracture retains a certain degree of cohesive or tensile strength. In particular, if a fracture propagates
slowly enough, healing may keep pace with propagation.
Joint termination and edges
At their fringes, joint surfaces split and twist into small out-of-plane fractures oblique (typically at
5-25°) and en échelon to the main joint. These fringe faces may individually bear secondary plumose
patterns nearly orthogonal to the direction of propagation of the parent joint. They connect by curved
or angular links. Fringe faces can be segmented by small fringe steps parallel to the joint axis. Joint
terminate with or without change in orientation. Branching and abutting against another joint are
other common descriptions.
Mechanics of jointing
A genetic classification of joints has been based on the size of inferred, imperceptible displacement
related to the three principal stress axes of a region. If the total displacement is normal to the fracture
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surface, it is an extension or dilatant joint (mode 1 fracture). If the shear component has some finite,
yet negligible value, the fracture called a shear joint is really a fault (modes 2 and 3 fracture), keeping
in mind that the shear component may have accumulated after formation of a former dilatant joint.
Relationship between joints and principal stresses
If we accept an average internal friction angle of 30° for general Coulomb type failure envelope, a
2θ angle of 135° on the Mohr diagram (i.e. dihedral angle ω = 45° between conjugate fractures)
divides fractures into subclasses. Conjugate fractures enclosing an angle of less than 45° involve
negative σ3 and σN ; when 45 ≤ ω ≤ 60°, σN is positive although σ3 remains negative. These
relationships have led to a simple classification:
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Failure mode Class ( )1 3σ − σ Dihedral angle
Tensile failure Extension fracture 04T< 0°
Hybrid shear failure Shear-extension fracture 0 04T 8T− Up to 60°
Shear failure Compressional shear fracture 08T> >60°
Note that there is a direct relationship between the magnitude of the differential stress ( )1 3σ − σ and
the tensile strength 0T of the rock, which is readily seen on the Mohr construction.
Extension joints
Linear elastic fracture mechanics predicts that the orientation of dilatant joints (genuine mode 1
fractures) in a relatively isotropic rock is controlled by the remote stress field at the time of fracture
propagation: joints are gaping planes parallel to the maximum compressive stress 1σ and
perpendicular to the direction of the least principal stress σ3. In other words, they form in the plane
containing σ1 and σ2 . Otherwise there would be a shearing stress and a corresponding finite shear
displacement on the joint plane. Triaxial experiments on brittle isotropic rocks confirmed this
theoretical consideration. Thus, regionally consistent joint sets are taken as effective proxies for stress
trajectories during joint growth: relatively closely-spaced, parallel and linear joints suggest that the
regional principal stress trajectories are rectilinear and remained parallel across the fractured area;
alternatively, complex joint orientations are related to stress trajectory variability.
The pattern of dilatant joints is commonly T-shaped, the younger joint abutting the older one. Given
suitable anisotropy of the tensile strength, it is however possible to get joints normal to σ2 or even
σ1.
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Hybrid joints
Hybrid joints show components of both extension and shear components. They are interpreted as
failure surfaces initiated in the transition from tensile to shear failure. They form when the stress
circle touches the (Griffith) failure envelope in the tensile (negative normal stresses) side of the Mohr
diagram. Dihedral angles between conjugate hybrid fractures are typically smaller than between shear
fractures (faults).
Shear joints
This term is unfortunate and ambiguous because shear joints actually are small faults. Conjugate
“shear joints” generally define X, Y or V shapes. The acute bisector of these shapes is parallel to σ1
, unless these patterns represent unrelated crosscutting or abutting fractures.
Joint initiation and origin
Geological observation indicates that joints initiate at rock “flaws” such as fossils, clasts,
porphyroclasts, cavities, etc. Hard and soft inclusions perturb the remote stress field, hence are sites
of joint nucleation in three ways:
- Amplification of a small remote tension so that the magnitude of local tensile stress at the flaw
exceeds the tensile strength of the rock;
- Conversion of remote compression into local tension.
- Local tension due to pore pressure.
Amplification of remote tension
The reason is that flaws have different elastic properties than the rock. For such conditions, fracture
mechanics consider a circular inclusion and a constant k, the ratio elastic shear modulus of the rock /
elastic shear modulus of the inclusion. The rock and the inclusion have the same Poisson's ratio. A
remote tension 3rσ induces a uniform tension 3iσ within the inclusion:
( )3i 3rσ =σ 3k 2k+1
and a tangential component tσ at two opposite points of the inclusion boundary:
( )t 3rσ =σ 3 2k+1
If k 0→ then the tangential stress is amplified by 3. Hard inclusions (high k ratio) amplify a remote
tensile stress by factors up to 1.5 inside the inclusion while the tangential stress outside the inclusion
is diminished. Near softer inclusions, the tangential stress is amplified, and for an open cavity or pore,
this amplification is a factor of 3.0. Griffith’s argument has already stated that the local stress can
exceed the remote tension by orders of magnitude at elliptical holes with very large axial ratio.
Conversion of compression into tension
Experiments in compression have shown that flaws can induce local tensile stresses. For example, for
a small angle of grain contact, 2φ , stress gσ at the grain center is tensile:
( )g 1r 2σ = −σ φ π
At the ends of the inclusion diameter parallel to the applied compression 1rσ the tangential stress is
( ) ( )t 1r- 1- k 2k 1σ = σ +
In both cases the change in sign shows that remote compression changes into a local tension.
Since compressive stresses are large in the Earth's crust, and the tensile strength of rock is small, this
conversion provides an attractive mechanism for joint initiation.
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Griffith’s argument has also demonstrated that sliding of the walls of an elliptical crack inclined to
the remote compression induces tensile stresses at its extremities (wing cracks at a larger scale).
Tension stresses due to pore pressure
Fluid pressures exceeding the least compressive remote stress produce tensile stresses greater than
the rock tensile strength, in particular at the extremities of elliptical cavities and micro-cracks.
Because the stress concentration increases with joint length (effect of aspect ratio) the tip propagation
velocity increases as joints grow and can propagate as long as adequate fluid pressure is maintained
(up to a limiting value).
Maximum depth of formation
Assuming that 1σ is vertical near the surface of the Earth, it is the weight of the rocks lessened by
the pore pressure fP . The pore pressure is commonly expressed by the fluid pressure ratio
fP gzλ= ρ . Hence 1σ can be expressed as a function of the depth z and the rock density ρ as:
( )1 gz 1σ = ρ − λ
Considering the unique solution for a “pure” joint on a Mohr plot (i.e. the single point where the
failure envelope cuts the Nσ axis, on its negative side) and that the differential stress is related to the
tensile strength ( )1 3 03Tσ − σ > , the maximum depth of formation of a tensile joint is:
( )
0
max
3T
z
g 1
=
ρ − λ
Taking the standard value of 0T 40MPa= for rocks, the maximum depth of formation of joints is
ca. 6 km. Exception occurs under high fluid pressure.
Exercise
Explain why one assumes that 1σ is vertical near the surface of the Earth. Under such
circumstances, 1σ is the lithostatic pressure that may be reduced by pore pressure;
justify this statement. Considering the unique solution for the nucleation of a tensile
joint, and accepting that the tensile strength of a rock is 40 MPa, calculate the
maximum depth at which jointing may occur.
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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 stresses, which
may exceed the tensile strength of rocks. In particular, hydrostatic confining pressure is released
during decompression and the buried rocks tend to expand radially. However, the differences in
compressibility between adjacent lithologies and mineral grains of different orientation or
composition trigger local deviatoric stresses. These local, non-hydrostatic stress generated by
decompression are such that the two principal stresses σ2 and σ3 may be tensile above a critical
depth.
Sheet (exfoliation) joints
Erosion relieves vertical stress, which must approach one atmosphere, but lateral stress (at least the
lithostatic pressure) is not reduced proportionally. Therefore, the state of stress becomes non-
hydrostatic and the vertical stress becomes minimum principal stress σ3 so that joints form
approximately parallel to the earth's surface. Dilatant joints formed during erosion of homogeneous
rocks such as granite, are sub-parallel to the topography, and this orientation results in sets of flat-
lying, curved and large joints referred to as sheeting or sheet structure. Spacing between sheet joints
increases with depth, down to 50-100m. Deeper sheet joints have a larger radius of curvature.
As soon as stress is relieved in the vertical σ3 direction, the original σ2 becomes the greatest tensile
stress. When the tensile strength is exceeded once more, a set of extension fractures perpendicular to
the original set will form, generally somewhat less well developed than the first. 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.
Bedding-parallel and bedding-contained joints
Pressure changes of 2 kbar, 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 dilatant joints
may form. Parting of bedding-parallel joints is also related to unloading.
It is likely that most of the bedding-contained joints also develop during decompression. Their
different spacing from bed to bed reflects differences in compressibility between different layers.
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Joints due to non-decompressional volume changes
On a regional scale, several rock types are juxtaposed in layers or other configurations. When these
are cooled, local deviatoric stresses will be set up within them because of the differences in thermal
contraction coefficients between different rock units and mineral grains in contact. Local non-
hydrostatic stress generated by cooling is particularly important in joint formation in magmatic rocks.
Regional deformation can impel material transfers that locally result in significant volume changes.
Stylolitic joints
Stylolitic joints have a characteristic saw-tooth profile and an interdigitating cone-like form in three
dimensions. The interlocking ‘teeth’ are normal or oblique to the joint surface. Stylolitic joints are
surfaces along which relatively soluble rock material has been removed by pressure-induced chemical
dissolution to accommodate shortening. Shortening is parallel to the teeth direction. Dissolution is
triggered by stress concentration at the contact between grains. Relatively insoluble residues (clay,
iron oxides, etc) remained accumulated in the joint. This deformation mechanism is called pressure
solution. Stylolites are particularly common in limestone.
Assuming that stylolitic joints initiate as flat planes and do not propagate out of plane, their
amplitudes represent a minimum estimate of the amount of shortening (compaction) that has
occurred. Assuming that insoluble material was initially evenly distributed in the rock and that there
has been no contamination by circulating fluids, the thickness of insoluble residue along a stylolitic
joint would be proportional to the amount of material dissolved and would therefore be proportional
to the shortening displacement across the stylolitic joint. Owing to this mode of displacements some
authors call them anticracks.
Columnar joints
Columnar joints are most prominent in basaltic sills and lava flows. They form a three-dimensional
network of interconnected fractures that dissect the rock in long and spectacular polygonal units
(commonly five- or six-sided columns). Minor, column-normal joint sets segment columns along
their length and generally terminate at the column-bounding joints. Thermal contraction during
cooling causes these column-bounding and column-normal joint sets to form and propagate
perpendicular to isotherms.
No stress occurs if the temperature of a homogeneous, isotropic, and unconstrained body changes.
Stresses arise if the body is prevented from expanding or contracting, or if there is an uneven
temperature distribution, as it is the case from the cold top to the warm bottom of a lava flow. A joint
begins to form when the local stresses are equal to or exceed the tensile strength of the rock (up to
485 MPa for basalt). Fracturing relieves thermal stresses along the joint sides, perpendicular to the
fracture plane, but concentrates stresses at its tip. Column-bounding fractures propagate in the
direction of the thermal gradient, following it as it moves through the cooling lava from the cool
outside to the hotter lava. Thus, fractures grow by successive addition of new segments to previous
ones.
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Propagation occurs whenever the stress concentration at the fracture tip is greater than or equal to the
tensile strength of the rock. Fracture growth ceases when insufficient thermal stress exists for
propagation or when the strain rate is too low to overcome viscous relaxation of stress where the
fracture tip is close to the cooling front, which is the transition between the cooled, brittle and the
warm, viscous or visco-elastic lava.
Three joint patterns are common to many cooled lava flows in which isotherms (hence thermal
stresses) are essentially horizontal. The lower part, called the colonnade, contains regular vertical
columns. The central, approximately elliptical zone named entablature, often located slightly below
the flow centre, displays irregular twisted columns expressing rapid convective cooling. The upper
zone may be regularly columnar (upper colonnade), crudely columnar (pseudo-columnar), or
bulky. This distribution may reflect the fact that the conductive cooling rate controls joint spacing:
fast cooling leads to narrow columns whose diameter increases inward from the margins of the flow.
Exercise
Two-dimensional thermal strain is linearly linked to the temperature change:
Tε = α ∆
where α is the coefficient of thermal expansion in the considered reference frame. One
assume that strain is homogeneously distributed. Consider a basaltic lava flow erupted
at 1200°C, which will cool down to 0°C (we are in Iceland). Openings along columnar
joints will compensate thermal retraction of a 5000m long flow that cannot shrink
because it adheres to its base. Calculate the linear strain with 6 1
2.510 C− −
α= ° .
Assuming the “width” of joint is 4
510 m−
, what would be the average size of the
columns? Is it geologically relevant?
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Jointing also occurs in intrusive igneous rocks because they contract more than the cooler country
rock. Like for lavas, thermal stresses arise because thermal shrinkage of the intrusive rock is not free.
Downward movement of the overlying country rock accommodates vertical contraction. If the
boundary between the igneous rock and the country rocks remains coherent, then compression
structures must develop in the country rock or extensional structures must develop in the cooling
magma to accommodate its horizontal contraction.
Mudcracks
Desiccation-related mud cracks in sediments are polygonal patterns, ideally hexagonal, similar in
many ways to columnar jointing. The incremental growth of desiccation joints is kinematically
similar to cooling joints, but the nature of joint growth due to loss of water in layered rocks is not
well known. The crack morphlology is influenced by drying gradients with strong gradients having
a strong orienting effect. The size of the polygons and the width and depth of the cracks depend on
the thickness of the layer of wet mud.
Why hexagonal? This symmetry is a simple geometrical rule. If the rock is perfectly homogeneous
and drying (or cooling) is also perfectly uniform, then the shrinkage centers are equally distributed.
The distance between all centers is equal, which in plan-view yields 6 circles tangent to each other
and centered on one imbricate circle. Joints develop orthogonal to the tensile stresses, which are equal
from center to center. Hexagons occur under these ideal conditions.
Joints due to regional deformation.
Many joints, and particularly those that cut through different lithologies, are related directly to folds
produced by regional deformation. The folds may be pronounced features or barely perceptible
regional upwarps or downwarps. Indeed, a conceivable cause of regional jointing is the very gentle
flexing of lithospheric plates to be expected when a plate changes latitude and thereby its radius of
curvature. Joints geometrically related to folds may originate during the folding. In that case, they
may reflect some elongation of the rocks, often parallel to fold hinge lines. If produced after folding,
their orientation must be accounted for by the mechanical anisotropy of the folded rock.
Veins
Veins are dilated fractures filled with oriented crystal fibres or non-oriented mineral deposits
(typically quartz, calcite or carbonates). Such secondary crystallizations have been transported into
and then deposited or precipitated along the fracture from solutions under favourable conditions of
temperature and pressure. Veins are thus taken as evidence for movement of fluids along fractures.
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They occur in rocks of all types and metamorphic grades with thickness from less than a millimetre
to several meters. They have accommodated an extension whose amount equals the space they occupy
unless there was wall dissolution during early fluid circulation.
Non-fibrous, massive crystals generally grow in open cavities and the veins themselves may contain
open gaps in which euhedral crystal terminations grow. They are sometimes called a fault-cast vein.
Mineralized veins occasionally contain economically important concentrations of metals or other
useful elements in ore deposits. Importantly also, they often contain datable material.
Veins are sealed fractures, hence evidence for reduced permeability. They also strengthen the rock
and restore continuity across the fracture. Groups of veins constitute a vein array.
Tension veins
Description; definition
Tension gashes and veins are filled mode 1 fractures formed in response to combined tectonic and
pore pressures. The latter have been high enough to enable a tensile effective minimum stress 3σ
normal to the plane of fracture.
Fibre minerals
Elongate, infilling minerals crystallize while the vein opens, with the long axis of the fibers tracking
the incremental extension direction. Therefore, fibre minerals track the opening trajectory and
document the formation of the tension fracture. The mineral nature of these fibres points to diagenetic
or metamorphic conditions and fluid compositions.
Three types of internal structure are used to relate the shape and orientation of veins with strain
orientation:
- Undeformed fibres are perpendicular to the margin of purely extensional veins.
- Undeformed (straight) fibres in shear veins are oblique to the vein margin according to the shear
component parallel to vein walls during vein opening.
- Fibrous crystals often grow at high angle to the vein wall in a curved shape, without any bending
of the lattice, indicating that the curved shape is a growth feature.
The originally curved fibres (they are not deformed) record a component of rotation during the
opening direction of the fracture. They indicate that the opening direction changed during vein
formation.
Three types of growth direction of vein crystals relative to wall rock are identified:
- Syntaxial (inward) growth adds material along the center of the vein. Fibres grow from the wall
in optical continuity with mineral grains of the same composition in the host rock. Crystallisation
is interpreted as progressive growth from wall to centre during vein opening, in the opening
direction. Fibres extending from opposite walls meet at a medial suture (sometimes on one side)
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where there is both a structural and optical discontinuity. The medial plane is the fracture plane
where successive cracking occurs but fiber separation is continuously sealed by new material
added along the center of the vein from its two sides.
- Antitaxial (outward) growth adds material along the vein wall. This occurs when the fibre
mineral is absent or uncommon in the host rock. Unique fibrous crystals run from wall to wall
and have grown outward from the median zone of the vein, which may contain wall rock
inclusions. This implies that there are two fracture / growth planes, between the vein and the two
walls. New material is continuously added along the vein walls.
- Ataxial growth refers to veins due to repeated opening and sealing of fracture planes, which jump
positions within the growing vein. Fibres are stretched (microboudinaged) crystals.
Crack-seal mechanism
Linear bands of regularly oriented and spaced solid inclusions parallel to the vein wall, across mineral
fibres, suggest repeated microfracturing of the fibres along their length followed by deposition of
optically continuous overgrowth that heals the fracture: the crack-seal mechanism. Each
microfracturing event records an incremental stage of vein opening. Growth preserves inclusions
indicating repeated fracturing.
Sediment-filled dykes
There are two types of sediment-filled fractures:
Intrusive clastic dykes contain fluidized or brecciated sediments that have been hydraulically
injected into fractures of the overlying sediment. Injection is due to high pore pressure in the
unconsolidated, source sediments. Clastic dykes commonly use joint patterns.
Neptunian dykes represent sediment filling, from above, of open fractures that may penetrate dip in
the layers. Neptunian dykes also are commonly consistent with joint patterns.
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Joints and veins in relations to other structures
Fracture sets often have consistent geometrical relationships to other structural orientations.
Tabular regions
In flat-lying sediments that have undergone little or no deformation the most prominent joints usually
are vertical and exhibit a marked consistency in their orientation patterns. The interpretation of these
joint systems almost orthogonal to layers rarely leads to unequivocal conclusions regarding the stress
or strain history of the area surveyed. However, such patterns are common in the forelands of
mountain belts, suggesting that portions of the upper crust are subject to rectilinear stress fields.
Folded regions
Joints are often a part of the deformation in regions where rocks have been folded. Although joints
normally are nearly perpendicular to bedding, they commonly form in a predictable pattern with
respect to the hinge trends.
- Longitudinal joints are roughly parallel to fold axes and often fan around the fold.
- Cross-joints are approximately perpendicular to fold axes. They are common and indicate axis-
parallel extension.
- Diagonal joints generally occur in paired, conjugate sets oblique to the fold axes, more or less
symmetrically arranged with reference to the longitudinal and cross-joints.
- Strike joints are parallel to the strike of fold axial planes, whereas cross-strike joints cut across
the axial plane.
The orientation and relative intensity of fracturing within folded structures can be predicted by
Gaussian Curvature or stress history analysis. The Gaussian curvature is the product (K) of the
principal curvatures (k1 and k2), which follow the trends of the principal strain axes (X and Z).
Extensional fractures will lie parallel to one of the principal curvatures, dependent on the stress field
that formed the fold. The intensity of fracturing is proportional to the degree of curvature (bending)
of the strata.
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. Joints that
are adjacent to faults commonly occur with one set parallel to the fault planes and one set oblique to
them, whatever the fault type. Joint density increases next to the fault.
En échelon and sigmoidal veins
The tips of small, lens-shaped veins (tension gashes) propagate in a direction perpendicular to the
incremental principal extension. En échelon veins are planar, regularly spaced and mutually parallel
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in an overlapping or staggered arrangement. Each vein is relatively short but collectively they form a
linear brittle shear zone delimited by two parallel, non-material enveloping surfaces. The strike of the
individual veins is oblique to the linear zone as a whole, which is interpreted as a discrete, potential
fault zone. En échelon veins are inclined against the sense of shear in agreement with extension
fractures being initiated normal to the incremental extension within the fault zone.
A potential shear fracture may be segmented in parallel and obliquely aligned dilatant joints or veins,
a peculiar set called en échelon arrangement. The alignment is parallel to the potential fault. The
attitude of extension fractures bisects the acute angle between potential conjugate shear fractures.
En échelon veins become sigmoidal (i.e. S-shaped) when the central part of the vein has rotated while
the vein was lengthening during deformation. Any new increment of vein lengthening or opening of
new veins tracks this direction. The sense of rotation of the central part of the vein with respect to the
tips indicates the sense of shear.
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Two geometrical types of conjugate arrays are identified:
- Conjugate pairs of en échelon arrays where the undistorted portions of veins in both arrays are
parallel;
- Conjugate pairs of en échelon arrays where the undistorted portions of veins in one array are not
parallel to the undistorted portions of veins in the other array. In that case, the pair is divergent if
the veins of both arrays diverge towards the intersection of the conjugate pair; conversely, the
conjugate pair is convergent.
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Tension gashes and “anticrack” stylolites
Vein boundaries move away from one another as the vein opens. Conversely, the surfaces of a
stylolitic joint move towards each other as it forms, which originated the name of “anticrack”. Paired
tension gashes and stylolites may occur as pinnate fractures on opposite sides of small faults, in
particular at the extremities of minor slip planes. The opening and closing directions help defining
the slip direction.
Pinnate joints
The pinnate joints are arranged diagonally, often in an en échelon array in the immediate vicinity of
the fault plane. They intersect the associated fault plane normal to the slip vector and subtend an acute
angle with it that closes in the direction of relative movement of the blocks containing the joints.
Depending on their orientation with respect to the relative movement of the fault block, they may be
either shear or tension joints. Pinnate fractures can form both before and during slip on the associated
fault. They have been produced in experiments on a wide variety of materials.
Intrusive bodies
Joint systems in igneous rocks can result from stresses set up during cooling of the rock mass within
or out of a regional stress field. Such joint systems may be quite different from joint systems in the
surrounding rocks.
Primary fractures of plutonic rocks
Primary fracture systems (primary joint systems) of plutonic rocks are dilatant veins and dykes
directly associated with the emplacement and with the flow fabric of plutonic bodies. They
commonly are veins of igneous differentiates (aplite to pegmatite) and veins coated by hydrothermal
and deuteric minerals. Four main systems have been identified and were awkwardly given the same
names as joint systems in deformed sediments.
- Cross-joints are consistently normal to the flow line and plane, they are considered as standard
extension fractures and many carry vein fillings (magmatic and mineral)
- Longitudinal joints (S-joints) are nearly always steeply dipping veins parallel to the flow lines
and orthogonal to the flow planes.
- Flat-lying joints (or stretch joints) are parallel to the flow planes. They often are confined to the
upper parts of intrusions.
- Diagonal joints (or marginal joints) form acute dihedral angles with the cross-joints. They are
steep veins oblique to flow lines and orthogonal to oblique to flow planes.
Brittle joints parallel to primary joints are often symmetrically related to the contacts of the body,
suggesting an origin during emplacement and cooling.
Dikes often form sets and swarms associated with plutonic bodies. Injection along joints is their main
mode of emplacement. Therefore, they serve to outline the failure pattern associated with intrusion.
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Radiating dikes are common around volcanic necks and shallow-depth (hypabyssal) intrusions.
Ring dikes and cone sheets are concentric around an intrusive center.
Summary
Joints are planar and curved fractures along which no appreciable shear displacement has taken place.
Surface marks show evidence for nucleation, very fast propagation and arrest of mode 1, opening
fractures. Joint propagation paths twist and tilt as they grow. Joint sets are formed in sedimentary and
crystalline rocks, early, intermediate and late in their histories. They form at shallow depth, under
low confining pressure and low temperature. Deep veins main form under high fluid pressure. Joints
and veins represent an elastic response of the rock to changing strain and stress conditions. For
example, some joints form during erosion unloading, on account of the greater ease with which
decompressed rock expands normal to, rather than parallel to, the free surface. However, there is no
general or single origin of joints.
At and near the surface, joints may either be weathered to produce open channels to fluid circulation,
or cemented by secondary minerals.
Recommended literature
Beach, A. 1975. The geometry of en-échelon vein arrays. Tectonophysics 28, 245-263.
Hancock, P. L. 1985. Brittle microtectonics: principles and practice. Journal of Structural Geology
7(3/4), 437-457.
Hodgson, R. A. 1961. Classification of structures on joint surfaces. American Journal of Science 259,
493-502.
Pollard, D. D. & Fletcher, R. C. 2005. Fundamentals of structural geology. Cambridge University
Press, Cambridge, 500 p.
Ramsay, J. G. & Huber, M. I. 1987. The techniques of modern structural geology - Volume2 : Folds
and fractures. Academic Press, London, 700 p.
Twiss, R. J. & Moores, E. M. 1992. Structural geology. W.H. Freeman & Company, New York, 532
p.
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