Brittle Deformation

1
Brittle Deformation

• Chapter 2 Structural Geology, 2nd Edition
• Robert J. Twiss and Eldridge M. Moores

2
Fracture

• Surfaces along which rocks or minerals have
  broken
• Fracturing creating two free surfaces where none
  existed before
• Surfaces across which the material has lost
  cohesion
• Provide information for
• tectonic events
• permeability of rock to fluid flow and the fluid
  storage properties
• aquifer characteristics, contaminant transport,
  migration of oil and gas.
• mechanical properties of the rock
• design of structures such as dams and tunnels

3
Fracture Geometry is Self-similar

• Fractures have the same geometry and distribution
  at a wide range of scales
• extend from microscopic to regional
• (see Box 2-1).

4
Two Basic Types of Fracture

• Extension fracture the relative motion, as the
  fracture propagates, is perpendicular to the
  fracture walls (mode I)
• Shear fractures the relative motion during
  propagation is parallel to the surface. Two
  end-member modes of shear fracture propagation
  are possible
• mode II propagation occurs if the sliding motion
  is perpendicular to the propagating tip, or edge,
  of the fracture
• mode III propagation occurs if the sliding motion
  is parallel to the propagating tip
• Oblique extension fracture has components of
  displacement both perpendicular and parallel to
  the fracture surface

5
Genetic vs. Descriptive Classification

• A descriptive classification based on the
  relative displacement across the fracture surface
  is more useful than a genetic classification
  based on interpretations of how the fractures
  formed
• The descriptive criteria
• orientation of the displacement on the fracture
• geometry of the fractures, including their
  orientation,
• extent of individual fractures
• distinctive patterns formed by associated
  fractures

6
Four categories of observations

• The distribution and geometry of the fracture
  system
• The surface features of the fractures
• The relative timing of the formation of different
  fractures
• The geometric relationship of fractures to other
  structures

7
Geologic history of fractures is difficult to
interpret

• Evidence bearing on the mode of fracture
  formation, and the relative time of formation of
  different fractures is often ambiguous
• As planes of weakness in the rock, fractures are
  subject to reactivation in later tectonic events,
  so some of the observable features of a fracture
  may be completely unrelated to the time and mode
  of its formation
• Careful study of fractures, however, has
  succeeded in making significant progress in
  understanding their origins and significance

1 From the Latin fractus, which means broken
8
Classification of Extension Fractures

• Fracture set Many fractures that occur in the
  same area and have a similar orientation and
  arrangement
• Joint Extension fractures that show very small
  displacement normal to their surfaces and no, or
  very little, displacement parallel to their
  surfaces
• A fracture with a small shear displacement,
  however, may be an extension fracture on which
  shear displacement has later accumulated

9
Systematic vs. Non-systematic

• Systematic joints have the attributes of roughly
  planar fracture surfaces, regular parallel
  orientations, and regular spacing
• Nonsystematic joints are curved and irregular in
  geometry, although they may be present in
  distinct aerially persistent sets, and are
  distinguished by nearly always terminating
  against older joints that belong to a systematic
  set
• The terms 'joint' or 'joint set' alone, however,
  usually refer to systematic joints unless
  specifically indicated otherwise

10
Joint Zone System, Lineament

• A joint zone is a quasi-continuous joint that is
  composed of a series of closely associated
  parallel fractures and that extends much further
  than any of the individual fractures
• Two or more joint sets affecting the same volume
  of rock constitute a joint system
• Joint sets and systems are nearly ubiquitous in
  rock outcrops, and may persist over hundreds to
  thousands of square kilometers, each set
  displaying a constant or only gradually varying
  orientation
• Such systems can show up as linear features, or
  lineaments, on high-altitude photographic and
  radar images

11
Sheet joints, sheeting, or exfoliation joints

• Extension fractures that are smoothly curved at a
  scale on the order of hundreds of meters
• They are subparallel to the topography and result
  in a characteristic smooth, rounded topography
  (Figure 2.5)
• Sheet joints occur in many kinds of rocks, but
  the characteristic topography is best displayed
  in plutonic rocks in mountainous regions where
  the joints appear to cut the rock into sheets
  like the layers of an onion
• Many sheet joints apparently formed later than
  other joint sets, although in some cases they
  predate late phases of intrusive activity, as
  indicated by dikes present along the joints

12
Columnar Joints

• Extension fractures characteristic of shallow
  tabular igneous intrusions, dikes or sills, or
  thick extrusive flows
• The fractures separate the rock into roughly
  hexagonal or pentagonal columns (Figure 2.6),
  which are often oriented perpendicular to the
  contact of the igneous body with the surrounding
  rock

13
Joints and other Structures

• Strike joints and dip joints are vertical joints
  parallel to the strike or dip of the bedding,
  respectively
• Bedding joints are parallel to the bedding
• Cross joints are systematic joints of a set that
  either consistently terminate against the joints
  of another set or that cut a fold or some other
  linear feature at high angles
• Oblique joints or diagonal joints - joints having
  other orientations relative to linear structures

14

• Unfortunately, there is no universally accepted
  definition of the term joint. The definition set
  down here is conservative in that fractures
  satisfying this definition would be called joints
  by every other definition of the term
• Note that the terms joint system and systematic
  joint have different meanings and should not be
  confused

15
Pinnate (feather) fractures

• Extension fractures commonly develop in
  association with shear zones in deformed rocks
• Pinnate fractures, or feather fractures, are
  extension fractures that form en echelon arrays
  along brittle shear fractures
• i.e., the extension fractures are parallel to one
  another but offset from one another along the
  trend of the shear fracture, which is oblique to
  the extension fracture plane
• The sense of rotation through the acute angle
  from the fault plane to the extension fracture
  plane is the same as the shear sense on the fault
  plane

16
Tension Gashes

• Gash fractures are extension fractures, usually
  mineral-filled, that form en echelon sets along
  zones of ductile shear in the same orientation as
  the pinnate fractures
• Are generally S- or Z-shaped, depending on the
  sense of shear along the zone
• The orientation of gash fractures relative to the
  shear zone can be used in the same way as feather
  fractures to determine the sense of shear on the
  associated shear zone
• For distinctly S- or Z-shaped gash fractures, the
  sense of shear is also indicated by the sense of
  rotation of the central part of the fracture
  relative to the fracture tips
• Extension fractures may also be associated with
  other structures, including folds and igneous
  intrusions

17
Veins

• Veins are extension fractures that are filled
  with mineral deposits
• The deposit may be massive or composed of fibrous
  crystal grains such as quartz or calcite
• The fibrous fillings can be very useful in
  interpreting the deformation associated with
  opening of the vein, as we discuss in detail in
  Sections 11.9 and 14.6

18
Geometry of Fracture Systems in 3D

• In studying of the origin of fractures in rocks,
  we collect data on the spatial pattern and
  distribution of the fractures in each fracture
  set which includes the
• orientation of the fractures,
• scale of the fractures
• spacing of the fractures
• relationship of the spacing to lithology and bed
  thickness

19
Fractal Geometry

• To the extent that systems of fractures are
  characterized by a self-similar geometry,
  however, the characteristics of spacing and size
  of the fractures are scale dependent and thus
  difficult to define precisely, except perhaps in
  terms of fractal geometry (See Box 2-1)
• Fractal geometry is a branch of mathematics that
  identifies and quantifies how patterns repeat
  from one scale to another in a system for which
  the geometry is independent of scale
• Fractures are features that have a fractal
  geometry

20

• Joints occur in rocks on a vast range of scales
  from master joints on the scale of kilometers or
  tens of kilometers down to small fractures on the
  mm scale
• The spacing between joints occurs with a
  corresponding range of scales
• The ability to describe accurately the
  characteristics of joint patterns, is important
  in predicting such phenomena as the fluid flow
  and storage properties of rock, necessary in
  calculating ground water flow and oil flow in
  reservoir rock, and the bulk mechanical
  properties of rock, important to numerous types
  of construction projects
• This complexity has lead to the proposal that the
  geometry of joint systems is fractal. We first
  introduce very briefly what a fractal object is,
  and then we describe how the concept of fractals
  might be applicable to joint patterns

21
What is a Fractal?

• A fractal object is something whose geometry is
  scale invariant
• The object is also said to be self-similar, which
  means that any part of the object viewed at a
  particular scale looks similar to the object
  viewed at any other scale
• Thus, for example, the pattern of joints viewed
  from an airplane at the scale of a quadrangle
  would look the same as the pattern of joints as
  seen close-up in an outcrop

22
Orientation of Fractures

• The determination of the preferred orientations
  of different fracture sets in a rock is one of
  the most common means of studying fracture
  systems
• To be objective, we should collect the
  orientations of all the fractures visible in an
  area of outcrop that is large relative to the
  spacing between fractures of the most widely
  spaced set. To establish regional patterns we
  measure numerous exposures over a large area
• We correlate fracture sets from one outcrop to
  another, assuming that fracture sets are related
  if their orientations are the same or are
  smoothly and consistently varying
• Other criteria, however, such as evidence
  establishing the relative timing of fractures,
  direction of fracture propagation, and whether or
  not the fractures are limited to individual
  strata can also be important is establishing
  correlations

23
Difficulties

• Many fracture planes are curved or twisted,
  making it difficult to decide what orientations
  are representative, and what range of
  orientations are associated with a particular
  fracture set
• Introducing subjective judgment into the process
  of choosing orientations to measure, however, may
  introduce bias into the sampling that could
  distort the description of the fracture geometry
  and could ignore fracture orientations that do
  not fit neatly into one of the sets with a clear
  preferred orientation

24
Warnings!

• Because shear fractures commonly intersect one
  another at an angle of roughly 60, it is often
  incorrectly assumed that all fractures that
  intersect at such an angle are shear fractures
• Similarly, the consistent orientation of joints
  relative to other structures is often taken to
  indicate a genetic relationship
• Although such interpretations cannot be ruled out
  a priori, they are unreliable unless corroborated
  by other evidence (see Box 1-1, 'Correlation vs.
  Causation').

25

• Some geologists produce detailed maps of all
  fractures exposed on a given area of outcrop to
  analyze the fracture patterns.
• This method, however, produces just a
  two-dimensional pattern, and does not record the
  three-dimensional orientations of the individual
  fractures and fracture sets.
• At this point in our understanding, the type of
  orientation analysis performed must be chosen to
  be consistent with the use intended for the data,
  and the geologist must keep in mind the
  limitations and potential biases of different
  data gathering methods

26
Fracture sets and events

• More than one orientation of fracture may be
  associated with a single fracturing event
• Genetically related fractures may differ in
  orientation as a result of segmentation and
  twisting of the fracture plane, curving of the
  plane, reorientation of the fracture into
  parallelism with a local planar weakness in the
  rock, or branching of the fracture into two or
  more orientations
• Some fractures may be of only local extent and
  may even result from human activity such as
  excavation or blasting. Careful study is
  required to identify the significant data

27

• The orientation of genetically related fractures
  may differ from one lithology to another on the
  other hand, the fractures in layers of different
  rock types may result from different events
• Orientation data on fractures are conveniently
  collected and compared by using orientation
  histograms, rose diagrams, or spherical
  projections

28
Scale and Shape of Fractures

• Individual fracture planes have definite tip
  lines where the fracture ends
• Field observations indicate that a joint may
  terminate by simply
• dying out
• curving and dying out
• kinking and dying out
• twisting and segmenting into an en echelon set of
  small extension fractures
• branching and dying out
• curving into a preexisting joint

29

• These intersecting and branching relationships
  may result from joints developed at different
  times
• The amount of displacement across the joint
  decreases toward the joint termination
• In a given joint set, individual joint traces or
  joint zones range in length from a few
  centimeters to many metersand even up to
  kilometers in the case of master joints.
  Fractures also exist at scales as small as the
  microscopic level such fractures are better
  referred to as microfractures than as joints

30
Shape of Individual Joints

• Depends largely on the rock type and on its
  structure
• In uniform rock, such as granite, argillite, or
  thin-bedded rocks of uniform composition, the
  boundary of an individual joint plane tends to be
  roughly circular to elliptical in shape, with the
  long axis horizontal
• In sedimentary sequences involving rocks of
  highly different mechanical properties, such as
  interbedded sandstone and shale, one dimension of
  a joint is commonly constrained by the upper and
  lower contacts of the bed in which the joint
  forms, and the joint tends to be of much greater
  extent parallel to bedding than across it
• Joints in individual beds of one lithology often
  end against beds of another lithology
• The shape of master joints is not well known
  because of the difficulty of seeing the third
  dimension. In areas of great vertical relief,
  however, joints can be traced to a depth of more
  than 1 km

31
Spacing of Fractures

• The spacing of fractures in a systematic set can
  be measured either as the average perpendicular
  distance between fractures or the average number
  of fractures found in a convenient standard
  distance normal to the fractures
• Such spacing measurement should be done only on
  joints of similar scale, because in some systems
  at least, the spacing is dependent on the size
  (length) of the joints (see Box 2-1)
• The average spacing of joints of a given scale
  tends to be remarkably consistent, and it depends
  in part on the rock type and on the thickness of
  the bed in which the fractures are developed

32
Spatial Pattern and Distribution of Fracture
Systems

• The most useful method of studying the pattern
  and distribution of fracture sets is to plot maps
  of the location and orientation of the fractures.
  
• In areas of very good exposure, it may be
  possible to map joints individually and to trace
  out the relationship of joints to one another and
  to lithology, and to analyze the anisotropy of
  the fracture pattern, the connectivity of the
  fractures, or their geometry.
• On such maps, we can also plot the strikes and
  dips of the fractures, their relationship to
  other local structures, and the amount and
  direction of shear (if any) on the fractures

33
Form lines (Trajectories)

• In most cases, there is neither enough exposed
  rock nor enough time available to permit such
  detailed mapping
• Usually data from outcrops scattered over a large
  area are plotted on a map
• From these data one constructs form lines, or
  trajectories, of the individual joint sets by
  correlating and matching the sets from one
  outcrop to the next and assuming the strikes of
  joints in the same set vary smoothly if at all
  from one outcrop to the next
• Figure 2.12 shows such a map for an area on the
  Appalachian plateau. The consistency of
  orientation of joints over such large areas
  indicates that they record regional tectonic
  conditions

34
Features of Fracture Surfaces

• The features on the surface of a fracture can
  provide information critical to interpretation of
  the fractures origin
• Many joints display a regular pattern of subtle
  ridges and grooves called hackle that diverges
  from a point or a central axis
• The pattern is known as plumose structure or as a
  hackle plume, named for its resemblance to the
  shape of a feather
• Plumose structure is present on joints in a
  variety of rock types, but it is most clearly
  displayed in rocks of uniform fine-grained
  texture, when the surface is illuminated at low
  angles

35

• Fringe faces at the edge of a joint should not be
  confused with pinnate fractures and gash
  fractures, even though they are all extension
  fractures that form en echelon arrays
• Fringe faces usually make a considerably smaller
  angle with the main joint face than pinnate or
  gash fractures make with the shear surface
• Moreover, in three dimensions, fringe faces are
  restricted to the edge of a joint surface, which
  commonly displays plumose structure, whereas
  pinnate and gash fractures occur along the entire
  shear fracture

36
Rib Marks

• In some cases, curvilinear features called rib
  marks and ripple marks cross the lines of hackle
  on the fracture face
• The rib marks either are cuspate in cross section
  (Figure 2.14B) or are composed of smoothly curved
  ramps connecting adjacent parallel surfaces of
  the joint face (Figure 2.14C see also Figure
  2.13B)
• They tend to be perpendicular to the hackle lines
• The ripple marks are rounded in cross section and
  oblique to the hackle lines (Figure 2.14D)

37
Kinematics

• Plumose structure is a unique feature of brittle
  extension fractures that distinguishes them from
  shear fractures
• The direction of divergence of the hackle lines
  is the direction in which the fracture
  propagated, and the lines of hackle form normal
  to the fracture front and parallel to the local
  propagation direction of the fracture front
• When traced back along the plume axis, the hackle
  is usually found to radiate from a single point,
  which is the point of origin of the fracture
• Rib marks are interpreted to be arrest lines
  where fracture propagation halted temporarily
• Ripple marks are interpreted to form during very
  rapid fracture propagation, in which case they
  are called Walner lines
• By careful study of the surfaces of joints,
  therefore, we can learn a great deal about where
  fractures initiated and how they propagated

38
Slickenside Lineation

• In some cases, a fracture displays slickenside
  lineations on its surface, indicating that shear
  has occurred on the fracture
• Slickenside lineations are either parallel sets
  of ridges and grooves, light and dark streaks of
  fine-grained pulverized rock, or linear mineral
  fibers
• Because extension fractures commonly accumulate
  small amounts of shear displacement during
  tectonic movements subsequent to their formation,
  such displacements do not necessarily, but may,
  indicate that the fracture formed by shearing

39
Mineral Deposits

• Joints and other fractures may have a thin
  mineral deposit such as quartz, feldspar,
  calcite, zeolite, chlorite, or epidote along
  their surfaces.
• These mineral layers indicate either that the
  fracture was open or that fluids under pressure
  were able to force it open, flow along the
  fracture, and deposit minerals from solution
• In some cases, a fracture is clearly associated
  with a zone of alteration in the surrounding
  rock, indicating diffusion of material into or
  out of the rock surrounding the fracture. Some
  joints have been affected by dissolution
  resulting in open fissures

40
Timing of Fracture Formation

• The interpretation of the development of fracture
  sets relies on determination of the timing of
  their formation relative to other fracture sets
  and structures. Although these relationships are
  often ambiguous and difficult to sort out,
  especially for extension fractures, we can make a
  few generalizations
• Where more than one set of joints are developed,
  younger joints must terminate against older
  joints, because an extension fracture cannot
  propagate across a free surface such as another
  extension fracture

41

• In Figure 2.3B, for example, the nonsystematic
  joints are clearly younger than the systematic
  ones
• Many such terminations are at a high angle,
  forming T-shaped intersections, and the younger
  extension fracture may curve toward a high-angle
  intersection where it approaches an older
  fracture
• Low-angle intersections also occur in some joint
  systems
• Where two fractures each curve toward a high
  angle intersections with the other, the fractures
  must be coeval

42

• Analyzing the abutting relations of extension
  fractures in a complex fracture pattern can
  reveal the sequence of development of fractures
  in the system. Such an analysis is illustrated
  in Figure 2.15.
• In the complete fracture map (Figure 2.15A), the
  abutting relations are used to assign each
  fracture to the oldest possible generation
• Note that the earliest generation of fractures
  forms a highly ordered pattern of systematic
  fractures that are long, parallel, and poorly
  interconnected (Figure 2.15B).
• Subsequent generations of fractures are shorter,
  less systematic, and tend to abut older fractures
  to form polygonal blocks. Younger generations of
  fractures progressively increase the connectivity
  of the fracture system

43

• This type of analysis only works for systems of
  extension fractures in which there has been no
  healing of fractures by mineral deposition
• The progressive fragmentation of the rock with
  the addition of subsequent generations of
  fractures, rather than the progressive
  development of several sets of systematic joints,
  suggests that healing of earlier fractures may be
  important in developing a joint system consisting
  of multiple sets of systematic joints

44
Cross-cutting Joints

• In many cases, joints cross-cut one another, a
  relationship that cannot be interpreted in terms
  of relative timing of fracture formation
• This relationship can arise in several ways
• If the first-formed joint is closed and has a
  high pressure keeping the joint faces together at
  the time a later joint forms, the later joint may
  be able to propagate across the earlier closed
  joint
• If the first-formed joint is cemented by mineral
  deposits, it no longer acts as a free surface,
  and a later joint can cut across the older one

45
Synchronous Joints

• Two joint sets could, in principle, also form
  during the same fracturing event
• For example, one set of joints could originate in
  one orientation at the top of a layer and
  propagate down, while another set of joints could
  originate in another orientation at the bottom of
  a layer and propagate up (Figure 2.17C)
• Their intersection would then show inconsistent
  relative-age relationships, because at different
  points along the intersection, the hackle lines
  would indicate that different joints had formed
  earliest
• A similar fracture geometry would be created if
  two pairs of coplanar joints formed at the same
  time and place (Figure 2.16D)

46
Fractures and Sediments

• Several structures indicate that extension
  fractures can form in sediments before they have
  consolidated into rock
• When such fractures form before the deposition of
  overlying sediments, the open fractures may be
  filled by the sediment subsequently deposited on
  top
• Mudcracks are one obvious example. If a steeply
  dipping fracture forms in uncompacted sediments
  and becomes mineralized before compaction is
  complete, the mineralized fracture may form a
  series of folds to accommodate the shortening
  associated with the subsequent compaction of the
  sediment

47

• Extension fracturing of unconsolidated sediment
  in the presence of high pore fluid pressure can
  result in the formation of clastic dikes
• The opening of the fracture creates a
  low-pressure area into which pore fluid rushes,
  carrying unconsolidated material of contrasting
  lithology
• The existence of such structures proves that some
  joints, at least, can form very early in the
  history of a rock

48
Cross-cutting Relationships

• Fractures that cross-cut a geologic boundary or a
  geologic structure clearly postdate the formation
  of that boundary or structure
• For example, a joint set that cuts across an
  intrusive contact is younger than the intrusive
  event, and joints that maintain a constant
  orientation across folded layers must have formed
  after the folding
• Fractures that are clearly affected by a geologic
  structure are older than that structure. A joint
  set that changes orientation over a fold but
  everywhere maintains the same angular
  relationship with the bedding could either
  predate or be synchronous with the folding, but
  it is not likely to be younger

49

• If one set of joints is consistently mineralized
  or has igneous rocks injected along the
  fractures, then it must be older than the
  mineralizing or intrusive event
• If a second set of joints in the same rocks is
  free of the mineralization or intrusion, then it
  probably formed after the mineralizing or
  intrusive event
• Applying criteria such as these has shown clearly
  that joints can form at any time in the history
  of a rockfrom the earliest time, when the
  sediment has not yet consolidated, to the latest
  time when the joints postdate all other
  structures in the rock. It is likely, therefore,
  that more than one mechanism produces joints. We
  discuss possible mechanisms in Chapter 9

50
Fractures Associated with Faults

• Fractures often form as subsidiary features
  spatially related to other structures. If such a
  relationship can be documented, the fractures can
  provide information about the origin of the
  associated structure
• In some cases, faults are accompanied by two sets
  of small-scale shear fractures at an angle of
  approximately 60 to each other with opposite
  senses of shear
• These are called conjugate shear fractures

51
Conjugate Fractures

• Figure 2.17 shows data for a system of conjugate
  fractures that developed in an area closely
  associated with a known fault.
• The rose diagram shown in Figure 2.17B is plotted
  in the vertical plane normal to the strike of the
  fault, and the distribution of fracture dips is
  plotted below the horizontal line. The
  orientation of the fault is also indicated on the
  figure. The major set of fractures is clearly
  parallel to the fault the second and less
  well-developed set is approximately 65 from the
  first set
• Extension fractures associated with faulting
  include pinnate fractures and gash fractures,
  which were described in Section 2.1

52
Fractures Associated with Folds

• Fractures often develop in rocks in association
  with folding
• It is convenient to refer the orientations to a
  mutually orthogonal system of coordinates (a, b,
  c) related to the fold geometry and the bedding.
  The b axis is parallel to the fold axis, which in
  general is the line about which the bedding
  planes are folded (Figure 2.18)
• A line parallel to the fold axis is parallel to
  the bedding regardless of where on the fold it
  lies. Thus the b axis has a constant orientation
  for all the (a, b, c) axes. The c axis is
  everywhere perpendicular to the bedding, and the
  a axis lies in the bedding plane perpendicular to
  the fold axis (b) and the c axis

53

• Figure 2.18 is a diagrammatic illustration of the
  orientations of fractures that have been reported
  from folds
• Fractures parallel to the plane of the a and c
  axes and to the plane of the b and c axes are
  called ac fractures and bc fractures,
  respectively
• The fractures shown in sets (A), (B), and (D) in
  Figure 2.18 are all perpendicular to the bedding.
  In sets (A) and (B), the ac fractures or the bc
  fractures, respectively, bisect the acute angle
  between the two other fracture sets, which are
  oblique fractures. In set (D), the bc fractures
  bisect the obtuse angle between the oblique
  fractures. The inclined fractures in sets (C)
  and (E) are parallel to the fold axis b. Those
  in set (C) make a low angle with bedding, and
  those in set (E) make a high angle with bedding

54
Caution!

• It is possible that the fractures of all these
  orientations formed in association with folding,
  that the ac and bc fractures are extension
  fractures, and that the oblique and inclined
  fractures are shear fractures
• That interpretation is not justified, however,
  simply on the basis of fracture pattern and
  orientation. Such fractures have been shown to
  predate the folding in some cases, and to
  postdate it in others, and as we have remarked
  before, the presence of shear displacement on a
  fracture does not necessarily mean that the
  fracture originated as a shear fracture
• In order to make a well-documented interpretation
  of complex fracture systems, it is critical to
  describe all the characteristics that we have
  discussed for the various fracture sets. This
  includes citing specific evidence for extensional
  or shear displacement and fracturing, the spatial
  distribution of fractures, and evidence
  suggesting the relative sequence of formation of
  the fractures in different sets

55
Fractures Associated w/ Igneous Intrusions

• Fractures can form in association with igneous
  intrusions, and some types occur only within the
  igneous rock
• We have already described columnar jointing in
  sills and thick flows and sheet joints or
  exfoliation joints in plutonic rocks
• In many cases, the internal structure of plutonic
  rocks is related in a simple manner to the
  orientations of other fractures that develop.
  Especially near the margins of plutonic bodies,
  platy minerals such as mica and tabular mineral
  grains may be aligned parallel to one another,
  creating a planar structure in the rock called a
  foliation
• Elongate mineral grains also may he aligned
  parallel to one another within the foliation,
  creating a linear structure called a lineation

56

• We can describe the orientations of fractures
  with respect to these structures by again using
  coordinate axes (a, b, c) where a is parallel to
  the lineation, b lies in the foliation
  perpendicular to a, and c is perpendicular to the
  foliation. Joints commonly form parallel to c
  and thus perpendicular to the foliation
• If they are also parallel to the lineation, they
  are referred to as ac joints if perpendicular to
  the lineation, they are called cross joints or bc
  joints
• Diagonal joints also occur, usually at an angle
  of about 45 to the lineation and normal to the
  foliation. Cross joints (bc) typically contain
  pegmatite dikes or hydrothermal deposits