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Joints and Veins
Earth Structure (2nd Edition), 2004
W.W. Norton & Co, New York
Slide show by Ben van der Pluijm
© WW Norton; unless noted otherwise
Lecture 7
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Joints
Three sets of systematic joints controlling erosion in
Cambrian sandstone (Kangaroo Island, Australia).
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Sketches of the fracture
geometries that form during
failure.
Land surface
~12 km
depth
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Systematic extension fractures in the Newark
Basin, Pa-NJ-NY
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Systematic
extension fractures in the
Newark Basin, Pa-NJ-NY
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Fracture Modes
http://www.naturalfractures.com/1.1.1.htm
Two modes of shear cracking
Mode II - sliding
Mode III - tearing
Two types of fractures normal
to σ1
Mode I - tensile cracks
Anti-Mode I - stylolite compaction seams
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Joints Surface MorphologyFig. 7.2
Plumes or Twist hackles
Shatter cones
Arrest lines
Wavy plumose structure on
a joint in siltstone.
Plumose structure in thin bedded
siltstone; pencil points to the point of origin.
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Plumose Structures Fig. 7.4
Straight plume
Plume with many arrest lines,
suggesting that it
opened
repeatedly.
Curvy plume
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Ideal joint structure Fig. 7.3
The face of joint 1 is exposed;joint 2 is within the rock
Block diagram showing the various
components of an ideal plumose structure on a joint.
Simple cross-sectional
sketch showing the dimple
of a joint origin, controlled
by an inclusion.
The twist hackle fringe
represents the breakup of
a joint into short segments
when it enters a region of
the rock with a different stress field.
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Dealing with Field Data about Joints Section 7.4.1
• In the inventory method, you
define a representative region
and measure all joints that
occur within that region.
• The problem with this method is
that a large number of
nonsystematic fractures in an
outcrop may obscure the
existence of sets of systematic
joints, especially if the systematic
joints are widely spaced.
There are basically two ways to carry out a
field study of joint orientation, spacing, and intensity.
• A more useful approach is the selection method where you visually scan the outcrop
and subjectively decide on the dominant sets , then measure a few representative joints
of each set and specify the spacing between joints in the set.
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• The selection
method results in
the filtering of measurements in
the field.
• While this technique won’t
permit
determination of
fracture density or provide statistics
on joint
orientation, it will allow you to define
fracture systems
in a region. • The hazard with this procedure is that a careless
observer may record what he or she wants to see, not what is really in the outcrop.
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Types of joint arrays Fig. 7.6
Traces of various types of joint arrays on
a bedding surface. (b) Idealized
arrangement of joint arrays with respect
to fold symmetry axes. The “hk0” label for
joints that cut diagonally across the
fold-hinge is based on the Miller indices
from mineralogy; they refer to the
intersections of the joints with the
symmetry axes of the fold.
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Columnar and Sheeting Joints Fig. 7.7
Sheeting joints (or exfoliation)
in granite of the Sierra Nevada.
Columnar joints in Massif Central, France
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Joint development Fig. 7.8
• A model of the sequence of development of joints.
• Time 1 refers to the time before the first joint forms, and time 7 is the present day.
• This scenario suggests that joints form in a random sequence, but with regular spacing.
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Stress shadow and joint spacing Fig. 7.9
Thins beds, shorter joints, smaller shadows, thus, closer spacing
Block diagram illustrating
stress shadow (shaded
area) around each joint.
Note how stress is transmitted across regions that are unfracturedin the third dimension.
Stresses are also exerted by tractions at bedding contacts.
Thin bedded sequence,
containing joints with
narrow stress shadows,
so that the joints are
closely spaced.
Thick bedded
sequence, containing
joints with wide stress
shadows, so that the
joints are widely
spaced.
The heavy vertical lines are joints
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Stress shadow Fig. 7.10
Cutting many springs in a row
causes a wider band of springs to
relax; thus, a larger area is
affected.
Why joint stress shadows exist.
A grid of springs.
Cutting one spring
causes only a few
springs to relax around
the cut, so only a
relatively small area is
affected, as indicated.
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Joints and Lithology Fig. 7.11
Young’s modulus, E:
stress related to strain (elasticity): σ = E.e
Large E, large σ, more fracturesSmall E, small σ, fewer fractures
Cross-sectional sketch
illustrating a multilayer
that is composed of
rocks with different
values of Young’s
modulus. The stiffer
layers (dolomite)
develop more closely
spaced joints.
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Elastic Constants
Joint associations and joint systems
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Orthogonal joint systems. (a) Traces defining a
ladder pattern. (b) Traces defining a grid pattern.
FIGURE 7.17 (a) Formation of joints in the hanging-wall block of a region in which normal faulting is taking place.
(b) Formation of joints above an irregularity in a (reverse) fault surface.
(c) Pinnate joints along a fault.
Joint associations and joint systems
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Traces
defining a grid
pattern.
FIGURE 7.19 Orthogonal joint systems.
Traces defining
a ladder pattern.
FIGURE 7.18 Block diagram showing
outer-arc extension
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Representation of joints Fig. 7.13
The three examples do
not portray the same data
sets.
Rose
diagram
Frequency diagram
(histogram).
Joint trajectory map
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Origin of joints: unroofing Fig. 7.14
Poisson effect (relaxing) occurs in rock that undergo elastic shortening in one direction and extension in the direction at right angles to the
shortening direction: Horizontal contraction accompanying vertical expansion
Joint formation during unroofing
As the block of rock
approaches the ground
surface, subsequent to the
erosional removal of
overburden, it expands in the
vertical direction and contracts
in the horizontal direction.
Cooling: Horizontal stress as contracting
vertically
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Joint terminations Fig. 7.21
Map view sketch illustrating
how joint spacing is fairly
constant because joints that
grow too close together
cannot pass each other.
Joints terminating without curving when they approach one another.
Joints curving into each
other and linking
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Veins (filled joints)
En echelon vein arrays can develop as a consequence of shear within a
rock body that is associated with
displacement across a fault zone
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En echelon and sigmoidal veins Fig. 7.23
Formation of a simple
en echelon array.
Formation of sigmoidal en echelon veins,
due to rotation of the older, central part of
the veins, and the growth of new vein material at ∼45°to the shear surface
En echelon veins in the Lachlan Orogen(southeastern Australia).
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En echelon veins in the Newark Basin
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En echelon veins in the Newark Basin
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En echelon veins in the Newark Basin
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Vein Morphology
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Vein opening Fig. 7.27
A change in extension direction leads to
the formation of sigmoidal fibers. In this
example, the opening is first perpendicular
to the vein wall, and then is oblique to the
vein. Because of the locus of vein fiber
precipitation, (c) antitaxial veins and (d)
syntaxial veins have different shapes.
Long axis of fibers in a vein tracks
the direction of extension, and a
change in extension direction leads
to the formation of sigmoidal fibers.
If the extension direction is perpendicular to the
vein wall then the fibers are perpendicular to the
grain.
If the extension direction is oblique
to the vein wall, then the fibers are oblique to the vein
wall.
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Interpretation of Fibrous Veins Sec. 7.73
Syntaxial veins typically form in rocks where the vein fill is the same composition as the wall rock; for example, quartz veins in a quartz sandstone.
The vein fibers nucleate on the surface of grains in the wall rock and grow inwards to meet at a median line.
Each successive increment of cracking occurs at the median line, because at this locality separate fibers meet, whereas at the walls of the vein, vein fibers and grains of the wall rock form single continuous crystals.
Each growth increment of a fiber is bounded by a trail of fluid inclusion.
Antitaxial veins form in rocks where the vein fill is different from the composition of the wall rock; for example, a calcite vein in a quartz sandstone.
In antitaxial veins, the increments of cracking occur at the boundaries between the fibers and the vein wall, probably because that is where the bonds are weakest.
Thus, antitaxial veins grow outward from the center. Increments of growth are sometimes bounded by trails of small dislodged flakes of the wall rock.
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Antitaxial and Syntaxial veins Fig. 7.26
Cross-sectional sketches, at the scale of individual grains, showing the contrast in the stages of crack-seal deformation leading to antitaxial veins and syntaxial veins.
Antitaxial veins.
The increments of cracking form along the margins of the vein, and the vein composition differs from the wall rock (i.e., the fibers are not in optical continuity with the grains of the wall).
During increments of cracking, tiny slices of the wall rock spall off. The slices bound the growth increments in a fiber.
Host rock in antitaxial should be different
Syntaxial veins.
During each increment, the cracking is in the center of the vein.
The composition of the fibers is the same as that of the grains in the wall rock (that is, the fibers are in optical continuity with the grains of the wall rock).
Optical continuity between fiber and grain means that the crystal lattice of the grain has the same orientation as the crystal fiber of the fiber.
Optically continuous fibers and grains go extinct at the same time, when viewed with a petrographic microscope.
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Interpretation of Fibrous Veins Sec. 7.73
• Growth persistently occurs at the same surfaces
• Growth occurs on vein - wall rock contact surfaces
• Vein mineral(s) often absent or minor in wall rock
• Often fibrous with youngest precipitate on vein - wall rock contact; oldest on median plane
• Individual crystals extend across median plane
• Growth persistently occurs at the same surface
• Growth occurs in middle of vein
• Growth often by syntaxial (crystallographic sense) overgrowth of wall rock grains
• Often elongate blocky with oldest precipitate on vein - wall rock contact; youngest on median plane
• Individual crystals do not extend across median plane
http://www.earth.monash.edu.au/Teaching/mscourse/lectures/lec5b.html
Antitaxial veins
Syntaxial veins
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fibrous veins, the crystals are long relative to their width, so that the vein has the appearance of being spanned by a bunch of hairs
blocky veins - the crystals of vein fill are roughly equant, and may exhibit crystal faces
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“Pressure” Fringes
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Antitaxial Fringes
Antitaxial fringe from Lourdes, Pyrenees
Rb-Sr ages: 87-50 Ma
strain rate from 10-15 to 10-14/sec(Müller et al., 2001)