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7. Joints & Veins
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Joints are a very common
rock structure.
They are fractures with no
offset.
Result from tectonic
stresses on rock mass.
Occur in parallel groups.
7.1. Joints
Basic Definitions
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Basic definitions...contd
Joints are reasonably continuous and through-going planar fractures, commonly on
the scale of centimeters to tens or hundreds of meters in length, along which there
has been imperceptible "pull-apart" movement more or less perpendicular to thefracture surface.
Joints are products of brittle failure, and they form when the tensile strength of
stressed rock is exceeded.
Joints are found in all outcrops of rock, and thus they are among the most
abundant of geologic structures.
The lengths and spacings of joints are related to the size and/or thickness, as well
as the stiffness, of the rock body in which the joints occur.
Weak, thin units are marked by very closely spaced joint surfaces; stiff, thick units
are marked by relatively widely spaced joints.
Distances between joints are commonly on the same order of magnitude as the
thickness of the rock layer in which the joints are found.
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Basic definitions.Contd
Usually planar
Usually form sets
Two or more sets are a system
Variable size
Spacing more or less consistent
Curved, irregular joints not part of a set are
nonsystematic joints
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TYPES
and CHARACTERISTICS OF JOINTS
Systematic joints: have a subparallelorientation and regular spacing.
Joint set:joints that share a similarorientation in same area.
Joint system: two or more joints setsin the same area
Nonsystematic joints:joints that donot share a common orientation andthose highly curved and irregularfracture surfaces. They occur in mostarea but are not easily related to arecognizable stress.
Some times both systematic and nonsystematic jointsformed in the same area at the same time butnons
ystematic joints usually terminate atsystematic joints which indicates thatnonsystematic joints formed later.
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NONTECONIC
JOINTS
Sheeting joints:
Those joints form subparallel to
the surface topography.
These joints may
be more
observed in igneous rocks
.
Pacing within these fractures
increases downward. These
fractures thought that they
form by unloading overlong
time when erosion removes
large quantities of the
overburden rocks.
Columnar joints and Mud
Cracks:
Columnar joints form in flows,
dikes, sills and volcanic necks
in response to cooling and
shrinking of the magma.
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Figure. Joint intersection patterns:-
(A) Y-intersections of joints in a lava flow.(Reprinted with permission from Science, vol. 239,
Aydin, A., and DeGraff, J. M., Evolution of polygonal
fracture patterns in lava flows. Copyright 1988
American Association for the Advancement of
Science.)
(B) X-intersections of joints in siltstone exposed at
Kimmeridge Bay, United Kingdom. (Photograph by
Terry Engelder.)
(C) T-intersection of joints in quartzite in the Salt
River Canyon region, Arizona. (Photograph by G. H.
Davis.)
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Examp les o f joint arrays
conjugate joint set in tilted Tertiary sandstones of
the Brule Formation in the Slim Buttes area of
South Dakota. The interpretation is that the
conjugate joint set happened before tilting of the
strata.
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Characteristics of Joints Plumose structure: is the structures formed
on the joint surface during its propagation
and provides information about the jointpropagation direction.
Hackle marks: indicate zones where the jointpropagate rapidly.
Arrest line: forms perpendicular to thedirection of propagation and is parallel tothe advancing edge of fractures.
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Characteristics of Joints
Bedding and foliation planes in coarse-
grained rocks constitute barriers to joinpropagation. Bedding in uniformly fine-grained rocks, such as shales andvolcanicalstic rocks, appears to be less ofbarriers.
In sandstone bed propagation of jointsthrough the bed is slightly offset from thelayers above or below.
Variation in bed thickness also affectspropagation direction.
In horizontal layering joints will notpropagate from sandstone into shale if theleast principle horizontal stress in shale is
greater than that in sandstone. Fractures will be terminated at the contact
between the two rocks.
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Joints Classified According to their Environment
and Mechanism of Formations (Engelder, 1985)
Tectonic fracture
Hydraulic fracture
Unloading fracture Loading fracture
All of these types are based on the assumption
that failure mechanism is tensile.
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Tectonic fractures:
Form at depth in response to abnormal fluid pressure and involvehydrofracturing. They form mainly by tectonic stress and the
horizontal compaction of sedimentat depth less than 3 km, wherethe escape of fluid is hindered by low permeability and abnormallyhigh pore pressure is created.
Hydraulic fractures:
Form as tectonic fractures by thepore pressure created due to theconfined pressed fluid during burial and vertical compaction ofsediment at depth greaterthan 5 km. Filled veins in low metamorphicrocks are one of the best of examples of hydraulic fractures.
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Unloading fractures:Form near sur face as erosion removes overburden
and thermalelastic contraction occurs. They formwhen more than half of the original overburden
has been removed. The present stress and tectonicactivity may serve to orient these joints. Verticalunloading fr actures occur during cooling andelastic contr action of rock mass and may occur atdepths of 200 to 500 m.
Release fractures:Simi lar to unloading f ractures but they form by
release of stress. Orientation of release joints iscontrolled by the rock fabric. Released joints formlate in the history of an area and are or ientedperpendicular to the original tectonic compressionthat formed the dominant fabr ic in the rock.
Release joints may also develop paral lel to the foldaxes when erosion begins and rock mass that wasunder buri al depth and li thi f ication begins to cool
and contract, these jointsstar t to propagateparallel to an existing tectonic fabri c.Sheared fractures may be straight or curved but
usually can't be traced for long distance.
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Origin and Significance of Joints
Join ts can be caused by a number of pro cesses that create tensional effect ive
st ress in rock:
Upli f t and erosio n
Residual stress
Tectonic deform at ion Natural hyd raul ic fractur ing
Factors Affecting the Formation of Joints
Rock type
Fluid pressure
Strain rate
Stress difference at a particular time
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Mode of formation
From the point of view of fracture mechanics, crack tips have been related tothree modes of displacement, namely extensional or Mode I displacement,and shear fractures of Modes II and III.
Mode I fracture (joints): it is the extensional fractures and formed byopening with no displacement parallel to the fracture surface. In extensionalfractures the fracture plane is oriented parallel to 1 and 2 andperpendicular to 3.
Mode II and Mode III are shear fractures. These are faults like fractures one
of them is strike -slip and the other is dip-slip. Same fracture can exhibit bothmode II and mode III in different parts of the region.
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Significance
- tectonics
- engineering
- economic geology- hazards
Chemical weathering tends to be concentrated along joints
Many important mineral deposits are emplaced along joint systems Highly jointed rocks often represent a risk to construction projects
Significance of Joints
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STRUCTURAL SIGNIFICANCE OF JOINTS
They are some of the simpler structures found in rocks, but can still get
complicated fast. Why are they important?
provide a mass wasting surface failure plane; joint analysis typically done
for slope stability, dam stability, tunnel stability.
produce a strength anisotropy: later reactivation?
provide fracture porosity/permeability - hydrologic modeling,
mineralization. important geomorphic control, contributing to drainage (e.g. trellis),
lineaments.
influence hydrocarbon migration.
they provide an easy to interpret paleostress system (with caution), but
difficult to date (igneous rocks provide an exception to difficulty in datingbecause of late juices that can fill the joints to produce veins).
pervasive phenomena, and one structure that occurs here in Nebraska.
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Bedding surface of massive Arkansas novaculite cut by numerous joints, Hot Springs.
Garland County, Arkansas. 1914. Plate 7 in U.S. Geological Survey. Folio 215. 1922."
How many joint sets can you see here?
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Fault Related Joints
Joints are also formed adjacentto brittle faults, and movementalong faults usually produces aseries of systematic fractures.
Most joints form by extensional fracturing of rock
in the upper few kilometers of the Earth's crust.
The limiting depth formation of extension fractures
should be the ductile-brittle transition.
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Fold Related Joints
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Joints within PlutonsFractures form in pluton in response to cooling
and later tectonic stress. Many of thesejoints are filled with hydrothermal mineralsas late stage products. Different types of
joints are present with pluton (i.e.
longitudinal, and cross joints)
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Figure. Ways in which individual
joints die out. (A) joint trace hooks
and stops. (B) joint traces hook andstop. (C) joint trace hooks radically to
form a T-intersection with adjacent
joint. (D) joint trace breaks up into en
echelon segments.
Straight Path
Twisted or Echelon Path
WHY JOINT STUDIES
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WHY JOINT STUDIES
Joints are studied and described because
They affect fluid flow (groundwater, petroleum) and erosion. Chemical
weathering tends to be concentrated along joints
Many important mineral deposits are emplaced along joint systems
Highly jointed rocks often represent a risk to construction projects
Fractures and layering are zones of weakness in rocks, especially if groundwater is
present. If they parallel slopes, saturated rock may break loose producing
catastrophic landslides.
The following are usually noted:
Systematic or non systematic; Number of joint systems
Orientations
Spacing and length of joints in each joint system
Cross cutting relationships and timing sequence of the joints
Joint surface features
Connectivity
Relationship to other tectonic structures
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ASPECTS THAT ARE OFTEN DESCRIBED JOINTS ARE:
geometry - planar, curved, irregular.
orientation - strike and dip.
sets - a group of fractures with a common cause and time of formation.
defined by preferred orientations + traits (orientation alone is not enough).
statistical description from a population perspective (distribution type, measure of
central tendency, measure of dispersion).
often displayed using circular histograms (e.g. rose diagrams) and/or stereoplots.
width, length, aspect ratio. spacing distribution for a set: regular or periodic versus
clustered, fractal. density of (how to measure?). intersections per unit length along atransect.
aggregate length per unit area.
aggregate area per unit volume.
spatial variation in density:
might expect that it increases as approach fault. fractal distribution of density a possibility.
the ease of which bias can creep in.
associated surface features = fractology (e.g. plumose marks and hackle structures).
Data to be collected
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Data to be collected Strike and dip. Or strike of linear features from aerial photos and Landsat images. Studies
of joint and fracture orientation from LANDSAT and other satellite imagery andphotographs have a variety of structural, geomorphic, and engineering applications
Width of the joint block and space
Identify the joints are open or filled
Establishing time relationships among joint systems
Data obtained from joints is plotted in rose diagram or equal area net. Equal area net for strike and dip and rose diagramfor strike only.
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A few criteria of establishing time relationships:
1. If joints beneath an unconformity have been opened
up by weathering and filled by rocks above theunconformity, the joints are older than the overlying
rocks.
2. Joints are older than dikes or veins that have utilized
them for emplacement.
3. Short joints that abut against longer joints areprobably the younger ones.
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REFLECTION!
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REFLECTION!
1. HOW MANY SETS OF JOINTS OBSERVE ON PICTURE?
2. ESTABLISH THE TIME RELATIONSHIPS AMONG THE JOINT SETS
7 2 VEINS
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7.2. VEINS
Veins are m ineralized fractu res . B ecause frac tu res channel f lu ids,
m in erals are common ly d epos ited fo rm in g veins . Term in olo gy fo r
veins is s im ilar to jo in ts , espec ial ly if the veins orig inated from
jo in t frac tures .
Veins: are filledjoints
and shear fractures
and the filling rangefrom quartz and
feldspar (pegmatite
and aplite) to quartz,calcite and dolomite.
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Two common occ urrences are:
en echelon v eins (r igh t) and
stockwo rk veins (below ).
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VEINSFractures filled with a minerals that precipitated from solution
Common precipitated minerals: quartz, calcite, zeolite, chlorite, ore minerals
Origin of Veins:
Joints
Shear ruptures
Vein Array- group of veins in a rock body.
Non-Systematic Vein Arrays- non-planar veins that
vary in width and orientation
Planar Systematic Vein Array- planar, parallel,
regularly spaced veins
En Echelon Vein Arrays- short, offset sub- parallel
veins lie between two parallel enveloping surfaces
inclined at an angle to enveloping surfaces.
Stockwork Veins-
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En Echelon Vein Arrays
Stockwork Veins
-cluster of irregularly shaped veins of
-highly variable orientation
-occur in a pervasively fractured rock body.
Vein Filling:Blocky (sparry)-
-equant vein filling crystals
-may display euhedral crystal faces
Blocky, euhedral crystal forms in a granitic vein
which cross-cuts syenite, Mosinee, Wisconsin
Fibrous- characterized by fibrous (linear) mineral
growth.
- fibrous- high (>10:1) length: width ratio
-may develop by repeated cycle of crack and
seal-mechanism whereby elevated fluid pore
pressures crack a vein, followed by sealing from
mineralized solution.
-may be useful for strain analysis
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Syntaxial Veins-
-vein fill is usually same mineral composition as the wall rock.
-Cracking occurs in the center of the veins
Step 1: Cracking along center
Step 2: New crack along the center
Antitaxial Veins- -veins may be of different
mineral composition than host rock. Crackingoccurs between host rock & vein material
Step 1- Tensional cracking
Step 2- shear cracking
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Shear fractures
en echelon tension gashes
-form ~45 degrees from plane of max. shear stress
-preexisting vein material rotates while new vein material grows
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Determining the sense of shear
V i filli d i k i
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Vein filling during crack opening
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Significance: Economic Geology
Alteration/Mineralization along fractures;
Veins preserve dilational separation
REFLECTION!
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What is it?
What are these structures?
What is the sense-of-shear?
Describe how the veins grew.
REFLECTION!
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en echelon tension gashes
right lateral or top-to-the-right
from center to tips during rotation