Chapter 7-Joints and Veins.pdf

7/26/2019 Chapter 7-Joints and Veins.pdf

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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 subparallel

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

Chapter 7-Joints and Veins.pdf

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