Joints and shear fractures

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Joints and shear fractures
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Structural Analysis Approach

• Geometry mapping, measurements of orientations

• Kinematics motions related to deformation
• Translation change in position
• Rotation change in orientation
• Distortion change in shape
• Dilation change in volume

Dynamics/Mechanics relating deformation to
stresses
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What is it??
Marker bed
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Structures from shallow to deep in the crust,
beginning with. Joints (D R p.
205-226) Next lecture Faults (DR p.269-279
286-296)
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Types of joints

• Opening (mode 1)
• Shearing (mode 2)
• Scissoring (mode 3)

Can be filled with veins materials (calcite,
quartz), Form networks that is preferentially
oriented in the regional stress field
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• Joint A fracture that forms by tensile loading
• walls of fracture move apart slightly no
  appreciable displacement
• forms perpendicular to tensile direction
• abundant structural element

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Joint A fracture that forms by tensile loading-
walls of fracture move apart slightly as joint
develops form perpendicular to tensile
direction abundant structural element
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Joint arrays
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Joints in 3-D commonly elliptical
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Joint surfaces

• Planar and often smooth, but not that smooth...
  Some texture

Moscow Kremlin - Bell Tower of Ivan the Great.
Fractured in 1737 due to uneven cooling
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Origin analogous to the focus of an earthquake
Plumose structure A subtle roughness on surface
of some joints resembles imprint of a feather.
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hackles in plumose structure.
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Joints Kinematics
ribs are arrest lines- opening is not
instantaneous, but rhythmic, like splitting wood
(ripping paper analogy)
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• GEOMETRY- planar, plumose structure
• KINEMATICS- very small pull-apart movements
  (dilation)
• ON TO MECHANICS...

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Griffith crack theory pre-existing microcracks
in a rock- act as stress "concentrators"
The largest properly oriented Griffith cracks
(perpendicular to tensile direction) propagate to
form a through-going crack
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But what produces the tensile stress?
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Thermal contraction can produce joints
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REMOVAL OF OVERLYING ROCKS (unloading) PRODUCES
JOINTS!
Unloading joints due to removal of overburden--
rock expands in vertical direction and contracts
in horizontal direction ("Poisson effect")
(marshmallow analogy) - forms near vertical and
horizontal joint arrays
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Jointing in response to unloading and "residual
stresses"
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Exfoliation joints Are parallel to topography
form by unloading of "residual" stress
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WHY ARE JOINTS IMPORTANT?
Joints related to regional stress
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Can also tell us about orientation of tectonic
stress
Can strongly influence the landscape weathering
is localized along joint surfaces-- hoodoos
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Significance for Engineering Planes of
weakness! Rock bursts in mines.
Significance for petroleum pathways for
hydrocarbons pump water into ground to
artificially produce joints! (hydrofracturing)
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Significance Economic Geology Alteration/Minerali
zation along fractures Veins preserve dilational
separation
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Geologic Hazards Rockbursts in mines
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Joints form/occur mainly in the uppermost crust
(upper few kilometers) WHY?
Stresses become more compressive with depth to
the point where rocks cant pull-apart
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Shear fracture A fracture with a component of
sliding motion due to compression
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Determining the sense of shear
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Shear fractures with appreciable displacements
FAULTS
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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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Next Geometry and Kinematics Faults (Read
DR, p. 269-279 286-296)