Brittle Deformation

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Brittle Deformation
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Fracture

• A planar or curviplanar discontinuity
• Forms as a result of brittle rock failure
• Under relatively low pressure and temperature
  conditions in the earth crust
• Rock fractures range in size from
• Microcracks Intragranular to intergranular
  (fraction of a mm)
• Faults - Extend for hundreds of kilometers

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Brittle Deformation

• Permanent change in rocks by fracture or sliding
  on fractures
• Fracture A discontinuity across which cohesion
  (Co) is lost
• The term fracture includes three basic types of
  discontinuities
• Extension fracture (type I)
• Relative movement normal to fracture surface
• Shear fracture (type II III)
• Relative movement parallel to fracture surface
• Oblique extension (hybrid) fracture
• Relative movement is oblique to the fracture
  surface
• Vein fracture filled by secondary minerals

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Four categories of fracture observation

• Distribution and geometry of fracture system
• Surface features of fracture
• Relative timing of fracture formation
• Geometric relation of fracture to other structures

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Fracture set and system

• Fracture set a group of fractures with similar
  orientation and arrangement
• Small extension fractures are referred to as
  joint
• Systematic joints have roughly planar surfaces,
  parallel orientation and regular spacing (vs.
  non-systematic joints)
• Fracture system two or more sets of fracture
  affecting the same volume of rock

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Sheet (exfoliation) joints

• Are parallel to topography
• Can form in any rock, but common in plutonic
  rocks that are exposed

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Columnar Joints

• Extension fractures characteristic of tabular
  extrusive igneous rocks
• i.e., form in lava flow, sill, dike
• Other types of joint
• Strike joint, dip joint, cross joint, oblique
  joints

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Extension fractures associated with shear
fractures

• Feather (pinnate) fractures form en-echelon to
  a main brittle shear fracture
• Gash fracture are simiar to feather fractures,
  but filled with mineral, in ductile shear
  fractures
• Are sigmoidal (S- or Z-shaped)

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What do we collect about fractures?

• Orientation (rose diagram, stereonet)
• Spacing
• Length
• Spatial pattern
• Relation to lithology
• Relation to layer (bed) thickness

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Joints

• Are a type of fracture which form due to tension
• They form parallel to the minimum tensile stress
• Perpendicular to maximum tensile stress
• Shearing is zero along joints when they form
• Also called cracks or tensile fractures
• Joints form under
• shallow depth, low confining pressure (Pc)
• elastic regime
• low temperature (T)
• high pore fluid pressures (Pf)

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Joints

• Joints form perpendicular to the maximum
  principal tensile stress
• i.e, along a principal plane of stress
• Therefore joints dominantly show separation or
  opening of the walls of the fractures with no
  appreciable shear displacement parallel to the
  plane of the fracture
• Joints form through the Mode I crack surface
  displacement
• Joints are commonly characterized by two
  matching, rough, discontinuous, and curved
  surfaces, although they are approximated to be
  smooth, continuous, and planar

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Tensile strength and jointing
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Joint sets
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Modes of crack surface displacement

• Individual cracks, when loaded, propagate,
  infinitesimally, in three different modes
• Mode I Tensile (Opening) Mode
• Tensile cracks form normal to the ?3 (parallel
  to the ?1?2 plane)
• Crack opens infinitesimally perpendicular to the
  crack plane
• Crack grows in its own plane no bending/changing
  orientation
• Mode II Sliding Mode
• One block moves parallel to the crack normal to
  the fracture front
• Mode III Tearing Mode
• One block moves parallel to the crack parallel to
  the fracture front

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Modes of crack surface displacement
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Modes of fracture
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Mode II and III

• Both are shear mode
• Do not grow in their own plane.
• As they start growing, they immediately either
• Curve
• Become mode I cracks
• Spawn new tensile, wing cracks
• However, shear fractures and faults are not large
  mode II or mode III cracks

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Joints

• The planar approximation is justified given that
  scale of geometric irregularities (e.g. joint
  surface morphologies, curvature amplitude) is
  commonly very small compared to the size of the
  fracture surface
• Termination of the two opposing surfaces at their
  distal edge or periphery (fracture front), i.e.,
  a finite extent of the two walls
• Displacement is zero at the fracture fronts
• Involve small relative displacement of the
  originally contiguous points compared to the
  in-plane dimensions of the fracture walls

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En-echelon Tension Gashes
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Vein
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Antiaxial vs. syntaxial vein
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Joint Spacing Bed Thickness
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Fracture Terminations
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Shear Fractures

• Fractures along which there has been shearing or
  displacement
• Shear fractures are small, with small
  displacement
• They occur in intact rock during brittle
  deformation
• If the amount of displacement is significant and
  measurable, the shear fracture is called fault

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Faults and joints
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Shear Fractures

• Shear fractures, form at an angle to the maximum
  compressive stress and show offset because of the
  shear traction along the fractures
• The hybrid shear fractures are discontinuities
  with a mixed mode of opening and shear and form
  oblique to the plane of the fracture as a result
  of both tensile normal stress and shear stress

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Terminology

• Fracture front The line separating the
  fractured region from the un-fractured part of
  the rock
• Fracture trace Intersection of the fracture with
  any surface
• Fracture tip The termination of the fracture
  along the trace of the fracture

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Joint Surface Morphology

• Surface morphology of joints show evidence for
  initiation, propagation, and arrest
• Theoretically, mode I loading in an isotropic,
  homogeneous material should lead to a smooth
  propagation with a mirror smooth surface
• Joints however are not smooth, because rocks are
  commonly neither homogeneous nor isotropic

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Out-of-plane Propagation

• The orientation of the maximum tensile stress in
  front of a single crack tip may not be parallel
  to the normal to the parent crack
• Cracks propagate so that new portions of the
  crack remain normal to the local maximum tensile
  stress
• This requires a crack to leave the plane of the
  parent crack in order to maintain its orientation
  relative to the local stress field
• This out-of-plane propagation is so common in
  microscopic scale that leads to the formation of
  rough, non-smooth fractures surfaces

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Crack Propagation Paths

• Out of plane propagation is characterized by a
  combination of two end member crack propagation
  paths
• Twist leads to segmentation of the crack into
  several smaller crack planes.
• Represents a rotation of the local maximum
  tensile stress in the initial yz plane.
• Rotation is about an axis in the crack plane
  to propagation direction
• Tilt Causes the crack tip to rotate without
  segmentation.
• Rotation is about an axis in the crack plane __
  propagation direction

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Mesoscopic Joint Surface

• Although microscopic out-of-plane propagation is
  common, joints appear smooth on the mesoscopic
  scale.
• This is due to the homogeneous nature of the
  remote stress field
• Even when the crack leaves its plane, the general
  propagation path remains normal to the remote
  maximum tensile stress
• The microscopic out-of-plane propagation leads to
  the development of joint surface morphology

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Plumose Surface Morphology

• Helps to interpret rupture nucleation,
  propagation, and arrest
• Develops largely due to local twists and tilts
  during propagation of a fracture which would
  otherwise be planar.
• Barbs surface irregularities
• In homogeneous rocks, barbs trace to the point of
  origin (an original crack)
• In inhomogeneous rocks (sandstone, shale), barbs
  radiate from either a bedding plane or an
  inclusion in the bed (e.g., fossil, concretion,
  clast)
• The point of origin may vary from bed to bed in
  shale or siltstone

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Joint Initiation and Beds

• If joints initiate from bedding plane, they often
  originate from irregularities such as ripples or
  sole marks
• Joints in a bed often initiate from a common
  feature such as the upper bedding plane
• Fossils and concretions often originate joints in
  adjacent beds

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Rupture Propagation

• The progress of rupture from the origin to the
  final arrest leads to the formation of some
  patterns that are printed on the surface of
  joints
• Mirror zone
• Area immediately adjacent to the point of origin.
• Forms under small tip stress values, not big
  enough to beak the material at oblique angles
• Mist zone
• Forms when larger stresses break bonds at oblique
  angles to crack plane.
• On the fine scale, this oblique cracking forms a
  non-smooth (misty) zone, separating the mirror
  from the hackle zone

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Hackle zone

• Forms when there are local components of twist
  during crack propagation.
• Form when propagation occurs at a critical
  velocity when cracks branch or bifurcate
• Hypothesis I High velocities shift the maximum
  local tension away from the existing crack plane
• Hypothesis II High velocities form secondary
  cracks. The main crack then branches to follow
  the secondary cracks

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Plumose Structures

• Also called feathers, hackle plumes, striations,
  or barbs.
• These record rupture motion
• Consists of an axis from which barbs mark the
  direction of the rupture front as portions
  diverge away from the plume axis
• Barbs become more pronounced toward the edge of
  the beds away from the axis
• Barbs represent long, narrow planes oblique to
  the main fracture plane.
• Barbs form similar to hackle marks due to twist

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Plumose Structures

• Plumose structures have different shapes
• Axes can be straight to curved
• Barbs vary from uniform to symmetrical to
  asymmetrical about the axis
• These variations reflect the degree to which
  rupture velocity was uniform during propagation

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Arrest of Rupture

• Arrest lines show up as ridges or cusped waves
  normal or subnormal to the direction of
  propagation of cracks
• At arrest lines a large component of tilt is
  involved in the out-of-plane crack propagation
• Arrest lines may be the boundaries between areas
  with perceptible barbs and areas with no barbs.
• Rock joints may show several arrest lines in a
  row or a single arrest line at the end of a long
  fracture
• Several arrest lines represent intermediate
  slowing or stopping points for a rupture as it
  moves through the rock to form a joint