Transcript Joints and Veins 1

Brittle Deformation
Fracture
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A planar or curviplanar discontinuity
 Forms as a result of brittle rock failure
 Under relatively low pressure and temperature
conditions in the earth crust
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Rock fractures range in size from:
 Microcracks – Intragranular to intergranular
(fraction of a mm)
 Faults - Extend for hundreds of kilometers
Brittle Deformation
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Permanent change in rocks by fracture or sliding on fractures
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Fracture: A discontinuity across which cohesion (Co) is lost
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The term fracture includes three basic types of discontinuities:
 Extension fracture (type I)
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Shear fracture (type II & III)
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Relative movement parallel to fracture surface
Oblique extension (hybrid) fracture
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Relative movement normal to fracture surface
Relative movement is oblique to the fracture surface
Vein: fracture filled by secondary minerals
Four categories of fracture
observation
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2.
3.
4.
Distribution and geometry of fracture
system
Surface features of fracture
Relative timing of fracture formation
Geometric relation of fracture to other
structures
Fracture set and system
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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
Sheet (exfoliation) joints
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Are parallel to topography
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Can form in any rock, but common in
plutonic rocks that are exposed
Columnar Joints
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Extension fractures characteristic of tabular
extrusive igneous rocks
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i.e., form in lava flow, sill, dike
Other types of joint:
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Strike joint, dip joint, cross joint, oblique
joints
Extension fractures associated with
shear fractures
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Feather (pinnate) fractures – form enechelon to a main brittle shear fracture
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Gash fracture – are simiar to feather
fractures, but filled with mineral, in ductile
shear fractures
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Are sigmoidal (S- or Z-shaped)
What do we collect about fractures?
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Orientation (rose diagram, stereonet)
Spacing
Length
Spatial pattern
Relation to lithology
Relation to layer (bed) thickness
Joints
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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
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Joints form under:
 shallow depth, low confining pressure (Pc)
 elastic regime
 low temperature (T)
 high pore fluid pressures (Pf)
Joints
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Joints form perpendicular to the maximum
principal tensile stress
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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
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i.e, along a principal plane of stress
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
Tensile strength and jointing
Joint sets
Modes of crack surface displacement
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Individual cracks, when loaded, propagate, infinitesimally,
in three different modes:
Mode I – Tensile (Opening) Mode
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Mode II – Sliding Mode
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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
One block moves parallel to the crack normal to the fracture front
Mode III – Tearing Mode
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One block moves parallel to the crack parallel to the fracture front
Modes of crack surface displacement
Modes of fracture
Mode II and III
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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
Joints
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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
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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
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Involve small relative displacement of the originally
contiguous points compared to the in-plane dimensions of
the fracture walls
En-echelon
Tension
Gashes
Vein
Antiaxial vs. syntaxial vein
Joint Spacing
&
Bed Thickness
Fracture
Terminations
Shear Fractures
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Fractures along which there has been shearing or
displacement
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Shear fractures are small, with small displacement
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They occur in intact rock during brittle deformation
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If the amount of displacement is significant and
measurable, the shear fracture is called fault
Faults and
joints
Shear Fractures
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Shear fractures, form at an angle to the
maximum compressive stress and show
offset because of the shear traction along
the fractures
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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
Terminology
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Fracture front: The line separating the
fractured region from the un-fractured part of
the rock
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Fracture trace: Intersection of the fracture
with any surface
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Fracture tip: The termination of the fracture
along the trace of the fracture
Joint Surface Morphology
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Surface morphology of joints show evidence for
initiation, propagation, and arrest
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Theoretically, mode I loading in an isotropic,
homogeneous material should lead to a smooth
propagation with a mirror smooth surface
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Joints however are not smooth, because rocks are
commonly neither homogeneous nor isotropic
Out-of-plane Propagation
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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
Crack Propagation Paths
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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
Mesoscopic Joint Surface
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Although microscopic out-of-plane propagation is
common, joints appear smooth on the mesoscopic
scale.
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This is due to the homogeneous nature of the remote
stress field
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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
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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
Joint Initiation and Beds
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If joints initiate from bedding plane, they
often originate from irregularities such as
ripples or sole marks
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Joints in a bed often initiate from a
common feature such as the upper
bedding plane
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Fossils and concretions often originate joints
in adjacent beds
Rupture Propagation
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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 nonsmooth (misty) zone, separating the mirror from the
hackle zone
Hackle zone
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Forms when there are local components of twist
during crack propagation.
Form when propagation occurs at a critical velocity
when cracks branch or bifurcate
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Hypothesis I: High velocities shift the maximum
local tension away from the existing crack plane
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Hypothesis II: High velocities form secondary
cracks. The main crack then branches to follow the
secondary cracks
Plumose Structures
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Also called feathers, hackle plumes, striations, or barbs.
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These record rupture motion
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Consists of an axis from which barbs mark the direction of the
rupture front as portions diverge away from the plume axis
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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
Plumose Structures …
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Plumose structures have different shapes:
 Axes can be straight to curved
 Barbs vary from uniform to symmetrical
to asymmetrical about the axis
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These variations reflect the degree to which
rupture velocity was uniform during
propagation
Arrest of Rupture
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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