Transcript Chapter 8: Joints and Shear Structuresy

Chapter 8
Joints and Shear Fractures
Joints
“Joints are the most ubiquitous structure in
the Earth’s crust, occurring in a wide
variety of rock types and tectonic
environments…They control the
physiography of many spectacular
landforms and play an important role in
the transport of fluids…”
Pollard and Aydin 1988
Joints
► Joints
– Fractures along which there is
no appreciable displacement parallel to
the fracture and only slight movement
normal to the fracture plane.
Importance of Joints
► Economically
important minerals can be
found in joints
► Joints act as the plumbing system for
ground-water
► Help to demonstrate the tectonic history of
an area which is important for the
construction of dams, bridges, power plants,
and buildings.
Three modes of fractures
► Three
types of fractures have been identified with
each one formed by a separate kind of motion.
 Mode I – Opening of fractures
 Mode II – Sliding of fractures
 Mode III – Tearing of fractures
Joint Systems
► Systematic
Joints – Parallel joints with
regular spacing
 Joint Set – Joints that share a similar
orientation
 Joint System – Two or more joint sets in the
same area
► Nonsystematic
Joints – Do not share a
common orientation, can be curved, and
can form irregular fracture surfaces
 They occur in many areas but do not appear
to be related to a recognizable stress field
Systematic and Nonsystematic
Joints
Joint Appearance
► Joints
 Unfilled – Generally recent
►May have smooth surfaces
►May have irregular surfaces
►May have concentric ridges
►May have a feathered texture called Plumose
Joints
Joint Appearance
► Joints
 Filled – Also called Veins
►Feldspar or Aplite – High temperature
►Quartz, Calcite, Chlorite, and Epidote –
Low temperatures
►Ore Minerals – Low temperatures
Calcite filled joints in Vermont
Joint Sets
► Conjugate
Joint Systems – Paired joint
sets that form at acute angles and are
thus shear fractures.
 Difficult to make certain that the acute
joints formed at the same time
 If you can prove they are conjugate then
σ1 bisects the acute angle
Fracture Analysis
► Study
of joint systems in an area reveals the
sequence and timing of tectonic events.
► The orientations of systematic fractures
provides information about the orientation
of the principal stress directions involved in
brittle deformation.
Joints and Principle Stress
Axes
Regional Tectonics
► Regional
joint-orientation patterns may be
determined by measuring strike and dip of
mesoscopic-scale joints over a wide area.
 Bearing of linear stream systems
►Satellite
imagery
►Topographic maps
►Aerial photos
Aerial photo of Precambrian granite in Wyoming
Graphical depictions of joint sets in Pigeon Forge,
TN
Lower-hemisphere equal-area plot
Rose diagram
Contour diagrams of a
Jointing related to the strain ellipsoid
Fracture and Lineament Orientation in Italy
The Anatomy of Joint Surfaces
► Various
features provide information on the rate
and direction of propagation of joints.
 Hackle Marks – Form in the zone where the joint
traveled rapidly
 Arrest Lines – For parallel to the advancing edge of
the fracture and perpendicular to direction of
propagation.
 Origin – Can often determine the initial site of the
joint. Joints always begin at a preexisting flaw in
the rock such as a grain of atypical size or hardness,
fossil, or concretion.
Plumose joint surface showing primary surface
features.
1. Main joint face
2. Twist hackle fringe
3. Origin
4. Hackle plume
5. Inclusion hackle
6. Plume axis
7. Twist-hackle face
8. Twist-hackle step
9. Arrest lines
10. Constructed fracture-front lines
Features associates with the propagation of a
joint
Plumose Joint Face, Ontario
Glacially produced joint surface in Killarney
Granite, Ontario
Controlling Factors of Joint Propagation
► Bedding
and foliation planes in coarse-grained
rocks act as barriers to joint propagation.
► Bedding in fine-grained rocks are often not
barriers.
► Variation in bed thickness also affects propagation
direction.
Four Categories of Joints
► Tectonic
– Form at depth and are driven by
tectonic forces.
► Hydraulic – Form at depth during burial and
compaction.
► Unloading – Form near the surface when ½
of the overlaying sediment is removed by
erosion.
► Release Joints – Form after the release of
horizontal stress and are controlled by
existing rock fabric.
Joints in Plutons
► Joints
form in plutons in response to
cooling and later tectonic stress
 Orientations of joints may be influenced by
the boundary of the pluton
Sheeting
► Sheeting
(Same as unloading joints) –
Form parallel to surface topography in
massive rocks.
 Spacing between sheets increases with
depth.
 Can be used in quarrying stone.
Columnar Joints
► Columnar
Joints – Form in response to cooling
and shrinkage in magma.
 Form in Flows, Dikes, Sills, and Volcanic Necks
 Hexagonal prisms are the most efficient geometric
shape.
Contraction to Form Columnar
Joints or Mud Cracks