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Brittle Failure: Joints
Ch. 7: p.123-‐124; 134-‐
1. A Mechanical View of Fractures: In the scientific discipline of linear elastic fracture mechanics, we define fractures based on mode.
Mode I – motion is perpendicular to the fracture walls and perpendicular to the fracture tipline.
Mode II – motion is parallel to the fracture walls and perpendicular to the fracture tipline.
Mode III – motion is parallel to the fracture walls and parallel to the fracture tipline.
Mode IV – motion is perpendicular to the fracture walls in a closing sense and perpendicular to the fracture tipline.
[Fig. 7.6. A mechanical view of fractures based on the fracture mode. The mode of fracturing is quantitatively related to the applied forces]
2. Fracture Modes: Surface fractures in Thingvellir National Park, Iceland.
[Fig. 7.7. At Thingvellir, Iceland, opening fractures are present at the surface (fissures and joints), indicating Mode I fracturing. The right side is lower, suggesting an underlying fault (Mode II or III fracturing)]
3. Fracture Modes: This limestone from the Appalachian Mountains shows stylolites along a pressure solution seam. This feature formed by dissolution during contraction, forming a closing fracture through Mode IV fracturing. 4. Kinematic Indicators: Fracture tiplines and motions relative to them are not directly observable in the field, so the kinematic approach to fracture classification is generally used.
A typical process of observing fractures involves:
- Distribution and geometry.
- Surface features of the fracture.
- Kinematic indicators along fracture.
- Relative timing of fracture sets.
- Relationship to other structures.
[Figure: Use of kinematic indicators to determine the type of fracture. Here, oblique opening suggests a combination of opening and shearing (mixed-‐mode fracturing)]
5. The Importance of Fractures: Brittle deformation by fracturing is extremely important to society, as most rocks are fractured near the Earth’s surface. We construct our societal infrastructure on these rocks and so must be aware of the implications of the fractures.
[Figure: A tunnel beneath Manhattan, New York City. Safety precautions are needed to prevent fractured rock collapses]
6. Fracture Representation: The challenge of constructing a tunnel through fractured rock masses.
[Figure: Image created using AMADEUS software (Bowman & McMahan, 2007)]
7. Engineering Concerns of Fractures: Construction in fractured rock masses can lead to disasters if not properly studied (e.g., Malpasset Dam disaster, France, 1959 – 420 fatalities).
[Figure: Construction of the Malpasset Dam occurred in a valley with walls made of fractured rocks]
8. Malpasset Dam Disaster: Images of the dam disaster.
[Figures: The dam failed when fractured rocks supporting the dam buttresses gave way during filling of the dam]
9. The Importance of Fractures: Fractures are vital to society in that they create pathways for the flow of fluids through the crust (e.g., water, oil, and natural gas) and can host mineral deposits.
About 10% of the US Gross National Product each year is produced through the breaking of rock (the highest on Earth).
[Figure: Extraction of oil from fractured rocks using the hydraulic fracturing technique (hydro-‐fracking)] [Figure: Pyrrhotite-‐pyrite-‐chalcopyrite mineral vein network in chloritized basalt (Natural Resources Canada)]
10. Opening Fractures: If the two sides of a fracture pull apart from each other, it forms an opening fracture, or tension fracture, or extension fracture. Mechanically, they are Mode I cracks.
[Figure: These opening fractures (fissures) formed within basalt lava flows on the Reykjanes Peninsula, Iceland]
11. Joints: If the amount of opening is very small compared to the length of the fracture, it forms a joint (the most common type of fracture). The opening is typically no more than a few mm. 12. Joints: Joints do not occur in isolation but are usually part of a large number of similarly oriented, parallel, evenly spaced fractures that define a joint set. Multiple joint sets are identified through their different orientations.
The joints typically extend through the entire thickness of a sedimentary bed or may cut through many beds. In 3D, they tend to be much longer than they are tall.
[Figure: Joint sets in sandstones in Arches National Park, Utah]
13. Joint Sets: When numerous regular, parallel joints exist to form a joint set, they are systematic joints. Aerially extensive joints that are more irregular (curved and with a range of orientations) are nonsystematic joints.
A number of almost coplanar joints define a joint zone. From a distance, they resemble one long continuous joint. Numerous joint sets in the same rock form a joint system.
[Figure: Joint terminologies]
22. Fault-‐Related Fractures: Shearing cracks may also create opening fractures at their ends/tips. These are called tailcracks, wing cracks, or horsetail fractures.
As with pinnate fractures, the geometry of the tailcracks indicates the sense of shearing.
[Figure: Tailcracks at the tip of a shear fracture in Aztec Sandstone, Valley of Fire State Park, NV]
23. Tailcrack Geometries: Tailcracks can be used to deduce the sense of motion in the absence of actual offset indicators.
[Figure: Clockwise from tip (RIGHT-‐LATERAL) / Counterclockwise from tip (LEFT-‐LATERAL)] [Fig. 7.28. Horsetail fractures at the end of a sheared joint. What was the sense of shearing?]
24. Tailcrack Veins: Tailcracks can also fill up with minerals to form veins.
[Figure: Vein-‐filled tailcracks in limestone at Languedoc, France. What is the sense of shearing? Try to pick out the pressure solution surfaces that also formed]
25. Fault-‐Related Fractures: Both pinnate fractures and tailcracks (wing cracks) can form in different locations around the edge of a shear fracture.
[Fig. 7.3. Formation of secondary cracks around the edge of a shear fracture]
26. Fracture Shape in 3D: Although opening fractures look like lines on a map that terminate at a point (the tip), remember they actually planes in 3D.
The tip must completely encircle this plane in 3D, marking the boundary between fractured and unfractured rock. This boundary is called the tipline (also fracture periphery and fracture front).
The motion sense relative to this tipline defines the fracture mode.
[Figure: Fracture tipline]
27. Fracture Shape in 3D: If a joint is completely contained within a rock layer, its tipline shape may be approximately circular or elliptical, with the long axis horizontal (confined within a bed or beds, especially where beds have variable lithologies).
[Figure: Joint tiplines in outcrop]
28. Fracture Surface Markings: A joint is comprised of 2 planar surfaces bounded by a tipline. They used to be seamlessly joined but split apart due to stresses applied to the rock.
As the surfaces pull apart, small jogs along the plane on the granular scale create a distinct ornamentation, also called surface markings.
[Figure: Surface markings along a joint in a thin bed] [Fig. 7.24a. Ornamentation on the surface of a joint in graywacke, Norway]
29. Fracture Surface Markings: It is mostly defined by plumose structure or hackle. It is best viewed when the lighting is oblique to the surface. Plumose usually diverges from a single point (the nucleation point of fracture growth), creating a fanning effect.
[Fig. 7.24a. Ornamentation on the surface of a joint in graywacke, Norway]
30. Fracture Surface Markings: If joint growth is confined to a single bed, the hackle may feather away from a central line of symmetry, fanning outwards in the direction of growth. Periods of no growth are marked by arrest lines or rib marks.
[Fig. 7.24b. Examples of rib marks on joint surfaces] [Fig. 7.25. Within a single bed, hackle fans out away from a central symmetry line. Periods of hesitation in growth create rib marks]
31. Fracture Surface Markings: Towards the edge of a joint, the fracture surface may break down into a number of smaller joints oriented at an angle to the main joint face, forming an en echelon pattern. This is twist hackle (or a twist hackle fringe) and forms because the stresses change in the region of the tipline before growth ceases.
[Fig. 7.24c. Twist hackle along the top and bottom of a joint confined to a single bed] [Fig. 7.26. Formation of twist hackle]
32. Fracture Surface Markings: Fracture ornamentation is very useful for determining joint propagation directions, providing insight into the sequence of events. Other useful tools are crosscutting relationships and interactions between joints. 33. Timing of Fracture Formation: A joint system may have multiple joint sets that formed at different times and under different stress conditions. We need to unravel the age sequence to determine the order of fracturing events. We use crosscutting relationships to determine age, but with joints, older joints cut younger joints. This is because younger joints cannot grow through the open space of an existing joint, so must terminate against it.
[Figure: Fracture sequence]
34. Joint Intersections: A joint will either termination orthogonal to an older joint (T-‐intersections) or curve parallel to it. Why does this happen?
Ambiguous crosscutting relationships can occur. If an old joint is closed by later stresses or becomes mineralized, a younger joint could crosscut it.
Joints can also propagate up and over an older joint in 3D or down and under.
[Fig. 7.29. A younger joint growing towards an older joint may curve to become either parallel or perpendicular to the older joint]
35. Ambiguous Joint Relationships: Younger joints can have ambiguous crosscutting relationships. (A) Growth under and around an older joint. (B) Growth in opposing directions away from an older joint. (C) Simultaneous growth where one goes up and over and the other goes down and under. (D) Simultaneous growth away from a common point. (E) Shear offsets create apparent terminations.
