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Styles of Brittle Failure
Ch. 7: p.119-‐125; 141-‐
1. Mixed Rheologies: Most rocks undergo a combination of different rheologies that depend on the time scale and the amount of strain. Examples include: - Elastic-‐plastic (Prandtl) - Viscoplastic (Bingham) e.g. lava with crystals - Viscoelastic (Kelvin) - Viscoelastic (Maxwell) e.g. mantle; ice
[Fig. 6.11. Geologic materials may have different rheological behaviors during the deformation process]
2. Brittle vs. Ductile Deformation: In structural geology, we commonly use the terms brittle and ductile to refer to rock behavior, but they do not explicitly imply specific rheologies. Ductile deformation preserves the continuity of originally continuous structures, but may involve a range of mechanisms (viscous, plastic, microfracture) that vary with conditions (scale, temperature, stress state, strain rate).
Brittle deformation involves actual breakage of the rocks, but this can actually happen in a number of ways…
[Fig. 6.16. Brittle and ductile deformation mechanisms. How these terms are used depends on the scale of observation]
3. Brittle Failure: Brittle deformation involves the loss of cohesion across a discrete, planar (or tabular) surface (or zone) within geologic materials. Deformation mechanisms in the brittle regime include fracturing, frictional sliding along grain boundaries, and rotation of grains.
“Brittle” is commonly thought to imply fracturing, but non-‐fracturing brittle deformation styles are possible, like granular flow in loose sediments.
[Fig. 7.1A. One possible process in the brittle regime is the frictional rotation of grains during granular flow in unconsolidated materials. Some ductile deformation can utilize this mechanism at the granular scale]
4. Brittle Failure by Fracturing: In consolidated rocks, brittle failure commonly involves fracturing. At the microscopic scale (microfracture), this may occur where individual grains impinge on each other, forming intragranular fractures. In nonporous rocks, fractures cross multiple grains and are called intergranular fractures.
The fracturing and crushing of grains is called cataclasis, and may result in a type of granular flow called cataclastic flow.
[Fig. 7.1B. Cataclastic flow in a zone where grains have been broken down by cataclasis] [Fig. 7.2. Microscopic fracturing in the brittle regime. (a) Intragranular fracturing in porous rocks. (b) Intergranular fracturing in non-‐ porous consolidated rocks]
5. Types of Fractures: The term fracture (or crack) refers to any discrete planar surface that manifests brittle deformation. Based on geometric observations, many fractures clearly look different to others and are thus recognized.
However, we properly distinguish different types of fractures based on their kinematics (relative motions). If the two sides of the fracture slide over each other, it is a shear fracture. Sliding occurs along the slip surface. Faults fall into this category (for offsets of ~10s of cm or more).
[Fig. 7.3. Different types of fractures based on the relative motions across the fracture surface]
6. Photo of a fault: Shear Fractures (Sliding Fractures) – in this case, a normal fault. 7. Types of Fractures: If the two sides of the fracture move apart from each other, it is a tension fracture (or extension fracture or an opening fracture).
If the opening is very small, it is a joint, but wider opening creates a fissure. Tension fractures filled with minerals are called veins, but if filled with magma are called dikes or sills.
Another possible type of motion is closing, where the two surfaces of the fracture interpenetrate through dissolution. These are closing fractures (or anticracks) and form pressure solution seams or stylolites.
[Fig. 7.4. Different types of fractures that may form during brittle deformation, and their relationship to principal stresses]
8. Photos of dikes and sills: (a) Dike feeding a fissure eruption, SW Iceland. (b) Jurassic-‐aged Ferrar dolerite sill intruded into sandstone, Dry Valleys, Antarctica. Photos by Simon Kattenhorn. 9. Photo of veins: Jurassic Morisson Formation, Utah. Photo by Simon Kattenhorn. 10. Deformation Bands: One type of shear fracture that forms through cataclasis of porous, granular rocks (especially sandstone) is a deformation band. They are a distinct type of fracture in that they are thicker (a few mm to cm) but undergo smaller slip displacements relative to their lengths than other shear fractures. Geometrically, they are described as tabular discontinuities.
[Fig. 7.38. Deformation band in sandstone (< 1 cm of offset)]
11. Photos of deformation bands: Deformation bands in hyalotuffs, Reykjanes Peninsula, Iceland. Photos by Jane Barnes. 12. Deformation Bands: Deformation bands may show increased dilation relative to the host rock, but usually experience cataclastic breakdown of grains and removal of pore space to create a highly impermeable zone. They are thus a barrier to fluid flow, unlike many other fracture types (important for oil migration).
[Fig. 7.39. Low permeability of a deformation band in sandstone, viewed in (a) hand sample, and (b) thin section. Note the crushing of grains and reduction of porosity]
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)]
20. 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]
21. Fracture Representation: The challenge of constructing a tunnel through fractured rock masses.
[ Figure: Image created using AMADEUS software (Bowman & McMahan, 2007)]
22. 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]
23. 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]
24. 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)]
