Lecture index: Brittle failure modes. / Structural significance of joints. / Examples of fracture and joint patterns. / Description of opening mode structures. / Some common joint associations. / Some models for joint formation. / Veins ('filled joints').
Brittle failure modes
A fracture is a brittle discontinuity. Fracture type is based on the type of movement associated with formation of the discontinuity. There are three end member geometric possibilities.
Hybrids (mixes) occur between 1) and 3), and between 2) and 3) of the above
Geometries for options 1. and 3. are shown here. Diagram altered from https://en.wikipedia.org/wiki/Fracture_mechanics
Image of cliff forming diabase sill that has intruded Triassic strata (specifically preferentially along a dark shale rich level), as seen in shoreline cliffs of western Edgeøya, Svalbard. Click on the image to see a larger version. Note how the sill is irregular and climbs to form a tee-pee like feature with a small dike that ascends from the top, and note the deformation of the overlying strata. A sill is a opening mode crack that is concordant to host rock anistropy. In this case the sill roughly followed the bedding. Note the deformation where the intrusive body cuts across layering. Magmatic pressure likely played an important role in its formation. The sill here is 10s of meters thick.
Extensional features are brittle phenomena where movement is perpendicular to the fracture surface in an opening mode. Since rocks are considerably weaker in tension than in compression (which we will delve into in a few weeks), extensional features are very common. Fluids under pressure are often involved in their formation.
Types of opening mode geologic features:
Crevasse field in a glacier at an ice falls. Note that while the crevasses are roughly aligned they do curve and branch.The increase in slope at an icefalls causes the overlying ice to move faster. That means it pulls away from the slower moving ice higher up, and extends in the direction of flow. The extension generates stresses that exceed the tensile strength of the ice and crevasses form. Glaciers as rock bodies that flow and deform at the surface of the earth on a human time scale provide a good learning opportunity in structural geology and material science.
A focus will be on joints since they are the most common type of structure.
They are some of the simpler structures found in rocks, but can still get complicated fast as they are considered in greater detail. Why are they important?
Above are some jointed basalts from Giant's Causeway, Ireland. Classic columnar jointing is developed here. Note that not all the columns are perfectly hexagonal. Also note how subhorizontal fractures also segment the columns along their length. These fractures have a distinctive disk shape, and are both convex upward and downward as can be seen in the photo to the right. The mechanics of the hexagonal columns are well studied, but those of the dish shaped fractures which segment the columns are not. Think of how fluids might flow through such an array of fractures.
This is a USGS photo looking northwest showing the joint pattern evident in the sandstones of Arches National Monument. Salt anticline valley is in the upper left, and the relatively smooth surface in lower right is a bedding plane that dips slightly to the right (east) as part of the anticline. The sandstones are exposed on the other side of the valley and are part of the west limb of the fold. The dominant joint set is parallel to the fold trace, a common relationship. It is this joint set that controls the development of the the striking erosional forms (fins and arches) that are a centerpiece of the valley, and favorite of tourists. If you observe the joints carefully you can see that they split and join other joints to form an anastomosing pattern.
The larger surface exposed here is a bedding surface in the Permian Kapp Starostin Formation as exposed on Treskelodden peninsula in Hornsund, southern Spitsbergen. The overlying softer shaley cherts have been stripped away to reveal two joint sets at right angles to each other in the limestone. The view direction is parallel to one of the joint sets. Orthogonal joint sets are fairly common. Note also how erosion is rounding off the corners of the blocks formed by the intersection joint sets. This represents a early stage in a process of what is known as spheroidal weathering, where the original sharp corners formed get rounded off.
Three related questions are: 1) what are traits of opening mode structures to be described, 2) how do you describe them, 3) what can you learn from documenting these traits?
For an individual fracture the following are some of the traits that can be documented:
Some important relationships of multiple fractures are:
This is a glacially smoothed subhorizontal surface on Cretaceous diabase sills from Edgeoya, Svalbard, showing an array of steeply oriented joints. Note how the rock is broken into rectangular blocks, a common phenomena when two orthogonal (at right angles) joint sets exist. Also note how the one set of joints truncates against the continuous joint running from side to side in the image. If this relationship is consistent, the continuous joint is the longitudinal joint, which is interpreted to have formed before the cross joints that truncate against it. Finally note that some fractures that can not be assigned to one of the two orthogonal sets exist. It is not uncommon at all that a rock body can have three or more joint sets developed within it. A particular challenge is to distinguish fractures that formed associated with surface weathering processes versus fractures that formed at depth and reflect tectonics and crustal stresses.
Describing arrays of planar fractures: orientations, spacings, and density.
Example of a stereonet plot of joints in Brule Formation (Tertiary age) strata from the Slim Buttes area of South Dakota. The dots represent the poles to the joints (the orientation of the line perpendicular to the plane). By default N is up and one is looking down in structural stereonet plots. At this point this type of plot may be unfamiliar, but you will become comfortable with producing and reading these types of plots. It is a basic tool of structural analysis you will learn how to conduct in one of your labs. You may be able to guess that the concentration of dots represents a concentration of measured joints that are close to each other in orientation, and thus help to define a set.
In different geologic situations you get different characteristic joint patterns.
Example of conjugate joint set in tilted Tertiary sandstones of the Brule Formation in the Slim Buttes area of South Dakota. The interpretation is that the conjugate joint set happened before tilting of the strata .The tilting is associated with normal faulting that is in symmetry with the normal faults.
Joints are polygenetic!
This photograph comes from mud flats in the deepest part of Death Valley at an unusual time where it had rained significantly. The layer on top is mud that has been recently deposited. The cracked layer below is older sediment with well developed cracks in it due to a previous episode of desiccation. They were recently exposed by stream erosion. Interestingly, the pattern is not the normal simple mudcrack pattern with triple junctions. Instead it is more organized with two preferred directions. Also significant, a close look shows the same pattern starting to develop in the the new overlying muds. This is an example of reactivation, where buried mud cracks determine the fracture pattern in new overlying muds as they go through another cycle of drying and cracking.
This is a view of a subhorizontal shoreline outcrop in subhorizontal Triassic strata on Edgeøya, Svalbard. The large tan rock material just below the walking stick is a large carbonate concretion. Other evidence indicates the concretions in these strata mostly formed before compaction of the shaley rocks. There are two well defined joint sets in the surrounding dark silty shales, at approximately 60 degrees to each other, but the fracture pattern in the concretion is different in orientation and pattern than that in the surrounding shales. Why? At least two possibilities exist. Because of their different cementation and ltihification histories (and therefore different mechanical histories), the joints may have formed at different times in an evolving/changing regional stress fields. Alternatively, because of the differing mechanical nature the stresses inside the concretion could have been different producing the alternate pattern. It does appear that the sub-orthogonal set in the concretion bisects the approximately conjugate set in the host sediment.
Veins differ from joints in that they are filled with hydrothermal precipitations, and can either form by wall rock replacement due to fluid-rock interactions or by dilation (opening up and volume increase). In the latter case they represent a greater magnitude of strain.
Simple calcite filled vein set in phyllite from Vanoise National Park in the French Alps. Both the extension direction and amount can be gauged here.
Veins with fibrous carbonate mineral growth in phyllites from Vanoise National Park in the french alps. Note how the fibers are perpendicular to the vein walls.
Image of gypsum veins forming around a cemented burrow in the Tertiary Brule Formation siltstones of Scotts Bluff National Monument. The view is of a subhorizontal surface. The pattern is partly radial, but with longer and thicker veins in one direction. The same pattern can be seen on a smaller scale in the upper left corner, and many examples of this geometry can be found in this area, all exhibiting the same preferred development direction. The radial vein component can be explained as a consequence of the surrounding material undergoing shrinking (syneresis) while the differently cemented (carbonate) core does not change volume. However, the preferred development of the veins in one consistent direction still needs to be explained. One possibility is that there is some inherent material strength anisotropy in the siltstone that makes it easier for fractures to open up in one direction, but there is little evidence of that. Another possibility is that while syneresis is occurring there is also an additional directed tectonic force that aids fracture development in a certain direction. In other words the pattern is a result of the combination of a local stress field localized on the 'hardened' burrow core with a more regional stress field that favors crack growth in the one direction.
Other filled tensile fracture features
Photography of shonkonite (potassic mafic dike of distinctive composition) dike cutting horizontal Cretaceous sandstones along the Missouri River in Montana (White Cliffs section). The width of the dike is the amount of opening of this tensile feature.
The bands cutting across the Tertiary strata in Badlands National Park (South Dakota) are clastic dikes, where the material that fills the tensile fracture is sediment (in this case both mud and sand). Two fundamental explanations for them are that they are earth fissures that filled in with loose sediment that washed in from above, or that they are injectites, where mobilized, wet sediment intrudes into the surrounding sediment from some subsurface source. One event that can cause such mobilization (liquefaction) of sediment is a large earthquake. Internal flow structures suggest these dikes are injectites.
Deformation, compaction bands and "anti-joints"
Deformation bands traits:
This is a close up view the Jurassic Aztec sandstone in Valley of Fire State Park in Nevada. Three planar discontinuities can be seen in the sandstone, and are compaction bands. They are more resistant to erosion because inside the band the grains have 'collapsed' to closer positions reducing porosity and enhancing cementation. The steeper bands (from upper left to lower right) do not show any offset of the delicate dune laminae here. This means that either side of the discontinuity has moved towards each other. This is opposite of a opening mode feature - an anti-joint. The more shallowly dipping layer here does offset both the earlier steep bands and the laminae, indicating that this has a shear component during its history. So this feature differs from the other and has been called a shear enhanced compaction band. These features are fairly rare and not particularly well understood at present, but they are interesting.
This is an image of two cross cutting sets of these compaction bands in the Aztec sandstone of Valley of Fire State Park (within easy striking range from Las Vegas). Note the staining pattern , which is somehow related to mobilization of iron compounds by groundwater. In each compartment a somewhat consistent pattern of the upper right corner being redder exists. Why should this be? One explanation in the literature is that bands are influencing the direction of water flow and ion transport by acting as natural membranes.
Several types of deformation bands exist including dilation bands. Deformation bands are a type of brittle feature only recently recognized, and of which there are several types. The compaction bands shown above are another type. A review of deformation bands can be found in Fossen et. al., 2007, Deformation bands in sandstone: a review; Journal of Geological Society, London; 164, 1-15.