J. Fractures

Introduction

Figure 1. Joints in limestone are often pathways for groundwater flow. Here the groundwater has dissolved the limestone surface to produce an irregular topography. Port au Port Peninsula, Newfoundland (boot for scale).

Most of the structures we have dealt with so far in the course are products of ductile deformation, which is favoured by high temperature, high confining stress, low strain rates, and low rock strength. In contrast, represent brittle failure of rocks in response to an applied differential stress. In general terms, fracturing is favoured by low values of pressure or mean stress, low temperatures (at which rocks are strongest), high strain rates, and strong or competent rocks such as well-cemented sandstones, limestones, and igneous intrusive rocks.

are planar or gently curved surfaces of failure produced by brittle failure of rocks. Where the rock masses on either side of a fracture have moved apart slightly, the fracture is called an . If the two rock masses have slid past each other, the fractures are . In the case of natural fractures, in the field, extension fractures are commonly called . Shear fractures are known as when the rocks on one side have been shifted significantly along the fracture surface. In this section we examine joints; later sections deal with faults.

Joints and veins in the Earth’s crust are of enormous importance to humans. Joints are planes of weakness in natural and artificial outcrops; failure along joint surfaces has led to catastrophic landslides and rock falls. In the subsurface, joints are pathways for the migration of fluids, including water, oil, and natural gas. An understanding of flow through fractures is necessary for the use of groundwater, and for the extraction of oil and natural gas. The space between the rock masses along a joint surface may become filled with minerals, deposited from groundwater, in which case the joint is known as a . Many economically important minerals are mined from veins.

Geometry of joints and veins

The orientation of a fracture, like any planar surface, can be defined by its and . At map scale the location and orientation can be represented by structure contours. The rocks immediately above a fracture constitute the and those immediately below, the (Figure 2). Such being the case, by definition the plane dips in the direction towards the hanging wall.  Clearly, these terms have no meaning if the fracture is vertical, in which case the walls are better identified as ‘north wall’ or ‘southeast wall’, etc. depending on the strike.

Figure 2. Block diagram of a dipping fracture, illustrating the hanging wall and the footwall.

If there is space between the two walls of a fracture its width is known as the fracture aperture. Some fractures show constant aperture over large distances, but in most there is some variation. Fractures do not extend forever through the Earth’s crust. The point at which the trace of a fracture ends is known as a fracture tip. In three dimensions, the tip of a fracture is a linear feature, known as a tip line.

Figure 3. Geometry of a joint. Left: in plan view. Right: in 3-D.

Joints are very common in many outcrops, but their orientations are typically not random. It is common to observe many joints in approximately the same orientation. These are known as a .

A combination of joint sets, cross-cutting each other in a regular way, is known as a .

Features of joint surfaces

Figure 4. Geometry of plume structures on a joint surface.

Joint surfaces may be perfectly planar, but in many cases they show feather-like markings known as  or plume structure.

 

 

Typical plume structure consists of two parts. A pattern of feather-like striations, called radiates from a central point or line, while a bolder series of concentric bands, known as is approximately perpendicular to the hackles. Towards the tip of a joint, in what is known as the fringe area, it is common to see that the surface splits into a number of smaller surfaces arranged like the blades of a fan. Each of the blades may have its own plume structure.

Figure 5. Plumose markings including hackle marks, Blairmore Group, Bow River Valley, Alberta.
Figure 6. Plumose markings: hackles and ribs, Blairmore Group, Bow River Valley, Alberta.

Plume structure can be simulated in laboratory experiments. It records the process of fracture propagation, and the dynamics of the fracture tip.

Fracture propagation is the process whereby a fracture grows within a rock, and involves the outward migration of the fracture tip. Fracture propagation may be very fast: speeds up to those typical of seismic waves (a few kilometres per second) are possible.

Because the fracture tip at any moment is the boundary between unfractured rock and rock that has been weakened by fracturing, the orientations of the stress axes are modified in the vicinity of the tip. This in turn causes the variations in fracture orientation as the fracture propagates.

The kinematics of fracture propagation may be interpreted from plume structure. Hackles diverge in the direction of propagation, while concentric ribs mark successive pauses in the position of the fracture tip.

Features of vein fills

Two contrasting types of vein fills are common.

When a joint aperture has been held open by fluid pressure during hydrothermal activity, mineral crystals may be deposited on the walls, producing a vein. Crystals that grow into the fluid-filled spaces may be euhedral (showing clear crystal surfaces) but more typically the crystals run into one another producing that are also approximately planar. There is often a tendency for crystals to get larger towards the centre of a vein.

Figure 7. Vein with non-fibrous fill and remaining unfilled void space
Figure 8. Fibre-filled veins, illustrating features of syntaxial (left) and antitaxial (right) fibre fills.
Figure 9. Fibrous quartz filling a vein. The quartz fibres grew simultaneously with vein opening, and track the relative movement of the two walls. SE Ireland.

Alternatively, when a joint has been filled by minerals deposited as the fracture opened, may be produced. Typically a very narrow crack is filled by a first layer of crystals. This is followed by renewed opening, and deposition of more mineral material and so on. This process is known as crack-seal. The result is that a series of fibres crosses the vein. The fibres track the kinematic process of vein opening.

There are actually two different styles of fibre fills that can be found in veins. If the repeated cracking occurs in the centre of the vein, the fibres are typically in crystallographic continuity with the grains in the wall. Such veins are described as . Sometimes in cross-sections of syntaxial veins it’s possible to pick out the line of separation running along the centre. In contrast, if the repeated cracking occurs at the edge, then the vein is . In antitaxial veins there is typically no single centre line, but there may be multiple strands of wall-rock inclusions parallel to the walls of the vein, marking successive cracks.

Dynamic interpretation of fractures

Figure 10. Typical relationships between stress and fractures in block diagram and stereographic projection. These examples show fractures formed in the gravity regime (σ1 vertical), but the same angular relationships between the structures and stresses apply in other stress regimes. Top: extension fractures. Middle: conjugate shear fractures. Bottom: distribution of poles to extension and shear fractures on a stereographic projection.

There are some characteristic relationships between the orientation of fractures and the stress axes that give us useful information.

Extension fractures

Where rocks have been fractured at low mean stress (or ‘confining pressure’), it is likely to be broken along fractures perpendicular to σ3. If we examine the fractures, we find that they are : the two sides of the fracture have been pulled apart.

Conjugate shear fractures

At higher pressure, failure typically occurs along two families of planes, breaking rocks into wedge-shaped fragments. The angle between the two families is about 60°. The planes intersect each other in a line parallel to σ2. The maximum principal stress σ1 bisects the acute angle between the planes, and the minimum stress σ3 bisects the obtuse angle.

If we examine the fractures, we find that they are : the two sides of the fracture have slid past each other. The sense of shear is such that the acute angles at the edges of the fragments have been pushed inward. Movement is inwards along the σ1 direction and outward along σ3.

Common types of joint and vein systems

Figure 11. Columnar joints in sill. Salisbury Crags, Edinburgh, Scotland.

Primary joints in igneous rocks

Primary joints in igneous rocks are typically the result of contraction during cooling from magmatic temperatures. Most such joints form perpendicular to the surfaces of lava flows or to the contacts of intrusions. In tabular igneous bodies such as lava flows, sills, and dykes, the joints may have a columnar appearance and may show a strikingly regular pattern of polygons, mainly hexagons, in cross-section. It has been shown that for a given amount of contraction, a hexagonal pattern minimizes the work in forming new surfaces.

Joints in the host rock of intrusions

Joints are also common outside igneous intrusions. Two types are common: joints and joints. These joints are typically the result of extension either during the forceful intrusion of magma or during the subsequent differential cooling of the intrusion.

Figure 12. Radial and concentric joints around an intrusion.

Joints related to erosion and exhumation

It’s common for joint sets to form parallel to the topographic surface in areas that have undergone rapid erosion. These are known as sheeting or sheet joints, and the process is known as exfoliation. These joints are most conspicuous in intrusive igneous rocks like granite, which are otherwise typically isotropic. In extreme cases, granite with many sheet joints may be confused with a bedded sedimentary rock when viewed from a distance!

There are several possible causes of exfoliation.

  • In regions where the crust is in horizontal compression, σ3 is vertical. As overburden is removed by erosion, and the mean stress is reduced, σ3 becomes negative (tensile). The rock tends to expand upward and does so by forming roughly horizontal joints.
  • Secondly, residual stresses may be locked into rocks at the time of formation, particularly during the cooling of intrusions. If tensile stresses are perpendicular to contacts, these may lead to the formation of joints parallel to the contacts when erosion reduces the overall pressure.
  • Finally, weathering of igneous rocks leads to significant volume changes, as water is absorbed, and clay minerals form. These volume changes may set up stresses that encourage exfoliation behaviour.

Joints related to faults and shear zones

Figure 13. En-echelon joints and veins related to sinistral shear (top) and dextral shear (bottom).

Faults and shear zones are typically associated with high densities of joints and other fractures. One common geometry reflects the distribution of stress in a developing fault zone. Repeated joints or veins are oriented at an angle of about 45° to a developing fault in a configuration called en echelon (derived from a diagonal military formation). The joints or veins are formed by extension, and are oriented perpendicular to σ3.

Figure 14. En-echelon quartz veins in schist. Amaliapolis, Greece.

Joints related to folds

Figure 15. Common configurations of joints and veins related to folds.

In folded layered rocks, particularly sedimentary rocks that were close to the brittle-ductile transition when they were folded, complex joint patterns may be present. Most common are that are parallel to fold hinges and roughly perpendicular to layers. They are most common on the outside parts of buckled competent layers, and probably form in response to stretching of the outer arc of the layer during folding.

A second set of joints is sometimes found parallel to the folded layers, occurring particularly at the inner surfaces of buckled competent layers. These probably result where the incompetent layers are insufficiently ductile to fill the space in the hinges between competent layers. Veins in this position have been particularly prominent sources of gold in several goldfields, notably in Australia and in Nova Scotia, where they are known as saddle reef veins.

A third type of fold-related veins occurs parallel to the of folds. Logically these are called profile veins but they are also often called ac-veins, from an older terminology for folds, involving three “axes” a, b and c. (What we now call the fold axis was axis b in this system). Profile joints and veins indicate that a component of extension occurred along the length of the fold hinges.

 

 

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Geological Structures: a Practical Introduction by John Waldron and Morgan Snyder is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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