1. Joints and Veins 2 Lecture 15 – Spring 2016
Structural Geology
Photo: Veining in the Painted Wall section, Black Canyon of the Gunnison N.P. , August, 1984 © D.L. Warburton
Joint studies in the field
Before attempting to study joints in the field, we need to know what questions we should be asking.
Systematic or nonsystematic? Is there a regular outcrop pattern of joints that allows us to define sets of planar joints or cross joints? Or is the pattern irregular, randomly oriented joints with short joint traces? Systematic joint sets are likely to reflect regional tectonic stress trajectories. If we see only random joints, we need to investigate and find out if that is localized or pervasive.
If joint sets are present, what are their orientations? If more than one set is present, is there a constant angle between them, so that a joint system may be defined? This sort of information is quite important for engineering geology studies. Some dams are designed to transfer the pressure of water to the canyon wall rocks. If joints with the wrong orientation are present, failure may result. Joints parallel to highways greatly increase the risk of rock falls on the road.
If different joint sets are present, what is the nature of the cross-cutting relationships between them? Are J or sigmoidal intersections present, or do the joints cross without apparent interaction?
What is the surface morphology of the joints? Are plumose patterns visible? If so, are the plumes straight or wavy? Are arrest lines visible? Plumose structures are regarded as proof of Mode I propagation. The geometry of the plume provides clues to the way the joint propagated (single pulse vs. multiple pulses). If field evidence varies from place to place, it probably tells us that the joint initiated at different times along its length. If growth starts and stops, it may indict fluid pressure was varying. Other surface features may indicate post-joint formation strain. Possible patterns include stylolitic pits, which indicate compression and pressure solution on the joint, and slip lineations, which may indicate the joint was reactivated as a fault.
What are the joint lengths? Long joints are the ones most of interest to structural geologists. Large joints can contribute to the collapse of rock. They also allow fluids to flow in a given direction much more rapidly.
What are the joint spacing and joint density in a given outcrop? Joint spacing tells us about the properties of the rock the joints are found in. Joint density can refer to two or three dimensions. In two dimensions, it refers to the trace length of joints per unit area of joint surface. In three dimensions, it means the area covered by joints in a unit volume of rock. Joint density helps define both fracture-related porosity and permeability of rock.
How does lithology affect joint distribution? For sedimentary rocks, are joints limited to a single bed, many beds, or beyond the outcrop? How does the bed composition affect joint spacing? In igneous rocks and the surrounding country rock, does proximity to the contact control joint spacing or style? This information can tell us about Young’s modulus, variations in fracture permeability, particularly as a function of position in the stratigraphic sequence, and can help determine the causes of joint formation.
Are joints isolated, or connected? This knowledge is critical in assessing permeability.
How are joints related to other structures, and to rock fabric? Is there a relationship to tectonic foliation? To fold elements? Do joints cut across folds, or were they reoriented by the folding? Do contemporary stresses fit the pattern of joints observed? Is joint style or spacing related to nearby faults? This sort of information is needed to assess the relationship of jointing and tectonic conditions.
Joints and Veins 2
Lecture 15 – Spring 2016

2. Data Collection in the Field
Data collection can be done as an inventory, collecting data on all joints within an area or crossing a line, or by selectively surveying an area, to visually assess what the major joints or joint sets are
Representative joints are measured
When working in the field, it is necessary to decide how to collect data.

3. Inventory Methods
Inventory methods can be quite time-consuming, and may be cluttered by data on nonsystematic joints
Inventory methods can provide data to assess joint density, joint orientation, joint spacing, and can be used with both systematic and nonsystematic joints
Figure 7.12 in text

4. Selective Method
The selective method works well when joints have large spacings, beyond the scope of a normal area survey, and does not clutter data collection with reams of information on nonsystematic joints which may not be useful
But it is a selective, and therefore subjective, process
People sometimes see what they want to see, data that match preconceived notions of what happened in a given region
Inventory methods can be quite time-consuming, and may be cluttered by data on nonsystematic joints. Inventory methods can provide data to assess joint density, joint orientation, joint spacing, and can be used with both systematic and nonsystematic joints.

5. Representation of Joint Data
Joint data can be represented by plotting the strike and dip of joints on geologic maps, or on topographic maps used as base maps

6. Joint Trajectories Figure 7.13a in text
Joint trajectories can be plotted to assess joint attitude in a given region
Trajectories represent the trend of joints; not necessarily the trace of individual joints

7. Statistical Maps
Statistical maps that show the orientation of many joints within a region can help show dominant joint orientations within the region
If joints are not vertical, they are best represented on equal area nets

8. Vertical Joints
If joints are vertical, so that only two-dimensions need be considered, a histogram plot of the joint density versus strike can be used (Figure 7.13b in text)

9. Rose Diagram The Rose diagram is a type of polar histogram
Bearings are shown directly on the diagram
Figure 7.13c in text
The circle is divided into pie slices, and the number of joints within each pie slice is determined. The radius of the plot is proportional to the number of joints, or the percentage of total joints, falling within the slice. Rose diagrams give an intuitive feel for the distribution of joint attitudes.
Tectonic Interpretation of Joints
Understanding the formation of joints and fractures in terms of stress gives geologists a basis for interpreting the field evidence available in the outcrops. Before attempting to interpret the rock record, we need to remember:
Rocks in different parts of the same outcrop may have joints that developed at different times, under different conditions. Once formed, it is not easy to “erase” a joint. Only strong shearing or metamorphism will eliminate the joint.
There can be strong local variations in stress fields, usually as the result of inhomogeneities in the rock.
We that in mind, we can examine the field evidence to see what it tells us about the origin of joints.

10. Unroofing The process is known as unroofing Figure 7.14 in text
As rock is buried, it becomes warmer, and experiences greater pressure. If rock at the surface experiences regional uplift, it will be eroded. Erosion removes weight and leads to isostatic rebound. The process is known as unroofing. (Fig.7.14) Unroofing and isostatic uplift produce three effects:
The process is known as unroofing
Figure 7.14 in text

11. Shrinkage As rock cools. it shrinks
Rock can contract in the vertical direction without causing secondary effects, since the surface of the earth is a free surface, and cannot transmit shear stress
Shrinkage in the horizontal directions causes horizontal tensile stresses to develop
Expansion in the vertical direction also leads to contraction in the horizontal directions, due to the Poisson effect
Membrane Effect - Each layer acts like a membrane, and uplift stretches the membrane and increases its radius of curvature, adding to the horizontal tensile stress

12. Tensile Stress
If the sum of the tensile stresses overcomes lithostatic burial pressure and the tensile strength of the rock, it causes the rock to crack and form vertical joints
Vertical joints indicate that σ3 is horizontal
Since the earth’s surface is a free surface, it must be a principal plane of stress
The other two principal planes must be vertical
This type of joint formation is likely to be important in continental interiors, especially in sedimentary basins.

13. Orogen
Belt of deformed rocks often accompanied by metamorphic and plutonic rocks
An extensive belt of rocks deformed by orogeny, associated in places with plutonic and metamorphic rocks

14. Epeirogenic Movements
Gentle vertical land movements of regional extent are called epeirogenic movements
Literally, making of continents
To be exact, it is an action of uplift or subsidence of large area of continent or ocean basins
Uplift creates domes
Subsidence forms basins
Sea level can change because of the movement of sea-bed
Orogens may be uplifted after convergent tectonism has ceased.
Formation of Sheeting Joints
In the upper few hundred meters of the earth’s surface, uplift and erosion can lead to the formation of sheeting joints. Sheeting joints are parallel to the topographic surface. They develop when rocks are crystalline, and lack bedding or schistoscity. Granite is the most common example.
Near the surface, there is a compressive load due to the overlying rock, but it is not strong. Yet sheeting joints are tensile fractures. So we seem to have a paradox.

15. Sheeting Joint Diagram
They appear to form where horizontal stresses are considerably higher than the vertical load pressure
Joint spacing less near surface
Figure 7.15a in text
With this type of stress field, joints propagate horizontally, and may be the equivalent of longitudinal cracks in vertical compression experiments in the laboratory. What causes the strong horizontal stress? Tectonic convergence is certainly one possibility, but doesn’t always seem to be present. Another possibility is residual stress, which may form in a number of ways. For example, unroofing of a pluton leads to residual stress because the coefficient of thermal expansion of the pluton and the wall rock differ. The pluton starts out hotter than the wall rock, and so cools more.

16. Development of Elastic Strain
Since the wall rock and pluton behave differently, elastic strains develop
The pluton has welded itself to surrounding rock, so the differential strain creates elastic stress in the pluton
Figure 7.15b in text

17. Unroofing
Commonly, the pluton shrinks more than the surrounding rock, and tensile stresses develop perpendicular to the wall
The resulting residual stress may exceed the weak load pressure, allowing tension joints to form
Figure 7.15c in text
The parallelism of the joints to the ground surface may be explained in two ways:
Topographic control on the joint surfaces, possibly because the vertical load is perpendicular to the ground surface.
Joint control of the topographic surface, due to rocks spalling off the surface at the joint surface.
The matter is still subject to debate.

18. Exfoliation in Granite
Image:
Tioga Pass Road, Yosemite National Park
Photography by Dr. Sharon Johnson, University of California, Berkeley

19. Granite Domes Formed by Exfoliation
Image:
Tanaya Lake, Yosemite National Park
Photography by Dr. Sharon Johnson, University of California, Berkeley

20. Natural Hydraulic Fracturing
We have seen that it is possible to inject fluids in the ground and cause fracturing, in a process called hydraulic fracturing
Sometimes, the process occurs naturally
The fluid can be water, petroleum, or natural gas
If the cracks are near the surface, they may intercept the surface, and allow fluid to leak out

21. Santa Barbara Channel
Natural oil seeps in the Santa Barbara channel off the coast of California lead to the discovery of oil reserves there
Hydraulic fracturing due to petroleum pressure may have played a role in developing the seepage

22. Opening Stress
A rock like a cemented sandstone may have both its pores and cracks filled with fluid
The fluid pressure within the crack creates an opening stress (σo)
Figure 7.16 in text

23. Inward Pressure
Fluid pressure also pushes inward, creating two types of closing stress, σcg and σcp
The “cg” subscript stands for a closing stress applied through the cracks in contact with the grain
The “cp” stress means a closing stress through the crack’s contact with a pore
Normally, the opening and closing stresses are equal and opposite. If the fluid pressure increases, the opening stress increases proportionally, as does σcp.
The σcg stress does not increase as fast, however, since the grains take some of the stress in elastic compression. Thus, a differential stress is created. This may cause the outward fluid pressure to exceed σ3, and create a tensional force. If the concentration of this force at the tip is sufficient, the crack propagates. Opening of the crack enlarges the pore, deceasing fluid pressure. The crack stops propagating. Cracks undergoing natural hydraulic fracturing might be expected to show many arrest lines, and this is observed.

24. Joints Related to Tectonic Deformation
Convergent or collisional orogenic events produce compressive stresses that affect rocks over broad regions
In the foreland region of the orogen, joints may form for several reasons
The maximum horizontal stress is approximately perpendicular to the trend of the orogen

25. Natural Hydrofracturing
Natural hydrofracturing produces joints that are parallel to the σ1 direction of tectonic features, such as folds
These joints may contain mineral infilling of a type consistent with depths and pressures of several kilometers below the surface, which indicates they are not formed by near-surface tension cracking
They are thought to result from an increase in fluid pressure due to overburden pressure from thrust sheets, or from deposition of material eroded from the continental interior
Since they form along with the orogenic event, they are syntectonic. Stress fields change during the course of orogenesis, and later formed joints may not parallel the early formed joints. This pattern is commonly observed in the foreland regions of orogens.
Joints are also commonly associated with faulting, and may fall into one of several categories. Country rock near the fault is affected by the same stress responsible for fault formation and movement.

26. Joints Not Parallel to Fault
Since faults are usually inclined to the remote σ1 direction, the joints formed in the stress field that caused the fault to move will not be parallel the fault
Figure 7.17a in text

27. Thrust Fault Joints
Movement along a thrust fault, particularly if the fault surface is not planar, may cause warping, with associated local tension and jointing
Note that the arrows show incorrect movement
Figure 7.17b in text

28. Pinnate Joints
Wall rock immediately adjacent to the fault may be affected by tensile stresses, creating short joints at angles between 30-45º to the fault
These are called pinnate joints
Figure 7.17c in text
Once the stress acting on a region is released, rocks relax elastically, and this may cause tensile stresses in parts of the rock. Release joints may form as a result.

29. Tension from Folding
Folding may produce local tensile stresses as layers bend
If metamorphic conditions are not produced, tension cracks may develop

30. Joint Convergence
A similar phenomenon can occur during crustal warping, with joints reflecting tensile stresses developed as the radius of curvature changes.
Orthogonal Joint Systems
In the foreland region of orogenic structures and in continental interiors, it is common to find two mutually perpendicular systemic joint sets. Two patterns are possible.
In outer-arc environments, this leads to joints that are parallel to the fold hinge, and which may converge at depth to the core of the fold
Figure 7.18 in text

31. Ladder Pattern
Ladder pattern has long joints with much shorter perpendicular joints, terminating at the long joints
Figure 7.19a in text

32. Grid Pattern
Alternatively, we can see joints which appear to be mutually cross-cutting defining a grid pattern
Figure 7.19b in text
How do we get tension in two perpendicular directions at the same time?
Recent research has suggested several answers.

33. Orogenic Foreland Region
Joint sets are usually a strike-parallel set with a set of orthogonal cross-joints
Cross-joints are parallel to the regional maximum horizontal stress trajectory
They are probably formed as syntectonic natural hydrofractures
The strike-parallel joints could be release joints formed by relaxing of stress, or out-arc extension of folds

34. Tensile Stress Regions
Joints may develop perpendicular to the regional tensile stress
If this stress relaxes, elastic rebound occurs, and slight expansion may occur in the direction perpendicular to the original stretching
This causes a new set of joints, perpendicular to the original joints, to form

35. Uplift During uplift, erosion may unload pressure from rock below
A joint set perpendicular to the regional σ3 develops
Continued uplift may open these joints further
Stress parallel to the existing joints cannot be relieved this way
It accumulates and forms a new set of joints orthogonal to the first

36. Alternating Stress
Grid patterns may be caused by two joint sets initiating at the same time, or alternative cracking episodes on each set
If both sets form in the principal plane perpendicular to σ3, it is clear that the stress field is changing with time
One possibility is that σ2 and σ3 are switching back and forth
This works if both stresses are similar in magnitude.

37. Conjugate Joint Systems
Some orogenic forelands display conjugate joint systems with the bisector of the dihedral angle being perpendicular to the fold axis.
Such folds often display plumose patterns, indicating they formed as Mode I fractures
Traditionally, it was thought that conjugate fractures were either shear fractures, formed at about 30̊ to σ1, or “transitional-tensile” fractures, formed at angles less than 30̊ to σ1
Shear fractures cannot propagate in their own plane
Transitional-tensile fractures may not exist
The result has been controversey

38. Cross-Strike Joints
Current thinking is that both sets of joints are cross-strike joints, initially formed perpendicular to σ3
The two sets would form at different times, with different stress fields

39. Devonian Joints in New York
In south-central New York, there are two joints sets, separated by an angle less than 60̊
The rocks are Devonian, and slightly folded
It is thought the joint sets formed during the late Paleozoic Alleghanian orogeny
Two different episodes orogenic activity took place
The maximum horizontal stress during the first and second episodes were not parallel
Slip lineations on such faults can then be interpreted as mesoscopic faults reactivated subsequent to their formation, rather than as shear fractures

40. Joints in Igneous Rocks
Igneous rocks can form joints, either within the igneous rock itself, or in the surrounding country rock
A plutonic intrusion can stretch the rock around it, causing tension fractures to form
The shape of the intrusion determines the joint pattern
Circular plutons often produce radial fracturing
The fractures may bend and become parallel to the regional maximum horizontal stress at a distance from the pluton.
Magmas or fluids from the pluton may intrude the joint, during or subsequent to the joint formation, and igneous dikes are the result
Thermal cracking of the country rock may form joints immediately around the pluton
This is more likely in shallow plutons, where the magma-country rock temperature difference is greater
Fluids from the pluton can also cause local hydrofracturing.

41. Hypabyssal Intrusions & Lava Flows
Hypabyssal intrusions or lava flows may also be subject to rapid cooling and contraction
Cooling takes the rock below the brittle/plastic transition, and elastic strain develops
When tensile stress exceeds the rock strength, it breaks
Shrinkage is equal in all directions, so several sets of joints form
Usually, there are three sets at about 120̊ forming to create a hexagonal pattern

42. Paleostress Indicators
Since joints propagate normal to σ3, their planes define the trajectories of σ3 within their region
If the joint is vertical, the strike of the joint defines the trajectory of maximum horizontal stress
However, we don’t know if this is σ1 or σ2

43. Large Joint
Limits of Growth of a Joint
The stress intensity at the tip of the crack depends on the length of the crack, and grows quickly as cracks become more elliptical. In large bodies of homogeneous rock, such as massive sandstone beds in Utah, joint surfaces may become huge.
Why do joints stop growing? Since we do not see joints growing without end, we know they stop. What can cause a joint to stop growing? A number of factors can stop a joint.
The joint may encounter a free surface. Since joints cannot propagate across free surfaces, they stop growing.
Source:
Walls of Entrada Sandstone border Park Avenue in the Courthouse Towers area, Arches NP
Joints in the thin bedded shale beneath the sandstone are much more closely spaced
USGS photo

44. Stress Shadows
Two joints may grow toward each other If they are not coplanar, they may enter each others stress shadow, and terminate
Figure 7.21a in text

45. Stress Shadow Overlap
Map view showing that joints cannot pass each other because they are too close together – their stress shadows overlap
Figure 7.21c in text

46. Joints Combine
The joint tips may interact, changing the local stress field, and the joints may grow together
Figure 7.21b in text

47. Other Joint Terminations
Joint growth can cause local drops in fluid pressure
The joint stops growing until fluid pressure builds up again
Multiple arrest lines show this type of behavior
The joint may grow into a different type of rock, where plastic yielding is easier
Plastic flow then terminates the joint
Alternatively, if the joint grows into a stronger rock, it may not be able to crack it

48. Veins and Vein Arrays
Veins are fractures that has been filled by precipitation of material from solution
By far the most common vein forming materials are quartz and calcite
Other substances found in veins include ore minerals, mostly sulfides, zeolites, and chlorite
The fractures may be either joints or shear ruptures
Veins range in size from the width of a hair to several meters, and can be many meters long
Veins can also form in groups, called vein arrays

49. Planar Systematic Array
Vein arrays can occur in a variety of ways
A planar systematic array has a group of veins, mutually parallel, with a nearly constant spacing
These form by mineralization during or subsequent to the formation of a systematic joint set
Figure 7.22a in text

50. Stockwork Vein Arrays
Stockwork vein arrays are the result of shattering of rock, either by extremely high fluid pressures, or locally pervasive fracturing associated with tectonic faulting and folding
Figure 7.22b in text

51. En Echelon Veins
En echelon veins may form by the infilling of en echelon joints in the twist hackle fringe of a larger joint
They can also form by shear across a fault zone
Figure 7.23a in text

52. En Echelon Vein Formation
The en echelon veins are initiated parallel to σ1, often at an angle of about 45º to the shear borders
As the shear develops, the fractures open
This allows them to fill with vein material
Once filled, they are material objects
Figure 7.23b in text

53. En Echelon Vein Formation
Further shearing will rotate them and the acute angle increases
If further vein growth occurs, it will be at 45º to the shear borders, and the veins will become sigmoidal
Figure 7.23c in text

54. Blocky Crystals Vein material may be blocky or fibrous
Blocky crystals form in open cavities
Open cavities form near the surface, where rock strength allows cavities to stay open, or fluid pressure if great enough to hold the fracture open
Figure 7.24a in text
Blocky crystals also form by recrystallization of earlier fibrous minerals
If few nucleation centers for crystal growth exist, it also favors blocky crystals.

55. Fibrous Crystals
The formation of fibrous crystals is somewhat more controversial
One mechanism is the crack-seal mechanism
A rock must contain pore fluid with dissolved minerals
If fluid pressure becomes great enough, the rock cracks a few microns
Fluid rushes in, locally lowering the pressure

56. Formation of Fibrous Crystals
Lower pressure can lower the solubility of the mineral, and initiate precipitation, which seals the crack
The process then repeats, over and over again
Figure 7.24b in text
Each time the crack widens a small amount
The existing crystals act as nuclei, and the crystals elongate
Another mechanism is the migration of ions by diffusion along fluid films on grain boundaries
At the tips of existing fibers, precipitation occurs, and gradually pushes the vein walls apart
Diffusion is a very slow process, so this would require a great deal of time.
Fibrous veins record information about the progressive strain history of a rock. Two cases exist, syntaxial and antitaxial.

57. Syntaxial Growth Cracking occurs at center of vein
Composition of fibers and wall rocks is identical
Figure 7.26b in text
Syntaxial growth requires that the vein material and the wall rock be the same mineral.
Vein fibers nucleate on the surface of the wall rock, growing inward to meet at the center of the fracture.
Successive increments of cracking are all at the median of the vein.
Each growth line shows a trail of fluid inclusions

58. Antitaxial Growth Vein and wall rock composition are different
Cracking occurs along vein margins
Figure 7.26a in text
Incremental cracks develop at the boundaries between the fibers and the wall rock, not at the median line.
Dislodged flakes of wall rock may indicate incremental growth boundaries

59. Fibrous Antitaxial Vein
Each time the crack widens a small amount
The existing crystals act as nuclei, and the crystals elongate, with veins growing outward from the center
Fibrous antitaxial vein in limestone – field of view about 1 mm

60. Antiaxial Calcite Veins
Tapley Hill Formation, Opaminda Creek, Arkaroola, South Australia
Width of view 13 mm, crossed polars
Tips of two parallel en échelon antitaxial fibrous calcite veins
Mean fiber width increases slightly from the median line outwards, which indicates that growth was outwards (antitaxial)
Fiber shape is symmetric around the median line, except near the tips
Growth and propagation of the veins caused bending of the shale "bridge" in between
Source:

61. Stretch Perpendicular to Vein
The direction of the fibrous elongation often marks the direction of stretching
Figure shows stretching perpendicular to the vein, with the long axis of the fibers parallel to stretching, and perpendicular to the wall
Figure 7.27a in text

62. Stretch Oblique to Vein
Figure shows the fibers at an oblique angle to the wall
Sigmoidal shaped fibers indicate that the extension direction rotated relative to vein wall orientation
Figure 7.27b in text

63. Change in Stretching Direction
Figure shows that the order of movement depends on whether the growth is syntaxial or antitaxial
Figure 7.27 cd in text

64. Linaments
Lineaments are linear features recognized on aerial photos, satellite imagery, or topographic maps
They exist at regional scale, but not as mesoscopic or microscopic features
They may not be initially recognizable from the ground

65. Lineament Photo
Structural lineaments are defined by structurally controlled alignment of features like ridges, depressions, or escarpments
Duncan lake region, Northwest Territories, Canada
Figure 7.28 in text
Sometimes lineaments cause changes in vegetation, and these changes often have structural control (water flow along cracks, for example)
Lineaments result from joint arrays, faults, folds, dikes, or contacts
Others do not appear associated with obvious structures
Lineaments “seen” in photos or imagery have sometimes been the result of reflections or of human origin, so it is important that lineaments be confirmed by ground observation (“ground truth”).
In other cases, analytical techniques involving computer enhancements may reveal lineaments.

66. Brockton-Froid Lineament, Montana
The Brockton-Froid lineament using analytical hillshading for visualization
The lineament is illuminated by a light source located at an azimuth of N35W and an angle from horizontal of 45º
Source:
“Using GIS for Visualizing Earthquake Epicenters, Hypocenters, Faults and Lineaments in Montana”
By Patrick J. Kennelly and Michael C. Stickney
“All magnitude 5.5 or greater earthquakes in Montana this century have occurred in the Intermountain Seismic Belt, except one - the May 16, 1909 earthquake in northeast Montana. Because of its early date, no local seismographs existed to record it; however, its widespread area of perceptibility and strong shaking near the epicenter suggest a magnitude of at least 5.5. Identifying fault scarps in the less seismically active eastern half of Montana is challenging. One fault candidate is the Brockton-Froid lineament in northeastern Montana. The lineament has been interpreted by field mapping efforts of Colton (1963a) of the U.S. Geological Survey as a northeast-southwest (N55E) trending fault zone more than 50 km (30 mi) in length. The entire zone is straight, with the northeastern-most portion consisting of a single lineament. In the central portion, the zone consists of two parallel traces defining a small graben-like structure. At the southwestern end, the zone splays into several less well defined lineaments. “
The lineament trends N55E, most visible in lower left corner toward upper right, where it is faint