1. Lecture 7 Joints
Form perpendicular to weakest stress, often tensile s3
Mostly Chapter 7

2. Joints and veins A fault offsets layers of sediment SS and clay layers
Joint: a fracture without measurable shear displacement (cracks or tensile fractures)
Fault: a fracture with measurable displacement
Vein: a fracture filled with minerals precipitated from solution
A fault offsets layers of sediment
SS and clay layers
Calcite veins fill joints

3. Surface morphology Plumose structure: wavy structure on joint
Spreads outward from joint origin

4. MODES
Divergent Shear (Transform) Dip-slip & rotation
See figure 6.11

5. Surface morphology Why does the plumose structure form?
Mode 1 loading: should yield smooth fractures perpendicular to s3.
BUT real joints are not perfectly smooth.
Rocks are not homogeneous.
these imperfections distort the local stress field
The stress field at the tip of the propagating crack changes
As the crack grows, the stress intensity at the tip of the crack also grows.
Also the velocity of the crack tip propagation is proportional to the stress intensity
Since the crack is really small at the origin, the stress magnitude may exceed a critical value for cracking.
Flaws at the origin

6. Joint spacing and bed thickness: Closely spaced in thin bedded rx
Joint Spacing in sedimentary rocks
Joints are mostly evenly spaced
Widely or closely spaced, partially depending on length of time tensile stress applied
Model of the sequence of development of joints.
Time 1, time before joint forms
Time 7, present day
Joints form in random sequence, but with regular spacing
Minimum distance (dm) joint relieves tensile stress for a critical distance – either side has a decrease in tensile stress.
Joint spacing and bed thickness:
Closely spaced in thin bedded rx
Wide spaced in thick bedded rx

7. Stress shadows Rigid grid
Cutting one string causes only a few remaining strings to relax.
Model of the sequence of development of joints.
Time 1, time before joint forms
Time 7, present day
Joints form in random sequence, but with regular spacing
Minimum distance (dm) joint relieves tensile stress for a critical distance – either side has a decrease in tensile stress.
Greater length of joint has a wider stress shadow
Cutting many strings in a row causes a wide band of strings to relax – larger area affected

8. Joint spacing and Lithology:
Stiffness = Elastic value E, Youngs’ modulus
Hookes law
where e is the elongation strain
Stiff dolomite fractures a few times before the sandstone fractured the first time.
Model of the sequence of development of joints.
Time 1, time before joint forms
Time 7, present day
Joints form in random sequence, but with regular spacing
Minimum distance (dm) joint relieves tensile stress for a critical distance – either side has a decrease in tensile stress.
Dolomite stiffness >> Sandstone
Stretch a block
Stress in each bed controlled by Hookes law (magnitude of stress depends on E)
Beds with large E (stiffness) develop a greater stress and fracture first.

9. Rocks with low tensile strength develop more closely space joints
AND
More tensile strain (stretching) yields more joints
Model of the sequence of development of joints.
Time 1, time before joint forms
Time 7, present day
Joints form in random sequence, but with regular spacing
Minimum distance (dm) joint relieves tensile stress for a critical distance – either side has a decrease in tensile stress.

10. Joint arrays Systematic vs nonsystematic joints Systematic joints:
Planar joints
Joints are parallel or subparallel.
Same average spacing
Solids are atoms/ions are connected by chemical bonds (bonds = tiny springs)
Nonsystematic joints:
Irregular spatial distribution
Not parallel to one another.
Different average spacing

11. Joints in the field Why study joints Tectonics (paleostress)
Geomorphology (drainage patterns)
Questions to answer in the field
Systematic or nonsystematic joints
Orientation of joints strike and dip
Cross-cutting relations Steno Relative Dating
Surface morphology planar plumose
Dimensions of joints
Joints and lithology which rocks thicknesses have closer
Relations of joints and faults and folds. DEMO FOAM Pyroxene
Model of the sequence of development of joints.
Time 1, time before joint forms
Time 7, present day
Joints form in random sequence, but with regular spacing
Minimum distance (dm) joint relieves tensile stress for a critical distance – either side has a decrease in tensile stress.
DEMO

12. Joints in the field Methods Inventory Sample fracture density
Sample joint orientation (strike and dip of joints)
Relate to tectonics
Joint trajectory map
Frequency diagram
Rose diagram
Each portrays different data sets

13. Categories of Brittle Deformation
Frictional Sliding on preexisting fractures
Cataclastic flow due grain scale fracturing
Shear rupture at acute angle to max. principle stress
Tensile cracking perpendicular to dir of min. stess

14. Cataclastic rocks
A Cataclastic rock is a type of metamorphic rock that has been wholly or partly formed by progressive fracturing. Rock fragments are reduced in size by crushing and grinding of existing rock, a process known as cataclasis. The process and is mainly found associated with fault zones.
Cataclasite is a type of cataclastic rock that is formed during faulting, consisting of angular clasts in a finer-grained matrix.
Cataclasite seen in thin section.
Scale is 200 mm

15. Stress Concentration and Griffith Cracks
A stress concentration is a location in an object where stress is concentrated. An object is strongest when force is evenly distributed over its area, so a reduction in area, e.g. caused by a crack, results in a localized increase in stress.
Griffith cracks are preexisting microfractures and flaws in the rock, weakening it. Reason rock failure less than theory

16. Origin and Interpretation of joints
Sheeting joints – uplift and exhumation
Rocks cool and contract with decrease of burial depth
Rocks shrink in the vertical direction (free surface – Earth Surface)
Since the rock can not shrink elastically in the horizontal-direction (its still confined)
Tensional stresses develop
As overburden decreases, the rock expands vertically – the Poisson effect.
It contracts in the horizontal direction
So the layer of rock stretches like a membrane
If the horizontal tensional stresses overcomes compressive stress, it will crack and form joints.
Sheeting joints form in a location where s1 is horizontal while s3 is vertical near the ground surface. Joints become more closely placed near the ground surface

17. Origin and Interpretation of joints
1) Sheeting joints – uplift and exhumation
Rocks cool and contract with decrease of burial depth
Rocks shrink in the vertical direction (free surface – Earth Surface)
Since the rock can not shrink elastically in the horizontal-direction (its still confined)
Tensional stresses develop
As overburden decreases, the rock expands vertically – the Poisson effect.
It contracts in the horizontal direction
So the layer of rock stretches like a membrane
If the horizontal tensional stresses overcomes compressive stress, it will crack and form joints.
A cooling pluton contracts more than country rock. Here, st (tensile stress) is oriented perpendicular to the intrusive contact.
After exhumation, joints form parallel to intrusive contact and creates an exfoliation dome.

18. Mechanical Exfoliation in Granite Yosemite National Park
Source: Phil Degginger/Earth Scenes

19. Origin and Interpretation of joints
2) Natural hydraulic fracturing
Stresses in the Earth’s crust are mostly compressive.
How do joints form in such a tectonic environment?
Effect of pore pressure on fracture.
Increase of pore pressure in a pre-existing crack pushes outward
Increase of st (tensile stress) that allow crack tip to propagate.
a) Stresses near the crack with high fluid pressure that exceed the magnitude of s3.
As a result, tensile stress (st) occurs along crack.
b) Crack enlargement. Opening stress >> Closing stress.

20. Origin and Interpretation of joints
3) Regional divergence
High pore pressures in blocks subject to divergence, weakens confining pressure
Formation of joints in hanging-wall block with normal faults
Tensile stress s3 weakest is horizontal, joints form perpendicular to s3
a) Stresses near the crack with high fluid pressure that exceed the magnitude of s3.
As a result, tensile stress (st) occurs along crack.
b) Crack enlargement. Opening stress >> Closing stress.

22. Veins and vein arrays Terminology (see table 7.2)
Vein: A fracture filled with mineral crystals precipitated from fluids.
Quartz or calcite are common vein fill
Ore minerals occur as vein fill
Vein Array: Groups of veins.
Vein array
Stockwork array of veins (rock shattered and filled by mineral precipitation)
a) Stresses near the crack with high fluid pressure that exceed the magnitude of s3.
As a result, tensile stress (st) occurs along crack.
b) Crack enlargement. Opening stress >> Closing stress.

23. Veins and vein arrays En echelon vein array Fill en echelon joints
Develop within a fault zone
a) Stresses near the crack with high fluid pressure that exceed the magnitude of s3.
As a result, tensile stress (st) occurs along crack.
b) Crack enlargement. Opening stress >> Closing stress.

24. Veins and vein arrays En echelon vein array Fill en echelon joints
Develop within a fault zone.
en echelon vein array
Fractures initiate parallel to s1, at an angle of about 45° to the borders of the shear zone.
Fractures open as displacement across the shear zone as it develops and fill with vein material.
Once formed, the veins will rotate. If a new increment of vein growth occurs, the new veins will initiate 45° to shear surface.
b) Sigmodial en echelon veins due to rotation of older, central part of veins and growth of vein material at ~45° to the shear surface

25. Calcite-filled wing crack with tip splay

26. Veins and vein arrays Vein fill: block and fibrous veins
Blocky – vein fill equant - open fracture when mineral precipitated.
b) Fibrous – vein crystals long relative to width.
Repeated cracking and filling, then sealing of vein. If pressure is high enough, vein cracks and slight openings develop. The crack fills with fluid and minerals precipitate.
The process repeats itself, hundreds of times.

27. Lineament:
A linear feature recognized on aerial photos, topographic maps or remotely sensed images.
Defined only on a regional scale.
Aligned topography, changes in vegetation
Represent faults, joints, folds, dikes, or contacts.
Lineaments are not always confirmed with ground truth.
Repeated cracking and filling, then sealing of vein. If pressure is high enough, vein cracks and slight opening (microns) develop. The crack immediately fill with fluid.
The process repeats itself, hundreds of times.

28. Summary Common questions you should ask when mapping
Should I pay attention to joints and veins?
It depends. What is the purpose of the map?
You should pay attention to them IF the map purpose is, for example:
to locate faults
to define variations in permeability
to define joint intensity for oil and gas exploration (maybe with drill core data)
to define and explore orientation of veins for ore deposits
to predict groundwater transport
But IF the purpose is, for example,
(1) understanding stratigraphy, or
(2) the history of folding in high-grade metamorphic rocks
then joint analysis will not help.