1. Brittle Deformation

2. Fracture A planar or curviplanar discontinuity
Forms as a result of brittle rock failure
Under relatively low pressure and temperature conditions in the earth crust
Rock fractures range in size from:
Microcracks – Intragranular to intergranular (fraction of a mm)
Faults - Extend for hundreds of kilometers

3. Brittle Deformation
Permanent change in rocks by fracture or sliding on fractures
Fracture: A discontinuity across which cohesion (Co) is lost
The term fracture includes three basic types of discontinuities:
Extension fracture (type I)
Relative movement normal to fracture surface
Shear fracture (type II & III)
Relative movement parallel to fracture surface
Oblique extension (hybrid) fracture
Relative movement is oblique to the fracture surface
Vein: fracture filled by secondary minerals

4. Four categories of fracture observation
Distribution and geometry of fracture system
Surface features of fracture
Relative timing of fracture formation
Geometric relation of fracture to other structures

5. Fracture set and system
Fracture set: a group of fractures with similar orientation and arrangement
Small extension fractures are referred to as joint
Systematic joints: have roughly planar surfaces, parallel orientation and regular spacing (vs. non-systematic joints)
Fracture system: two or more sets of fracture affecting the same volume of rock

6. Sheet (exfoliation) joints
Are parallel to topography
Can form in any rock, but common in plutonic rocks that are exposed

7. Columnar Joints
Extension fractures characteristic of tabular extrusive igneous rocks
i.e., form in lava flow, sill, dike
Other types of joint:
Strike joint, dip joint, cross joint, oblique joints

8. Extension fractures associated with shear fractures
Feather (pinnate) fractures – form en-echelon to a main brittle shear fracture
Gash fracture – are simiar to feather fractures, but filled with mineral, in ductile shear fractures
Are sigmoidal (S- or Z-shaped)

10. What do we collect about fractures?
Orientation (rose diagram, stereonet)
Spacing
Length
Spatial pattern
Relation to lithology
Relation to layer (bed) thickness

11. Joints Are a type of fracture which form due to tension
They form parallel to the minimum tensile stress
Perpendicular to maximum tensile stress
Shearing is zero along joints when they form
Also called cracks or tensile fractures
Joints form under:
shallow depth, low confining pressure (Pc)
elastic regime
low temperature (T)
high pore fluid pressures (Pf)

12. Joints
Joints form perpendicular to the maximum principal tensile stress
i.e, along a principal plane of stress
Therefore joints dominantly show separation or opening of the walls of the fractures with no appreciable shear displacement parallel to the plane of the fracture
Joints form through the Mode I crack surface displacement
Joints are commonly characterized by two matching, rough, discontinuous, and curved surfaces, although they are approximated to be smooth, continuous, and planar

13. Tensile strength and jointing

14. Joint sets

15. Modes of crack surface displacement
Individual cracks, when loaded, propagate, infinitesimally, in three different modes:
Mode I – Tensile (Opening) Mode
Tensile cracks form normal to the 3 (parallel to the 12 plane)
Crack opens infinitesimally perpendicular to the crack plane
Crack grows in its own plane; no bending/changing orientation
Mode II – Sliding Mode
One block moves parallel to the crack normal to the fracture front
Mode III – Tearing Mode
One block moves parallel to the crack parallel to the fracture front

16. Modes of crack surface displacement

17. Modes of fracture

18. Mode II and III Both are shear mode Do not grow in their own plane.
As they start growing, they immediately either:
Curve
Become mode I cracks
Spawn new tensile, wing cracks
However, shear fractures and faults are not large mode II or mode III cracks

19. Joints
The planar approximation is justified given that scale of geometric irregularities (e.g. joint surface morphologies, curvature amplitude) is commonly very small compared to the size of the fracture surface
Termination of the two opposing surfaces at their distal edge or periphery (fracture front), i.e., a finite extent of the two walls
Displacement is zero at the fracture fronts
Involve small relative displacement of the originally contiguous points compared to the in-plane dimensions of the fracture walls

20. En-echelon Tension Gashes

21. Vein

22. Antiaxial vs. syntaxial vein

23. Joint Spacing & Bed Thickness

24. Fracture Terminations

25. Shear Fractures
Fractures along which there has been shearing or displacement
Shear fractures are small, with small displacement
They occur in intact rock during brittle deformation
If the amount of displacement is significant and measurable, the shear fracture is called fault

26. Faults and joints

27. Shear Fractures
Shear fractures, form at an angle to the maximum compressive stress and show offset because of the shear traction along the fractures
The hybrid shear fractures are discontinuities with a mixed mode of opening and shear and form oblique to the plane of the fracture as a result of both tensile normal stress and shear stress

28. Terminology
Fracture front: The line separating the fractured region from the un-fractured part of the rock
Fracture trace: Intersection of the fracture with any surface
Fracture tip: The termination of the fracture along the trace of the fracture

29. Joint Surface Morphology
Surface morphology of joints show evidence for initiation, propagation, and arrest
Theoretically, mode I loading in an isotropic, homogeneous material should lead to a smooth propagation with a mirror smooth surface
Joints however are not smooth, because rocks are commonly neither homogeneous nor isotropic

30. Out-of-plane Propagation
The orientation of the maximum tensile stress in front of a single crack tip may not be parallel to the normal to the parent crack
Cracks propagate so that “new’ portions of the crack remain normal to the local maximum tensile stress
This requires a crack to leave the plane of the parent crack in order to maintain its orientation relative to the local stress field
This out-of-plane propagation is so common in microscopic scale that leads to the formation of rough, non-smooth fractures surfaces

31. Crack Propagation Paths
Out of plane propagation is characterized by a combination of two end member crack propagation paths:
Twist: leads to segmentation of the crack into several smaller crack planes.
Represents a rotation of the local maximum tensile stress in the initial yz plane.
Rotation is about an axis in the crack plane || to propagation direction
Tilt: Causes the crack tip to rotate without segmentation.
Rotation is about an axis in the crack plane _|_ propagation direction

32. Mesoscopic Joint Surface
Although microscopic out-of-plane propagation is common, joints appear smooth on the mesoscopic scale.
This is due to the homogeneous nature of the remote stress field
Even when the crack leaves its plane, the general propagation path remains normal to the remote maximum tensile stress
The microscopic out-of-plane propagation leads to the development of joint surface morphology

33. Plumose Surface Morphology
Helps to interpret rupture nucleation, propagation, and arrest
Develops largely due to local twists and tilts during propagation of a fracture which would otherwise be planar.
Barbs: surface irregularities
In homogeneous rocks, barbs trace to the point of origin (an original crack)
In inhomogeneous rocks (sandstone, shale), barbs radiate from either a bedding plane or an inclusion in the bed (e.g., fossil, concretion, clast)
The point of origin may vary from bed to bed in shale or siltstone

34. Joint Initiation and Beds
If joints initiate from bedding plane, they often originate from irregularities such as ripples or sole marks
Joints in a bed often initiate from a common feature such as the upper bedding plane
Fossils and concretions often originate joints in adjacent beds

35. Rupture Propagation
The progress of rupture from the origin to the final arrest leads to the formation of some patterns that are printed on the surface of joints
Mirror zone:
Area immediately adjacent to the point of origin.
Forms under small tip stress values, not big enough to beak the material at oblique angles
Mist zone:
Forms when larger stresses break bonds at oblique angles to crack plane.
On the fine scale, this oblique cracking forms a non-smooth (misty) zone, separating the mirror from the hackle zone

36. Hackle zone
Forms when there are local components of twist during crack propagation.
Form when propagation occurs at a critical velocity when cracks branch or bifurcate
Hypothesis I: High velocities shift the maximum local tension away from the existing crack plane
Hypothesis II: High velocities form secondary cracks. The main crack then branches to follow the secondary cracks

37. Plumose Structures
Also called feathers, hackle plumes, striations, or barbs.
These record rupture motion
Consists of an axis from which barbs mark the direction of the rupture front as portions diverge away from the plume axis
Barbs become more pronounced toward the edge of the beds away from the axis
Barbs represent long, narrow planes oblique to the main fracture plane.
Barbs form similar to hackle marks due to twist

38. Plumose Structures … Plumose structures have different shapes:
Axes can be straight to curved
Barbs vary from uniform to symmetrical to asymmetrical about the axis
These variations reflect the degree to which rupture velocity was uniform during propagation

39. Arrest of Rupture
Arrest lines show up as ridges or cusped waves normal or subnormal to the direction of propagation of cracks
At arrest lines a large component of tilt is involved in the out-of-plane crack propagation
Arrest lines may be the boundaries between areas with perceptible barbs and areas with no barbs.
Rock joints may show several arrest lines in a row or a single arrest line at the end of a long fracture
Several arrest lines represent intermediate slowing or stopping points for a rupture as it moves through the rock to form a joint