Geol341
2011
Two modes of Brittle Failure
σ
3
σ
1
Outline
• Review
• Tensile Fractures
– Joints, cleat
– Systematic sets
– Bedding/thickness relationships
– Interpreting joint sets
– Origin of orthogonal tectonic joints
– Origin of exfoliation joints
– Veins and ductile shear zones
• Antifractures: Stylolites
Once a fracture exists deformation continues by frictional sliding of cataclastic flow
Failure= rupture of atomic bonds
1

σ
1
σ
3
Role of Holes
Stress Concentration around holes
C=2(b/a)+1
10:1
Stress concentration= 21X
Stress field around a hole
σ
3
σ
1
Microscopic flaws control the macro strength
Role of Fluid Pressure
Formation of Shear Fractures
σ
3
σ
1
2

Otter Creek, WV
Joints
• The most common type of tensile fracture
• Form near the Earth’s surface
• Control the bulk strength of the rock
– Coal Cleat
• Important fluid conduits
– Groundwater
– Hydrocarbons
Jointed Rock Face, NH
Jointed limestone bed at Lilstock Beach on the southern coast of the Bristol Channel, England (Pollard and
Fletcher, 2005).
Joints in every rock outcrop
Monument Valley, AZ-
Landscapes controlled by Joints
3

Arches National Park-
Landscapes controlled by Joints
Joints are systematically oriented over huge areas
Related to regional stress fields
Surface Joint Morphology
Fracture surface in Plexiglas with fracture propagation textures (Pollard and Fletcher, 2005)
4

Joint Features record fracture propagation
Fracture Spacing/Bedding
Join Spacing
•Spacing decreases with layer thickness
•More joints in stiffer layers
•Often joints are confined to specific beds
•Spacing increases with strain
Photo: J. Olson, UT Austin
Cross cutting relationships
Secon d Set
C
What are the relative ages of the joint sets?
B
A
Appalachian Joint Sets
5

Systematic Joint Sets Appalachian Joints
• Set 1 Parallel to folds –strike joints
• Set 2 Perpendicular to fold – cross-fold joints
• What is going on?
Strain pattern during bucking of a single layer
Competition between local and remote stresses
Orthogonal Sets 2
• What if the rocks are not visibly folded
(Otter Creek)?
Secon d Set
Orthogonal set formation
σ
1 Strain during development of set
1 causes σ 1 and σ 3 to flip. This is only possible if differential stress is low (shallow conditions).
Displacement during set 1
6

Other Common Mechanisms for
Joint formation
• 1. Unroofing – exfoliation
– Rock is elastic!
– It springs back when the load is removed
• 2. Cooling Joints
Exfoliation Joints
Exfoliation Joints in Yosemite National park
Exfoliation Joints
Hexagonal cooling joints
Columnar Jointing
Basalt
7

Cooling Joints in Basalt
Vein systems
•Arrays of fractures filled by mineral
•Lead to bulk volume increase
•Common in low grade metamorphic rocks
•Contain many important mineral deposits
Veins often record multiple episodes of cracking and opening.
Vein Fill
Veins are opening mode fractures filled with new minerals.
Crystals growth is often controlled by the progressive opening of the vein.
En Echelon Veins in a shear zone
Strain pattern in a shear zone
Think about the strain ellipse in order to interpret the sense of shear.
8

S-shaped veins in a shear zone
Veins acquire an S shape because of progressive rotation due to non-coaxial strain (simple shear)
First vein set
Second set
Conjugate shear zone
Stylolites in Marble
σ
1
Pressure solution features - Anticracks
σ
3
Take home ideas
• Two types of brittle failule:
– Shear fractures
– Tensile or opening-mode fractures
• Shear fractures form at +/- 30 o to σ
1
• Tensile fractures form parallel to σ
1
• Microcracks and defects control rock strength
• Fluid pressure offsets σ
1 and σ
3 driving
Mohr circle into the tensile domain
More take home ideas
• Joints often control weathering and subsurface fluid flow
• Systematic joint sets are due to both regional and local stresses during deformation
• Joint spacing is controlled by bed thickness
• Orthogonal joint sets require a flip of the principal stresses
• Rapid unroofing causes joints
• Cooling and contraction also causes joints.
9