joints & shear fractures. remember: three “directions” of stress compression extension shear...
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Joints & Shear FracturesJoints & Shear Fractures
Remember: Three
“directions”of stress
• Compression• Extension• Shear
How are these stress conditions created? What are their effects?
Focus on Extension TodayFocus on Extension Today
Modes of Fracture - Definitions
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3
1
MODE I - TENSILE FRACTURE
JOINT
• Straight up tensile failure (direction of displacement is perpendicular to fracture plane, displacement is parallel to 3. Fracture on 1-2 plane.)
3=90
MODE I - TENSILE FRACTURE
MODE I - TENSILE FRACTURE
Fracture plane (ideally) tangent to failure envelope.
2 = 180
3
1
MODE II - SHEAR FRACTURE
MODE II - SHEAR FRACTURE
• Shear fracture (all displacement parallel to fracture surface)
• Much more on this when we talk about faults!
3
Ideal=60
MODE II - SHEAR FRACTURE
= 602 = 120
3
1
MODE I/II - TENSILE & SHEAR
MODE I/II - TENSILE & SHEAR
• Tensile failure with some shear component - motion components both parallel and perpendicular to fracture.
3
>60
MODE I/II - TENSILE & SHEAR
Fracture plane (ideally) tangent to failure envelope.
>60180 > 2 > 120
Focus on Joints - Mode I & I-II
VEINS are just joints filled with something (either mineral cement, soft sediment, or even liquid hot magma). Veins are more
common than empty joints in many environments!
Mud-filled joints in
siltstone, Panther
Beach (JCM)
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Columnar joints, Devil’s
Postpile Basalts (JCM)
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Exfoliation Joints, Sierra Batholith, “onion peels”
Systematic & Asystematic - Purisima Fm. at Pt. Reyes
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Joint spacing controlled by layer thickness
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Mud cracks - two sets - two layers
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MODE I - TENSILE FRACTURE
Criteria for falling in realm of jointing:
3 < 0
1 is small
(differential stress is therefore small, mean stress is also small.)
Coulomb failure doesn’t apply here!
How do we drive 3 into negative values? Remember “negative” 3 means pressure acting outward from within a body of rock.
1. Unroofing - pressure “frozen” into a pluton or metamorphic rock is released when overburden eroded away
2. Cooling/drying compaction - i.e. columnar jointing in a basalt flow or mud cracks in a puddle
3. Fluid pressure - pore pressure pushes out from inside, drives all forces more negative…
Fluid Pressure is homogeneous -
Cannot support directionality!
WithFluids
Failure!
WithoutFluids
No Failure
Tectonic stresses haven’t changed - butEffective Stress (*) is reduced!
11*
You find a fracture in the field…
How can you tell whether it’s a joint (mode I), a shear fracture (mode II)
or has components of both?
1. Surface decoration
• Joints often have “plumose structure”
Anatomy of Joint Surface features
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1b. Surface Decoration on shears
• Shear fractures often have linear striations - either grooves (slickenlines) or “antigrooves” where fibers have grown (slickenfibers) which record direction of shear motion on fracture face
Angles of intersection
• Joints may form along 1-2 plane and along 1-3 plane, therefore are often in perpendicular sets.
May be combo of
systemic and non-systemic
- which set came first?
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Shear fractures ~60° to 3
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3
Joints: Sandstone (lighter gray) was extended, joints filled with Qtz veins.
3
Natural Bridges Introduction
Print your own for reference:es.ucsc.edu/~crowe/structure/
natbridges.html