Joints & Shear Fractures. Remember: Three “directions” of stress Compression Extension Shear...

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