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Fractures, Joints and Veins

Geol341

2016

Outline• 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• Reading Fossen Ch. 7

Two modes of Brittle Failure

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Opening ModeTensile CracksMode 1

Shear FracturesModes 2 and 3

30o

Role of Holes

Stress Concentration around holes

C=2(b/a)+1

10:1

Stress concentration= 21X

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Stress field around a hole

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Microscopic flaws control the macro strength

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Triaxial Testing Apparatus

Pollard and Fletcher, 2005

Brittle failure in triaxial tests of Ohtawa basalt (Hoshino et al., 1972).

P= 0.1 MPa P= 49 MPa P= 98 MPa

Axial Splitting Shear Fracture Distributed shearing

Orientation of Different Fracture Types to the Stress Field

Formation of Shear Fractures

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3 Situations

About to Fail Failure!

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Mohr-Coulomb Fracture Criterion

Cohesive Strength

TensileStrength

Angle of internal friction

Lithostatic and Hydrostatic Pressure

Role of Fluid Pressure in Fractures(effective stress)= (stress - fluid pressure)

Fracking a Well

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Hydraulic fracturing in a wellbore. a) Vertical cross section in the plane of the fracture. b) Horizontal cross section through the fracture and wellbore. State of stress in absence of fracture and wellbore is (Sv, SH, Sh). Zoback (2007) Fracture surface in Plexiglas with fracture propagation

textures (Pollard and Fletcher, 2005)

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Hydraulic fractures form parallel to 1

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Frac Parameters

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Pressure

Slurry Rate

Propant Seismic Events

Time (hours)

Microseismic Monitoring of Frac Job

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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

Otter Creek, WV

Jointed limestone bed at Lilstock Beach on the southern coast of the Bristol Channel, England (Pollard and Fletcher, 2005).

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Jointed Rock Face, NH Joints in every rock outcrop

Monument Valley, AZ-

Landscapes controlled by Joints

Arches National Park-

Landscapes controlled by Joints

Joints are systematically

oriented over huge areas

Related to regional stress fields

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Surface Joint Morphology

Fracture surface in Plexiglas with fracture propagation textures (Pollard and Fletcher, 2005)

Joint surface featuresJoint Features record fracture

propagation

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Relationship Between Stress Field and Joint Morphology

Ziegler et al. 2014

Fracture Spacing/Bedding

Photo: J. Olson, UT Austin

Join Spacing

•Spacing decreases with layer thickness

•More joints in stiffer layers

•Often joints are confined to specific beds

•Spacing increases with strain

Cross cutting relationships

What are the relative ages of the joint sets?

A

B

C

Evidence for fluid flow along Fractures in Sandstone

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Sealed Fractures

• Fractures filled with cement

Photo: J. Olson, UT Austin

Systematic Joint Sets

Appalachian Joint SetsAppalachian 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 asingle layer

Competition between local and remote stresses

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Orthogonal Sets 2• What if the rocks are not visibly folded

(Otter Creek)?

Orthogonal set formation

Strain during development of set 1 causes σ1 and σ3 to flip. This is only possible if differential stress is low (shallow conditions).

Displacement during set 1

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 in Yosemite National park

Exfoliation Joints Mechanical Explanations of Exfoliation Joints: 1. Release of Elastic Strain during erosional

unloading

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Mechanical Explanations of Exfoliation Joints: 2. Outward force caused by surface curvature

Martel 2011

Hexagonal cooling joints

Columnar JointingBasalt 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

Vein FillVeins are opening mode fractures filled with new minerals. Crystals growth is often controlled by the progressive opening of the vein.

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Veins often record multiple episodes of cracking and opening.

En Echelon Veins in a shear zone

Strain pattern in a shear zoneThink about the strain ellipse in order to interpret the sense of shear.

S-shaped veins in a shear zoneVeins acquire an S shape because of progressive rotation due to non-coaxial strain (simple shear)

First vein set

Second set

Conjugate shear zone

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Stylolites in Marble

Pressure solution features - Anticracks

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Stylolite showing dissolution of white calcite

B. Railsback photo

Horse Cave, KY

Take home ideas• Joints form parallel to 1

• 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 cause joints.