Fractures, Joints and Veins

Home , Columnar jointing, Fracture (geology), Joint (geology), Pressure solution

Geol341 Outline • Tensile Fractures – Joints, cleat Fractures, Joints – Systematic sets – Bedding/thickness relationships – Interpreting joint sets and Veins – Origin of orthogonal tectonic joints – Origin of exfoliation joints – Veins and ductile shear zones

• Antifractures: Stylolites 2016 • Reading Fossen Ch. 7

3 Two modes of Brittle Failure Role of Holes Opening Mode Tensile Cracks Shear Fractures Mode 1 Modes 2 and 3 Stress Concentration around holes 3 C=2(b/a)+1 1

30o

10:1 1 Stress concentration= 21X

Stress field around a hole Microscopic flaws

3 control the macro strength

1

1 P= 0.1 MPa P= 49 MPa P= 98 MPa Triaxial Testing Apparatus

Axial Splitting Shear Fracture Distributed shearing Pollard and Brittle failure in triaxial tests of Ohtawa basalt (Hoshino et al., 1972). Fletcher, 2005

Orientation of Different Fracture Types to the Stress Field

Formation of Shear Fractures 3 Situations

3

About to Fail Failure!

1

2 Mohr-Coulomb Fracture Criterion Lithostatic and Hydrostatic

Angle of Pressure internal friction

Cohesive Strength

Tensile Strength

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

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 Fracture surface in Plexiglas with fracture propagation wellbore is (Sv, SH, Sh). Zoback (2007) textures (Pollard and Fletcher, 2005)

3 Frac Parameters Slurry Rate 3

3 1 Time (hours)

Microseismic Monitoring of Frac Job Joints

• The most common type of tensile fracture 1 • Form near the Earth’s surface • Control the bulk strength of the rock – Coal Cleat 1 • 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).

4 Jointed Rock Face, NH Joints in every rock outcrop

Monument Valley, AZ- Arches National Park- Landscapes controlled by Joints Landscapes controlled by Joints

Joints are systematically oriented over huge areas

Related to regional stress fields

5 Surface Joint Morphology

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

Joint Features record fracture Joint surface features propagation

6 Relationship Between Stress Field and Fracture Spacing/Bedding Joint Morphology

Ziegler et al. 2014 Photo: J. Olson, UT Austin

Cross cutting relationships Join Spacing

•Spacing decreases with layer thickness •More joints in stiffer layers •Often joints are confined to specific beds •Spacing increases with strain

What are the relative ages of the Evidence for fluid flow along Fractures joint sets? in Sandstone

7 Sealed Systematic Joint Sets Fractures

• Fractures filled with cement

Photo: J. Olson, UT Austin

Appalachian Joints Appalachian Joint Sets • 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 Competition between local and single layer remote stresses

8 Orthogonal set formation Orthogonal Sets 2 Strain during • What if the rocks are not visibly folded development of set (Otter Creek)? 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 Exfoliation Joints in Yosemite National park Joint formation • 1. Unroofing – exfoliation – Rock is elastic! – It springs back when the load is removed • 2. Cooling Joints

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

9 Mechanical Explanations of Exfoliation Joints: Hexagonal cooling joints 2. Outward force caused by surface curvature

Martel 2011

Columnar Jointing Basalt Cooling Joints in Basalt

Vein Fill

Vein systems Veins are opening mode fractures filled with new minerals. Crystals growth is often controlled by the progressive opening of •Arrays of fractures the vein. filled by mineral •Lead to bulk volume increase •Common in low grade metamorphic rocks •Contain many important mineral deposits

10 Veins often record En Echelon Veins in a shear zone multiple episodes of cracking and opening.

Strain pattern in a shear zone S-shaped veins in a shear zone Veins acquire an S shape because of progressive rotation due to Think about the strain ellipse in order to interpret the sense of shear. non-coaxial strain (simple shear)

Conjugate shear zone First vein set

Second set

11 Stylolites in Marble Stylolite showing dissolution of white calcite 1

3

Horse Cave, KY Pressure solution features - Anticracks B. Railsback photo

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.