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Fractures, Joints and Veins

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Fractures, Joints and Veins 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) © 2010 MicroSeismic. All Rights Reserved. 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) Pressure Propant Seismic Events Hydraulic fractures form parallel to 1 © 2010 MicroSeismic. All Rights Reserved. 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 © 2010 MicroSeismic. All Rights Reserved. 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 C A B 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. 12.
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