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WVU GEOL 341 - Fractures, Joints and Veins

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1Fractures, Joints and VeinsGeol3412011Outline•Review• 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: StylolitesTwo modes of Brittle Failureσ3σ1Once a fracture exists deformation continues by frictional sliding of cataclastic flowFailure= rupture of atomic bonds2Role of HolesStress Concentration around holesC=2(b/a)+110:1Stress concentration= 21Xσ3σ1Microscopic flaws control the macro strengthStress field around a holeσ3σ1Role of Fluid PressureFormation of Shear Fracturesσ3σ13Joints• 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– HydrocarbonsOtter Creek, WVJointed limestone bed at Lilstock Beach on the southern coast of the Bristol Channel, England (Pollard and Fletcher, 2005). Jointed Rock Face, NHJoints in every rock outcropMonument Valley, AZ-Landscapes controlled by Joints4Arches National Park-Landscapes controlled by JointsJoints are systematically oriented over huge areasRelated to regional stress fieldsSurface Joint MorphologyFracture surface in Plexiglas with fracture propagation textures (Pollard and Fletcher, 2005)5Joint Features record fracture propagationFracture Spacing/BeddingPhoto: J. Olson, UT AustinJoin Spacing•Spacing decreases with layer thickness•More joints in stiffer layers•Often joints are confined to specific beds•Spacing increases with strainCross cutting relationshipsSecond SetWhat are the relative ages of the joint sets?ABCAppalachian Joint Sets6Systematic 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 stressesOrthogonal Sets 2• What if the rocks are not visibly folded (Otter Creek)?Orthogonal set formationSecond SetStrain 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 1σ17Other Common Mechanisms for Joint formation• 1. Unroofing – exfoliation– Rock is elastic! – It springs back when the load is removed• 2. Cooling JointsExfoliation JointsExfoliation Joints in Yosemite National parkExfoliation JointsHexagonal cooling jointsColumnar JointingBasalt8Cooling Joints in BasaltVein systems•Arrays of fractures filled by mineral•Lead to bulk volume increase•Common in low grade metamorphic rocks•Contain many important mineral depositsVein FillVeins are opening mode fractures filled with new minerals. Crystals growth is often controlled by the progressive opening of the vein.Veins often record multiple episodes of cracking and opening.En Echelon Veins in a shear zoneStrain pattern in a shear zoneThink about the strain ellipse in order to interpret the sense of shear.9S-shaped veins in a shear zoneVeins acquire an S shape because of progressive rotation due to non-coaxial strain (simple shear)First vein setSecond setConjugate shear zoneStylolites in MarblePressure solution features - Anticracksσ3σ1Take home ideas• Two types of brittle failule:– Shear fractures– Tensile or opening-mode fractures• Shear fractures form at +/- 30oto σ1• Tensile fractures form parallel to σ1• Microcracks and defects control rock strength• Fluid pressure offsets σ1 and σ3 driving Mohr circle into the tensile domainMore take home ideas• 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 causes


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