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Geology 3120 - Geology 3120 - Kinematic Indicators in Shear ZonesKinematic Indicators in Shear Zones
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OutlineOutline
• What to look forWhat to look for
• Kinematic indicatorsKinematic indicators
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What to look forWhat to look for
• Lines or PlanesLines or Planes
• RotationRotation
• Mineral GrowthMineral Growth
• Cross-Cutting RelationshipsCross-Cutting Relationships
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Offset and Deflected MarkersOffset and Deflected Markers
How would you describe the kinematics of this system?How would you describe the kinematics of this system?
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Offset and Deflected MarkersOffset and Deflected Markers
Right-lateral offsetRight-lateral offset
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FoliationFoliation
How would you describe How would you describe
the kinematics of this the kinematics of this
system?system?
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FoliationFoliation
Left-lateral offsetLeft-lateral offset
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Foliations (S-C fabric)Foliations (S-C fabric) Foliation develops as a result of simple shearFoliation develops as a result of simple shear Dominant simple-shear parallel to walls of shear zone develops a Dominant simple-shear parallel to walls of shear zone develops a
S-surface foliationS-surface foliation New foliation develops (C-surface) at an angle (18-25 to the shear-New foliation develops (C-surface) at an angle (18-25 to the shear-
zone walls)zone walls)
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S-C FabricS-C Fabric
• S-surfaces - foliation planesS-surfaces - foliation planes
• C-surfaces - shear bands (C-surfaces - shear bands (useful for determining the sense of shearuseful for determining the sense of shear))
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S-C FabricS-C Fabric
Right-lateral motionRight-lateral motion
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Foliation (mica) fishFoliation (mica) fish A layer with oblique internal fabric becomes detached A layer with oblique internal fabric becomes detached
along S-fabric to form an asymmetric ‘fish’.along S-fabric to form an asymmetric ‘fish’.
Fish “flash” - oriented sample reflected sunlight…Fish “flash” - oriented sample reflected sunlight…
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Mica FishMica Fish
• If you can stack the fish up, you’ve got the sense of shear If you can stack the fish up, you’ve got the sense of shear
correct. Or, think of the fish as thrust surfaces…correct. Or, think of the fish as thrust surfaces…
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Mica FishMica Fish
Right-lateral movementRight-lateral movement
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Porphyoblasts / PorphyroclastsPorphyoblasts / Porphyroclasts PorphyroblastsPorphyroblasts = large grains that have grown in the = large grains that have grown in the
rock mass during deformation and metamorphismrock mass during deformation and metamorphism PorphyroclastsPorphyroclasts = relict earlier large grains that have = relict earlier large grains that have
survived ductile deformation (e.g. feldspar)survived ductile deformation (e.g. feldspar)
Blast: grain size Blast: grain size increaseincrease
Clast: grain size decreaseClast: grain size decrease
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TailsTails on on porphyroclasts are porphyroclasts are asymmetric in the asymmetric in the direction of ductile direction of ductile flow. flow.
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RotationRotation
• Works for inclusions, porphyroblasts, and porphyroclastsWorks for inclusions, porphyroblasts, and porphyroclasts
• Tails and fabric in the rotated grains are usefulTails and fabric in the rotated grains are useful
?
?
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RotationRotation
• Right-lateral shear (dextral)Right-lateral shear (dextral)
CW
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Mylonitic orthogneiss - Nash Fork Shear Zone, Medicine Bow Mtns, So. Wyoming.
K-spar porphyroclasts with asymmetric tails What is sense of shear here?
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Pressure Shadows with Fiber Pressure Shadows with Fiber
GrowthGrowth
• Minerals will grow in the direction of maximum extensionMinerals will grow in the direction of maximum extension
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Pressure Shadows with Fiber Pressure Shadows with Fiber
GrowthGrowth
Maximum extensionMaximum shortening
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Fractured GrainsFractured Grains
How would you describe the kinematics of this system?How would you describe the kinematics of this system?
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How would you describe the kinematics of this system?How would you describe the kinematics of this system?
Fractured GrainsFractured Grains Large grains of some Large grains of some
minerals (e.g. feldspars, minerals (e.g. feldspars, pyroxenes) continue to pyroxenes) continue to deform brittley within a deform brittley within a ductile matrix.ductile matrix.
The fractures indicate a The fractures indicate a sense of motion opposite sense of motion opposite that of actual shear sensethat of actual shear sense
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Fractured GrainsFractured Grains
Right-lateral offsetRight-lateral offset
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VeinsVeins
• Calcite extensional veinCalcite extensional vein
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Tension GashesTension Gashes
• Initial shearingInitial shearing
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Tension GashesTension Gashes
• Directions of maximum extension and maximum shorteningDirections of maximum extension and maximum shortening
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Tension GashesTension Gashes
• Mode I (opening) fracture developsMode I (opening) fracture develops
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Tension GashesTension Gashes
• Continued development of a tension gashContinued development of a tension gash
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Tension GashesTension Gashes
How would you describe the kinematics of this system?How would you describe the kinematics of this system?
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Tension GashesTension Gashes
Finally, left-lateral motion!!!Finally, left-lateral motion!!!
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Tension GashesTension Gashes
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Tension GashesTension Gashes
How would you describe the kinematics of this system?How would you describe the kinematics of this system?
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Tension GashesTension Gashes
Left-lateral motionLeft-lateral motion
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ReferencesReferences
Slide 1Slide 1: : http://earth.leeds.ac.uk/shearzones/gallery/odhair.htm
Slides 4, 5Slides 4, 5: : http://earth.leeds.ac.uk/shearzones/gallery/apliteshear.htm
Slides 6, 7:Slides 6, 7: http://earth.leeds.ac.uk/shearzones/gallery/kohi1.htm http://earth.leeds.ac.uk/shearzones/gallery/kohi1.htm
Slides 8, 9:Slides 8, 9: http://www.earth.monash.edu.au/Teaching/mscourse/lectures/lec4b.html http://www.earth.monash.edu.au/Teaching/mscourse/lectures/lec4b.html
Slides 10,11:Slides 10,11: Image 30 at http://talc.geo.umn.edu/orgs/struct/microstructure/sec2.html Image 30 at http://talc.geo.umn.edu/orgs/struct/microstructure/sec2.html
Slides 12, 13:Slides 12, 13: http://www.uni-mainz.de/~mezger/Text/Mezger_metamorphic_geology.html http://www.uni-mainz.de/~mezger/Text/Mezger_metamorphic_geology.html
Slides 14, 15:Slides 14, 15: http://earth.leeds.ac.uk/strain/gallery/presshadbig.html http://earth.leeds.ac.uk/strain/gallery/presshadbig.html
Slides 16, 17:Slides 16, 17: http://www.earth.monash.edu.au/Teaching/mscourse/lectures/lec4b.html http://www.earth.monash.edu.au/Teaching/mscourse/lectures/lec4b.html
Slide 18:Slide 18: http://earth.leeds.ac.uk/minor/dmt/reprecipitation.htm http://earth.leeds.ac.uk/minor/dmt/reprecipitation.htm
Slides 19 - 22:Slides 19 - 22: http://earth.leeds.ac.uk/minor/tensiongash/gash1.htm http://earth.leeds.ac.uk/minor/tensiongash/gash1.htm
Slides 23 - 24:Slides 23 - 24: http://earth.leeds.ac.uk/minor/tensiongash/tgash.htm http://earth.leeds.ac.uk/minor/tensiongash/tgash.htm
Slide 25:Slide 25: http://earth.leeds.ac.uk/minor/tensiongash/gash3.htm http://earth.leeds.ac.uk/minor/tensiongash/gash3.htm
Slides 26 - 27:Slides 26 - 27: http://earth.leeds.ac.uk/minor/tensiongash/gashpic2.htm http://earth.leeds.ac.uk/minor/tensiongash/gashpic2.htm