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Geology 3120 - Kinematic Indicators in Shear Zones

Geology 3120 - Kinematic Indicators in Shear Zones. Outline. What to look for Kinematic indicators. What to look for. Lines or Planes Rotation Mineral Growth Cross-Cutting Relationships. Offset and Deflected Markers. How would you describe the kinematics of this system?.

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Geology 3120 - Kinematic Indicators in Shear Zones

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  1. Geology 3120 - Kinematic Indicators in Shear Zones

  2. Outline • What to look for • Kinematic indicators

  3. What to look for • Lines or Planes • Rotation • Mineral Growth • Cross-Cutting Relationships

  4. Offset and Deflected Markers How would you describe the kinematics of this system?

  5. Offset and Deflected Markers Right-lateral offset

  6. Foliation How would you describe the kinematics of this system?

  7. Foliation Left-lateral offset

  8. Foliations (S-C fabric) • Foliation develops as a result of simple shear • Dominant simple-shear parallel to walls of shear zone develops a S-surface foliation • New foliation develops (C-surface) at an angle (18-25 to the shear-zone walls)

  9. S-C Fabric • S-surfaces - foliation planes • C-surfaces - shear bands (useful for determining the sense of shear)

  10. S-C Fabric Right-lateral motion

  11. Foliation (mica) fish • A layer with oblique internal fabric becomes detached along S-fabric to form an asymmetric ‘fish’. Fish “flash” - oriented sample reflected sunlight…

  12. Mica Fish • If you can stack the fish up, you’ve got the sense of shear correct. Or, think of the fish as thrust surfaces…

  13. Mica Fish Right-lateral movement

  14. Porphyoblasts / Porphyroclasts • Porphyroblasts = large grains that have grown in the rock mass during deformation and metamorphism • Porphyroclasts = relict earlier large grains that have survived ductile deformation (e.g. feldspar) Blast: grain size increase Clast: grain size decrease

  15. Tails on porphyroclasts are asymmetric in the direction of ductile flow.

  16. Rotation ? ? • Works for inclusions, porphyroblasts, and porphyroclasts • Tails and fabric in the rotated grains are useful

  17. Rotation CW • Right-lateral shear (dextral)

  18. Mylonitic orthogneiss - Nash Fork Shear Zone, Medicine Bow Mtns, So. Wyoming. K-spar porphyroclasts with asymmetric tails What is sense of shear here?

  19. Pressure Shadows with Fiber Growth • Minerals will grow in the direction of maximum extension

  20. Pressure Shadows with Fiber Growth Maximum extension Maximum shortening

  21. Fractured Grains How would you describe the kinematics of this system?

  22. Fractured Grains • Large grains of some minerals (e.g. feldspars, pyroxenes) continue to deform brittley within a ductile matrix. • The fractures indicate a sense of motion opposite that of actual shear sense How would you describe the kinematics of this system?

  23. Fractured Grains Right-lateral offset

  24. Veins • Calcite extensional vein

  25. Tension Gashes • Initial shearing

  26. Tension Gashes • Directions of maximum extension and maximum shortening

  27. Tension Gashes • Mode I (opening) fracture develops

  28. Tension Gashes • Continued development of a tension gash

  29. Tension Gashes How would you describe the kinematics of this system?

  30. Tension Gashes Finally, left-lateral motion!!!

  31. Tension Gashes

  32. Tension Gashes How would you describe the kinematics of this system?

  33. Tension Gashes Left-lateral motion

  34. References Slide 1: http://earth.leeds.ac.uk/shearzones/gallery/odhair.htm Slides 4, 5: http://earth.leeds.ac.uk/shearzones/gallery/apliteshear.htm Slides 6, 7:http://earth.leeds.ac.uk/shearzones/gallery/kohi1.htm Slides 8, 9: http://www.earth.monash.edu.au/Teaching/mscourse/lectures/lec4b.html Slides 10,11: Image 30 at http://talc.geo.umn.edu/orgs/struct/microstructure/sec2.html Slides 12, 13:http://www.uni-mainz.de/~mezger/Text/Mezger_metamorphic_geology.html Slides 14, 15:http://earth.leeds.ac.uk/strain/gallery/presshadbig.html Slides 16, 17:http://www.earth.monash.edu.au/Teaching/mscourse/lectures/lec4b.html Slide 18:http://earth.leeds.ac.uk/minor/dmt/reprecipitation.htm Slides 19 - 22:http://earth.leeds.ac.uk/minor/tensiongash/gash1.htm Slides 23 - 24:http://earth.leeds.ac.uk/minor/tensiongash/tgash.htm Slide 25:http://earth.leeds.ac.uk/minor/tensiongash/gash3.htm Slides 26 - 27: http://earth.leeds.ac.uk/minor/tensiongash/gashpic2.htm

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