1. Geology 360 - 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, S. 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 shortening
Maximum extension

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:
Slides 4, 5:
Slides 6, 7:
Slides 8, 9:
Slides 10,11: Image 30 at
Slides 12, 13:
Slides 14, 15:
Slides 16, 17:
Slide 18:
Slides :
Slides :
Slide 25:
Slides :