1. Shear zones and shear sense indicators Please read (D&R, pp. 493-551)

2. Shear zone: zone of highly strained rocks A fault zone is a shear zone formed in the brittle regime Can also have a purely ductile shear zone Or even a zone with a mixture of brittle and ductile deformation- due to composition (feldspar or qtz) or strain rate (silly putty analogy)

4. Shear zones form in the deeper crust of all structural systems: thrust, strike-slip and normal But how are they brought to the surface (exhumed)?

5. Thrusting must be accompanied by erosion to cause exhumation

6. "Tectonic Exhumation" Removal of overburden by normal faulting. Erosion plays a role, but is not required!

7. The architecture of many extensional shear zones brittle detachment fault

8. A ledge of fault breccia below the detachment

9. thick zone of mylonitic rocks below the detachment

10. Ahh, sweet mylonites! What is sense of shear?

11. Even deeper- a less deformed "injection complex"

12. Undeformed granite in core Mylonite

13. Shear sense indicators in shear zones

14. brittle fault offsets- be careful!!!! 1) faults postdate ductile deformation 2) sense of shear not always clear

15. ductile offsets within shear zone- better!

16. another example of a small ductile shear zone

17. Folds, transposition, and ambiguous sense of shear

18. sheath folds (tongues that point in direction of shear)

19. Cross section of a sheath fold

20. Mylonites are EXCELLENT! What is sense of shear? Porphyroclasts

21. Sigma structure- wings step up in direction of shear

22. Delta structure: porphyroclast rotates faster than wings

24. A delta + sigma structure- what is sense of shear?

25. Mica-fish fabrics. Typical for sheared rocks with muscovite and/or biotite. A special form of S-C fabric. Again wings/tails step up in direction of shear

26. Summary of shear fabrics

27. So far, we have talked about shear fabrics related to noncoaxial deformation (simple shear). Some "shear" zones are not due to simple shear, but rather coaxial deformation (pure shear) What do some of these structures look like?

28. symmetric boudins due to flattening/stretching

29. Symmetric pressure shadows due to flattening (pure shear) strain

30. Next Lecture: Metamorphism, deformation and time