1. Determining Stress State and Sense of Shear
Joints and Shear Zones
Determining Stress State and Sense of Shear

3. Twiss and Moores [1992]

4. Tensional Joints - Mode I
The 3 direction is perpendicular to joint face
P-waves affect propagation of fracture, producing plumose pattern
Tensional Joints need true tension or highly elevated fluid pressures

7. Shear Joints - Mode II and III
The 3 direction is not perpendicular to the joint face
Joint face undergoes both tension and shear

8. Shear Zones and Riedel Shears
Shear zone with maximum principal stress oriented 45° to orientation of shear zone
If the fault zone is brittle, get Reidel shears forming

13. R

14. Veins and Shear Zones
Veins can form and then rotate within shear zones
Tip of vein points in the direction of maximum principal stress 1

16. Allmendinger (Cornell)

17. From Allmendinger (Cornell)

18. S-C Fabrics S-C fabrics is type of foliation formed within shear zones
S is foliation attributed to finite strain
C is the shear plane

19. S-C Fabrics

22. C S