Joints & Shear Fractures

1. Joints & Shear Fractures

2. Focus on Extension Today Remember: Three “directions”of stress • Compression • Extension • Shear How are these stress conditions created? What are their effects?

3. Modes of Fracture - Definitions

4. JOINT MODE I - TENSILE FRACTURE s1 s3

5. MODE I - TENSILE FRACTURE • Straight up tensile failure (direction of displacement is perpendicular to fracture plane, displacement is parallel to s3. Fracture on s1-s2 plane.) s3 q=90

6. 2q = 180 MODE I - TENSILE FRACTURE Fracture plane (ideally) tangent to failure envelope.

7. MODE II - SHEAR FRACTURE s1 s3

8. MODE II - SHEAR FRACTURE • Shear fracture (all displacement parallel to fracture surface) • Much more on this when we talk about faults! s3 Ideal q=60

9. q = 60 2q = 120 MODE II - SHEAR FRACTURE

10. MODE I/II - TENSILE & SHEAR s1 s3

11. MODE I/II - TENSILE & SHEAR • Tensile failure with some shear component - motion components both parallel and perpendicular to fracture. 90>q>60 s3

12. 90>q>60 180 > 2q > 120 MODE I/II - TENSILE & SHEAR Fracture plane (ideally) tangent to failure envelope.

13. Focus on Joints - Mode I & I-II VEINS are just joints filled with something (either mineral cement, soft sediment, or even liquid hot magma). Veins are more common than empty joints in many environments!

14. Mud-filled joints in siltstone, Panther Beach (JCM)

15. Columnar joints, Devil’s Postpile Basalts (JCM)

16. Exfoliation Joints, Sierra Batholith, “onion peels”

17. Systematic & Asystematic - Purisima Fm. at Pt. Reyes

18. Joint spacing controlled by layer thickness

19. Mud cracks - two sets - two layers

20. MODE I - TENSILE FRACTURE Criteria for falling in realm of jointing: s3 < 0 s1 is small (differential stress is therefore small, mean stress is also small.) Coulomb failure doesn’t apply here!

21. How do we drive s3 into negative values? Remember “negative” s3 means pressure acting outward from within a body of rock. • Unroofing - pressure “frozen” into a pluton or metamorphic rock is released when overburden eroded away • Cooling/drying compaction - i.e. columnar jointing in a basalt flow or mud cracks in a puddle • Fluid pressure - pore pressure pushes out from inside, drives all forces more negative…

22. Fluid Pressure is homogeneous - Cannot support directionality!

23. Tectonic stresses haven’t changed - but Effective Stress (s*) is reduced! s1* s1

24. You find a fracture in the field… How can you tell whether it’s a joint (mode I), a shear fracture (mode II) or has components of both?

25. 1. Surface decoration • Joints often have “plumose structure”

26. Anatomy of Joint Surface features

27. 1b. Surface Decoration on shears • Shear fractures often have linear striations - either grooves (slickenlines) or “antigrooves” where fibers have grown (slickenfibers) which record direction of shear motion on fracture face

28. Angles of intersection • Joints may form along s1-s2 plane and along s1-s3 plane, therefore are often in perpendicular sets.

29. May be combo of systemic and non-systemic - which set came first?

30. s3 Shear fractures ~60° to s3

31. s3 Joints: Sandstone (lighter gray) was extended, joints filled with Qtz veins.

32. Natural Bridges Introduction Print your own for reference: es.ucsc.edu/~crowe/structure/natbridges.html