1. Anderson’s theory of faulting
Goals: 1) To understand Anderson’s theory of faulting and its implications. 2) To outline some obvious exceptions to Anderson’s theory and some possible explanations for how these exceptions work.

2. Primary assumptions
Surface of the earth is not confined, and not acted on by shear stresses.
Also, tectonic plates move parallel with Earth’s surface (unknown in 1951)
Homogenous rocks
Coulomb behavior

3. Three possible stress combinations
Hypothetically requires 2 of the 3 principal stresses to be parallel with the surface of the earth
What are they?
What kind of faults would you expect at each?

4. σ1 horizontal, σ3 vertical — reverse faults
σ1 vertical, σ3 horizontal — normal faults
σ1 horizontal, σ3 horizontal — strike-slip faults

5. Most rocks have an angle of internal friction ≈ 30°
What dip angles does Anderson’s theory predict for
σ1 horizontal, σ3 vertical — reverse faults?
σ1 vertical, σ3 horizontal — normal faults?
σ1 horizontal, σ3 horizontal — strike-slip faults?

6. Hypothetically Can you think of any exceptions??
Reverse faults: should form at ~30° dip
Normal faults: should form at ~60° dip
Strike-slip faults: should form at ~90° dip
Can you think of any exceptions??

7. Common exceptions Thrust faults — mechanically unfavorable
Low-angle normal faults — mechanically very unfavorable

9. Possible explanations
Elevated pore fluid pressure
Pre-existing weaknesses
Rolling-hinge model for low-angle normal faults

10. 1. Elevated pore fluid pressure (Pf)

11. High Pf can lower effective stressσ1 σn
σ3eff σ3

12. This can activate slip on a low-angle fault
σ3eff
σ1eff

13. However, if cohesive strength is sufficiently low...
σ3eff
σ1eff

14. Pore-fluid-pressure mechanism requires low σeff on fault, but high σeff in surrounding rocks

15. It also doesn’t work well for low-angle normal faults
σ3eff
σ1eff

16. 2. Pre-existing anisotropy
Bedding
Weak layer (salt, shale)
Foliation

17. Donath (1961) produced shear fractures at very low angles to σ1 in anisotropic rock

18. 3. Rolling-hinge model for low-angle normal faults

19. Cartoon cross section illustrating the rolling-hinge model

21. East Humboldt
Range
Ruby
Mountains

22. Geologic map of the Ruby Mountains and East Humboldt Range

24. Cross section of a low-angle normal-fault system

25. Cartoon cross section illustrating the rolling-hinge model