Anderson's theory of faulting predicts that the orientation of faults depends on the principal stresses. It assumes reverse faults dip at 30 degrees, normal faults dip at 60 degrees, and strike-slip faults are vertical. However, exceptions like low-angle normal faults exist. Pore fluid pressure or pre-existing weaknesses in the rock can allow faults to form at shallower angles. The rolling-hinge model also explains how low-angle normal faults can develop.

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
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
1. Elevated pore fluid pressure
2. Pre-existing weaknesses
3. Rolling-hinge model for low-angle normal
faults

10. 1. Elevated pore fluid
pressure (Pf)

11. High Pf can lower effective stress
σs
σn
σ1
σ3
σ1eff
σ3eff

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

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

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
σs
σn
σ1eff
σ3eff

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. Ruby
Mountains
East Humboldt
Range

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