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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. Primary assumptions.

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anderson s theory of faulting
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.

primary assumptions
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
three possible stress combinations
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?

slide4
σ1 horizontal, σ3 vertical — reverse faults
  • σ1 vertical, σ3 horizontal — normal faults
  • σ1 horizontal, σ3 horizontal — strike-slip faults
slide5
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?
hypothetically
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??

common exceptions
Common exceptions
  • Thrust faults— mechanically unfavorable
  • Low-angle normal faults— mechanically very unfavorable
possible explanations
Possible explanations
  • Elevated pore fluid pressure
  • Pre-existing weaknesses
  • Rolling-hinge model for low-angle normal faults
slide11

σs

High Pf can lower effective stress

σ1eff

σ1

σn

σ3eff

σ3

slide12

σs

This can activate slip on a low-angle fault

σn

σ3eff

σ1eff

slide13

σs

However, if cohesive strength is sufficiently low...

σn

σ3eff

σ1eff

slide15

σs

It also doesn’t work well for low-angle normal faults

σn

σ3eff

σ1eff

2 pre existing anisotropy
2. Pre-existing anisotropy
  • Bedding
  • Weak layer (salt, shale)
  • Foliation
slide21

East Humboldt

Range

Ruby

Mountains