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 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 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?

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

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 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??

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

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

1. Elevated pore fluid pressure (Pf)

High Pf can lower effective stress σ1 σn σ3eff σ3

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

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

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

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

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

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

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

Cartoon cross section illustrating the rolling-hinge model

East Humboldt Range Ruby Mountains

Geologic map of the Ruby Mountains and East Humboldt Range

Cross section of a low-angle normal-fault system

Cartoon cross section illustrating the rolling-hinge model