Anderson’s theory of faulting: In-class thought exercise
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.
1. Anderson’s (1951) theory of faulting: Explain first two bullets, have
students work out second two bullets.
 Surface of the earth is not confined, and essentially is not acted on by
shear stresses.
 Also, tectonic plates generally move parallel with Earth’s surface
 This hypothetically requires 2 of the 3 principal stresses to be parallel
with the surface of the earth.
 Three possible combinations:
σ1 horizontal, σ3 vertical — reverse faults
σ1 vertical, σ3 horizontal — normal faults
σ1 horizontal, σ3 horizontal — strike-slip faults
 According to Coulomb’s Law, fractures develop at θ ≈ 30° in most
rocks because most rocks have φ ≈ 30°. Therefore:
Reverse faults should dip at ≈ 30°
Normal faults should dip at ≈ 60°
Strike-slip faults should dip at ≈ 90°
2. Exceptions to Anderson’s theory of faulting: Have students brainstorm
and work out.
 Looking for low-angle normal faults and thrust faults. These are
commonly documented structures
 Anderson’s theory stricto sensu also does not predict oblique-slip
faults
3. Mechanisms that can explain low-angle faulting
 Elevated pore fluid pressure (Hubbard and Ruby, 1959)
a. Must have very low σeff at fault surface — illustrate with Mohr circle
b. σeff must be relatively high elsewhere, otherwise fault block will
deform internally and low-angle fault will not slip
c. To my knowledge, nobody has fully explained how this can work
 Pre-existing weakness in rocks
Experiments by Donath (1961) show that a pre-existing anisotropy
such as bedding or cleavage can allow fractures to form at 10–15°
to σ1
We know that faults commonly follow weak layers
 Rolling-hinge model for crustal-scale low-angle normal faults —
widely accepted, but still somewhat controversial
a. Illustrate core complex geometry with Ruby-East Humboldt range
b. Fault initiates at surface at high angle and soles into a low-angle
fault (mylonite zone) below the brittle-plastic transition
c. As fault slip progresses and crust is thinned, hot, buoyant crust
rebounds isostatically
d. This geometry allows the slipping segment of the fault to undergo
frictional sliding
e. May be aided my large additions of mantel-derived basaltic
magma to the lower crust and/or solid-state flow into the domain
high extensional strains