Presentation is loading. Please wait.

Presentation is loading. Please wait.

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.

Similar presentations


Presentation on theme: "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."— Presentation transcript:

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 σ 1 horizontal, σ 3 vertical — reverse faults σ 1 vertical, σ 3 horizontal — normal faults σ 1 horizontal, σ 3 horizontal — strike-slip faults

5 Most rocks have an angle of internal friction ≈ 30° What dip angles does Anderson’s theory predict for –σ 1 horizontal, σ 3 vertical — reverse faults? –σ 1 vertical, σ 3 horizontal — normal faults? –σ 1 horizontal, σ 3 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

8

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 (P f )

11 High P f can lower effective stress σsσs σnσn σ1σ1 σ3σ3 σ 1 eff σ 3 eff

12 This can activate slip on a low-angle fault σsσs σnσn σ 1 eff σ 3 eff

13 However, if cohesive strength is sufficiently low... σsσs σnσn σ 1 eff σ 3 eff

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

15 It also doesn’t work well for low-angle normal faults σsσs σnσn σ 1 eff σ 3 eff

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

17 Donath (1961) produced shear fractures at very low angles to σ 1 in anisotropic rock

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

19 Cartoon cross section illustrating the rolling-hinge model

20

21 Ruby Mountains East Humboldt Range

22 Geologic map of the Ruby Mountains and East Humboldt Range

23

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

25 Cartoon cross section illustrating the rolling-hinge model


Download ppt "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."

Similar presentations


Ads by Google