Andersonian faulting

REVIEW

Kinematic Analysis

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In rift systems, a convenient manner of describing the amount of extension: e= Lf-Lo/Lo

DEFINITIONS
 
 

READINGS

 Ch. 9

LECTURE
 
 

Why are there faults?

On average strike-slip faults dip vertically, normal faults dip at 60deg. and thrust-slip faults dip at 30deg..
 
 

At depth all rocks are in comrpession from all directions. A general stress field can always be broken down into three simple principal stress directions (all compressional). Earthquakes occur when there is a large stress imbalnce between these principal stresses.
 
 

Anderson (1951) recognised that one of these stresses pointed down and the other along a plane parallel to the earth's surface. The type of fault depends onthe relative orientation of the principal stresses in space.

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Fig. 9. 62

Fig. 9.64

Fig. 9.72
 
 

These principal stresses can be decomposed into stresses along the future fault plane (shear stresses) and at right angles to the fault plane (normal stresses).

Shear stresses tend to make the fault move and normal stresses tend to retard its movement.
 
 

Experimental Observations

Through repeated laboratory experiments we can plot the required set of normal and shear stresses and the ratio of each at which faulting will start to take place. The conditions are described by a linear relation of shear and normal stresses. It appears that the strength of the rock is derived from two sources. First it must break the rock and then move the fault:

 In order to break the rock the stresses must overcome the cohesive strength of the rock ([[sigma]]0) . Granites, basalts and quartzites are more tightly bonded that calcite-cemented sandstones.

 Once the rock is unbonded movement must take place and the internal frictional resistance to faulting must be overcome. (tan [[phi]]) . The internal resistance varies as a funtion of the normal stresses. That is, if there is greater normal stress pushing to keep the naging and footwall together, there is a greater shear stress required to move the fault.
 
 

In summary:

 [[sigma]]s = [[sigma]]0 + tan [[phi]] ([[sigma]]n)

 [[sigma]]s - critcial shear stress neede to create the faulting

 [[sigma]]0 - cohesive strength

 tan [[phi]] - coefficient of internal friction

 [[sigma]]n - normal stress

 The shearing resistance is greater as the normal stress is greater.