Faults, fault zone, mylonites, ductile strain

REVIEW
 
 

DEFINITIONS
 
 

READINGS

 Ch. 9

LECTURE
 
 

Definitions

Fault is a fracture along which there is visible offset by shearing

Field Recognition of Faults

Sometimes a fault surface may appear to be sharp but at a small enough scale you can always smaller faults (microfaults). In fact faults do not really occur alone but in groups (fault systems, fault zones).
 
 

The San Andreas Fault is a famous fault but the whole faults is composed of a series of faults within a very broad zone.
 
 

For example, a conceptual strike-slip fault at depth would look like:
 
 

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Above a certain depth (i.e., P and T) the rocks break by crushing.

 Below a certain depth, brocks deform without losing cohesion. The minerals change size and shape but there are no major clear fractures.
 
 

Field identification of Faults

 e.g., Faults are not always obvious in the field.

in the best of cases you may be able to see the actual fault surface, a sharp break in slope. A fault surface that is in uneroded condition and exposed may have visible striae (frictional surface sratches), chatter marks (steplike features oriented perpendicular to the striae) The steps are no more than about 5mm high.

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Fault classification

Faults terminology can seem puzzling at first. There are several ways to describe faults. One method is used when the complete 3-dimensional description of a fault can be made and another when only apparent views of the fault contact are available. (Analogous to last lab where we examined apparent vs. true dip)
 
 

Faults can be classified based on the true slip, the actual relative displacement (slip is a vector, magnitude, sense and direction).

 Faults can also be classified based on the apparent relative displacement. In fact, the offset observed in faults in outcrops, in map patterns and in structural profile is the 'separation' - sense and magnitude.

 Another measure of apparent displacement is throw. You will use throw more often if you enter the oil industry. a throw is the separation without the sense, i.e. apparent the stratigraphic displacement but without saying .
 
 

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Were we to know the true total displacement, then we could use the following classification scheme based on the slip.

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However, we do not always have a 3-dimensional view of faults. How can you find the slip from an apparent slip (separation)

 Normally you'll deal with faults in outcrop faces.
 
 

In this case the classification is a little different.

Determining Fault movement
 
 

Regional Faulting