Week 8

Joints and Shear Fractures II. Read pages 261-268 in Chapter 5: Joints and Shear Fractures.

Faults I. Read pages 269-303 in Chapter 6: Faults.

You are expected to read all the sections listed below. Information from the sections in italics will be discussed in class. You are expected to read the other sections and you may be called on in class to answer questions based on that material.

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You should become familiar with the following terms during this weeks lectures and readings:

asperites breccia cataclasite conjugate set
chatter marks dip-slip fault drag folding fault
fault-line scarp fault rocks fault scarp fault surface
fault zone footwall gouge hanging wall
hydraulic joints left-handed strike-slip fault left-lateral fault listric fault
microfaults normal fault normal-slip fault oblique-slip fault
offset overlap paleoseismic pseudotachylite
release joints repetition/omission of strata reverse fault reverse-slip fault
right-handed strike-slip fault right-lateral fault rollover anticline separation
slickensided surfaces slickensides slickenlines slip
slip-fiber lineations stratigraphic throw striations tectonic joints
thrust-slip fault tip-line loop triangular facets unloading joints

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You should be able to answer the questions below following this week:

  1. What are the potential differences between a left-handed strike-slip fault and a left-lateral fault?
  2. What characteristics of the fault plane can be used to argue that a left-lateral fault could be reclassified as a left-handed strike-slip fault?
  3. Two wells were drilled in an area of faulting. Both wells reached the same depth, both began at the same elevation, and both were spudded (rested) in the same formation. Well A drilled through an inclined fault plane and penetrated Unit X twice. Well B drilled through a second inclined fault and did not encounter Unit X. Sketch a possible structural explanation of these observations.
  4. Describe a situation in which a fault could be classified as both left-lateral and aas a reverse fault.
  5. What criteria could you use to determine fault slip and fault separation?
  6. What are the criteria that could be used to recognize the presence of a fault?
  7. Sedimentary rocks in a deformed area are inclined 70 E. Describe the repetition or omission of stratigraphic units that would result from offset on reverse or normal faults that dip 30 E or 30W. (Note: you should have 4 possible fault/bedding relationships. Sketch cross sections to illustrate your answer).

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Joints and Shear Fractures II

Influence of Pore Fluid Pressure

Hubbert & Rubey showed that high pore pressures can decrease the effect of normal stress

In both cases, the critical shear stress is reduced

fluid pressure ratio = fluid pressure/lithostatic (normal) pressure L = Pf/Pl (Pf = LPl = L x normal stress)

Elevated fluid pressures can explain the formation of joints at substantial depths (Fig. 5.49).

As joints open, they may be filled with vein material to form crystal fiber veins, which may preserve a record of the veins opening.

 

Regional Jointing

The orientation of joint sets can be used as aids to decipher tectonic history of a region.

 

Chapter 6 - Faults

 

The fault surface itself

fault - fracture along which there has been visible offset by displacement parallel to the fracture surface (hand specimen)

fault scarp - steps in land surface formed by active faults, with weathering and erosion the step does not represent an accurate representation of the fault plane, but an erosional relic, termed a fault-line scarp (example from Tetons, Wyoming)

fault surface - typically non-planar, elliptical in shape with an aspect ratio (width:height) of 2:1 or 3:1 in some normal faults measured in coal fields

tip-line loop - connects points along the edge or tip of the fault plane

 

Things on the fault surface

slickensides - smooth, polished fault surface (hand specimen) due to effects of slip or the presence of a neomineral coating formed during movement

slickenlines - lineations on fault surfaces that record (last) direction of slip (hand specimen)

slip-fiber lineations - crystals grow in shadow of small steps on fault surface (hand specimen)

chatter marks - small steps in the fault surface, oriented perpendicular to slip direction, usually step down in direction of slip

 

Fault rocks

formed by brittle fracture during slip adjacent to or within plane of fault

gouge - fine grained, clay-rich, "rock flour" formed along the fault surface, <0.1 mm grain size

breccia - angular rock fragments in finer matrix, fragments more abundant than matrix, various sizes, from microbreccia (>0.1 mm - <1 mm) to megabreccia (>0.5 m fragments)

cataclasite - very fine grained, strongly indurated rock made up of rock fragments and matrix, typically formed under higher temperatures and pressures than breccias

pseudotachylite - dark, glassy rock formed as a product of frictional melting

 

Faults on maps

Truncation, offset, repetition or omission of rock units may indicate the presence of a fault.

if a fault plane is not observable, we may interpret its presence by the repetition or omission of strata which will depend on:

 

Fault Classification

faults may be named based upon the separation of units across the fault surface or the absolute sense of slip on the fault surface. The former is easier to determine than the latter.

Slip classification

Strike-slip faults

Dip-slip faults

Oblique-slip faults

Separation classification

 

How to determine slip on faults

1. using slickenlines, striations give direction of slip

cut-off - line formed by the intersection of a bed with the fault plane

2. using drag folds to determine slip

 

Strain significance of faults

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