Faults: Basics

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Faults Basics
Goal To understand the basic terminology for
describing faults and to recognize faults in the
field.
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• Dip-slip faults Slip up or down the dip.
• Normal fault Hanging wall down indicates
  extension
• Reverse fault Hanging wall up indicates
  shortening

Reverse
Normal
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Strike-slip faults

• Slip parallel with earths surface
• Typically have subvertical dip

• Sense of motion
• Dextral right-lateral right-handed
• Sinistral left-lateral left-handed

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Slip vs. Separation

• Slip Total movement along fault surface.
• Vector lying in fault surface
• Separation Total apparent offset along fault
  when viewed in 2-D (either map or cross section).

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Fault zones showing separation
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Character of faults

• Discrete, single plane
• Zone of anastomosing, closely spaced faults
  (fault zone)
• Wide zone of penetrative, plastic deformation

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B
C
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Fault Rocks

• Frictional/brittle fault rocks Mechanical
  disaggregation and grinding
• Plastic fault rocks Plastic flow of minerals at
  atomic scale
• grain-size reduction due to deformation-driven
  dynamic recrystallization
• Very strong foliation in fault zone

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Recognizing faults

• Truncation of rock units
• Visible off-set of rock units
• Omitted or repeated stratigraphy or
  biostratigraphy
• Juxtaposition of seemingly unrelated rock units

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Fault kinematic indicators

• Visible off-set of rock units
• Shear-sense indicators Rotated, asymmetric
  objects that form in fault zonesmost often
  plastic

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