Faults: Basics
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Faults: BasicsGoal: 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 earth’s 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 faultsa) Discrete, single plane
b) Zone of anastomosing, closely spaced faults (fault zone)
c) Wide zone of penetrative, plastic deformation
A 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 zones—most often plastic
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