Faults: BasicsGoal: To understand the basic terminology for describing faults and to recognize faults in the
field.
• 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
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
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).
Fault zones showing separation
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
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
Recognizing faults• Truncation of rock units
• Visible off-set of rock units
• Omitted or repeated stratigraphy or biostratigraphy
• Juxtaposition of seemingly unrelated rock units
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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