Faults: Basics
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Faults: Basics
Goal: To understand and use the basic terminology for describing faults.
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Basic TerminologyHanging wall and footwall: Come from 18th-
century English coal mines
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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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Oblique-slip faults• Strike-slip and dip-slip components
• Most faults are oblique-slip, but are often dominantly strike-slip or dip-slip
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Slip vs. Separation• Slip: Total movement along fault surface.
– Vector lying in fault surface
– Direction of vector (slip-line) expressed as trend and plunge or rake in fault plane
• Separation: Total apparent offset along fault when viewed in 2-D (either map or cross section).
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Same separation, different slipDip-slip fault Strike-slip fault
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To determine slip, you need a piercing point
– Piercing point: Line that intersects fault surface and is off-set by fault
– Match hanging-wall cutoff with footwall cutoff
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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 zone showing separation
Near Sheep Creek, Utah
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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
Watch deformation movies
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Frictional/brittle fault rocksFault gouge: Clay-sized particles
Fault breccia: Angular chunks surrounded by gouge and/or vein material
Cataclasite: Indurated version of fault gouge
Pseudotachylyte: Glass formed from frictionally generated melt
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Breccia/gouge zone
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Plastic fault rocksProtomylonite: Up to 10% dynamically
recrystallized material
Mylonite: 10–90% dynamically recrystallized material
Ultramylonite: 90–100% dynamically recrystallized material
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Brittle-Plastic transition
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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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Visible off-set and damage zone