Faults: Basics addendum

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Faults Basics addendum
Goal To understand and use the basic terminology
for describing faults.
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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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Fault zones showing separation
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Same separation, different slip

• Dip-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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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

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Frictional/brittle faulting
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Plastic faulting
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Frictional/brittle fault rocks

• Fault 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 rocks

• Protomylonite Up to 50 dynamically
  recrystallized material
• Mylonite 5090 dynamically recrystallized
  material
• Ultramylonite 90100 dynamically recrystallized
  material

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Brittle-plastic transition
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Brittle-plastic transition