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Faults: Basics

Faults: Basics. Goal : To understand and use the basic terminology for describing faults. Basic Terminology. Hanging wall and footwall : Come from 18th-century English coal mines. Dip-slip faults : Slip up or down the dip. Normal fault : Hanging wall down — indicates extension

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Faults: Basics

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  1. Faults: Basics Goal: To understand and use the basic terminology for describing faults.

  2. Basic Terminology Hanging wall and footwall: Come from 18th-century English coal mines

  3. 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

  4. Slip parallel with earth’s surface Typically have subvertical dip Sense of motion Dextral = right-lateral = right-handed Sinistral = left-lateral = left-handed Strike-slip faults

  5. Oblique-slip faults • Strike-slip and dip-slip components • Most faults are oblique-slip, but are often dominantly strike-slip or dip-slip

  6. 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).

  7. Dip-slip fault Strike-slip fault Same separation, different slip

  8. 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

  9. Character of faults • Discrete, single plane • Zone of anastomosing, closely spaced faults (fault zone) • Wide zone of penetrative, plastic deformation A B C

  10. Fault zone showing separation Near Sheep Creek, Utah

  11. 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

  12. 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

  13. Breccia/gouge zone

  14. Plastic fault rocks Protomylonite: Up to 10% dynamically recrystallized material Mylonite: 10–90% dynamically recrystallized material Ultramylonite: 90–100% dynamically recrystallized material

  15. 1 3 2

  16. Brittle-Plastic transition

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

  18. Visible off-set and damage zone

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