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. 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

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. Same separation, different slip Dip-slip faultStrike-slip fault

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 a)Discrete, single plane b)Zone of anastomosing, closely spaced faults (fault zone) c)Wide zone of penetrative, plastic deformation ABC

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 2 3

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