Faults: Basics

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

Basic TerminologyHanging 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

– 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

Oblique-slip faults• Strike-slip and dip-slip components

• Most faults are oblique-slip, but are often dominantly strike-slip or dip-slip

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).

Same separation, different slipDip-slip fault Strike-slip fault

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

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 zone showing separation

Near Sheep Creek, Utah

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

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

Breccia/gouge zone

Plastic fault rocksProtomylonite: Up to 10% dynamically

recrystallized material

Mylonite: 10–90% dynamically recrystallized material

Ultramylonite: 90–100% dynamically recrystallized material

1

2

3

Brittle-Plastic transition

Recognizing faults

• Truncation of rock units

• Visible off-set of rock units

• Omitted or repeated stratigraphy or biostratigraphy

• Juxtaposition of seemingly unrelated rock units

Visible off-set and damage zone