Faults: BasicsGoal: To understand the basic terminology for describing faults and to recognize faults in the

field.

• 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

Slip vs. Separation• Slip: Total movement along fault surface.

– Vector lying in fault surface

• Separation: Total apparent offset along fault when viewed in 2-D (either map or cross section).

Fault zones showing separation

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 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

– Very strong foliation in fault zone

Recognizing faults• Truncation of rock units

• Visible off-set of rock units

• Omitted or repeated stratigraphy or biostratigraphy

• Juxtaposition of seemingly unrelated rock units

Fault kinematic indicators• Visible off-set of rock units

• Shear-sense indicators: Rotated, asymmetric objects that form in fault zones—most often plastic