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Faults: Basics

Faults: Basics. Goal : 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.

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Faults: Basics

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  1. Faults: Basics Goal: To understand the basic terminology for describing faults and to recognize faults in the field.

  2. 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

  3. 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

  4. 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).

  5. Fault zones showing separation

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

  7. 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

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

  9. Fault kinematic indicators • Visible off-set of rock units • Shear-sense indicators: Rotated, asymmetric objects that form in fault zones—most often plastic

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