Lecture Index: Notebook terms./ Description of idealized fault components. / Fault zone rocks and structures. / Fault recognition at map scale. / A traditional fault classification. /

Readings:

• p. 145-168 Chapt. 8 Faults and faulting in van der Pluijm & Marshak, 1997, Earth Structure - An introduction to structural geology and tectonics. McGraw-Hill.
• suggested: Scholz, C. H., 1987, Wear and gouge formation in brittle faulting; Geology, v. 15, p. 493-495. This is an introduction to the scaling of various fault parameters.

• strike and dip
• hanging wall and footwall
• cutoff lines
• net slip, dip slip, strike-slip
• piercing points
• dextral vs. sinistral
• fault breccia, gouge, flinty crush rock, pseudotachylites
• breccias as polygenetic
• mylonites
• fault scarps
• striae, slickensides
• dilatancy
• drag folds
• types of fault terminations
• fault recognition criteria in the field
• Andersonian classification
• fault reactivation.

Faults or a portion of a fault can sometimes be idealized as a planar surface with a vector of slip in it and offset and truncated layers.

• In this case, the following components are used to describe the fault geometry:
  • strike and dip of fault plane.
  • orientation of slip vector.
  • magnitude of slip vector.
  • sense of motion.
  • cutoff lines and piercing points.
• Net slip = strike slip plus dip-slip component. This is just one place that trig functions come in useful.
• Offsets:
  • heave, throw and separation.
  • imp. distinction between separations and slip. You can have a horizontal separation and no strike-slip movement or a throw and no dip-slip movement.
• In reality faults are typically non-planar! This has some very important implications.
  • fault bend geometries. Often found in layered materials (laminate behavior).
  • listric: curved.
  • non-planar geometries require fault block internal deformation in addition to translation, or require significant fault zone dilation or volume loss.
• fault terminations:
  • offset dispersal splays (tri-shear termination?). A fault can be thought of as more than a surface, but more of a volume, one whose constraining surfaces do not need to be parallel.
  • transferrence (change of character).
  • hinged faults: implications for slip directions.
  • intersection with surface.
  • elliptical offset gradient patterns on indiviudal fault may be family of transferrence. structures below resolution.

Some faults are pretty much a surface, but most faults are manifest as a zone with distinctive rocks and features related to the fault movement within them. Below are some examples.

• breccias:
  • sedimentary breccias (e.g. talus accumulations).
  • solution/collapse breccias.
  • volcanic or intrusive breccias.
  • impact breccias.
  • fault breccias.
  • how do you distinguish between various types?
  • associated dilatancy.
• damage zones.
  • usually intense fracturing, but not alot of slip.
  • deformation bands in sands.
  • precursor or syn-slip?
• gouges.
• flinty crush rocks.
• pseudotachylite.
• variety of types of mylonite (discussed later since ductile in many ways).
• striae.
• slickensides.
• veining (fibrous especially informative).
• drag folds.

How do you recognize faults in the field?

• fault zone and rocks often not exposed.
• structural discontinuities do not necessarily equate to a fault:
  • intrusive contact.
  • unconformity.
  • fault.
  • composite.
• layer parallel detachments:
  • difficult to recognize.
  • disparity in deformation on either side.
  • presence of fault rock types, and associated structures.

Andersonian classification: This classification is based both on observation of what types of faults are common, and on theory guided by the idea that the earth's surface tends to shape fault orientations.

• normal faults.
• thrust faults.
• reverse faults.
• wrench faults.

• reverse faults and reactivation.
• kinematic classification:
  • contractional.
  • extensional.
  • strike-slip.
  • transpressional.

Fault images to feed your eyes.

Copyright Harmon D. Maher Jr., This may be used for non-profit educational purposes as long as proper attribution is given. Otherwise, please contact me. Thank you.