Discrepencies

1
Discrepencies

• Different researchers use different cut-off
  points for what we term low-angle faults. We will
  go with the text definition of dips lt30
• Fault workers commonly refer to lenses in fault
  zones as horses when they are m-km scale
  structures
• Triangular features developed by erosion of fault
  surfaces on fault-bounded mountain fronts are
  fault facets. Flatirons are morphologically
  similar structures developed by erosion of tilted
  sedimentary layers

2
Finding fault(s)

• How to evaluate whether a fault is present in the
  subsurface or an area of poor exposure

3
Fault .but may be

• Abrupt lithologic change
• Abrupt change in orientation of bedding or
  foliation
• In both cases, look for evidence of fault-zone
  deformation with proximity to contact

• Nonconformity - look for evidence such as basal
  conglomerate
• Angular unconformity (see above) or Fold (look
  for lithologic continuity)

4
Use of stratigraphy

• Faults can cut out (omit) or stack (repeat)
  stratigraphic intervals. Stratigraphic omission
  or repetition can be evident through mapping or
  drill core.

5
Sense of slip in the absence of piercing points

• Relative ages of hanging wall and footwall
  materials constrains sense of slip
• Metamorphic grade of hanging versus footwall
  constrains kinematics

6
Heterogeneity and anisotropy

• Fractures typically nucleate on heterogeneities
  (e.g., fossil)
• Low-angle reverse, or thrust, faults localize in
  mechanically weak layers such as shale and
  evaporite. They cut across stronger layers in
  short steps
• As long as it is favorably oriented, it is
  apparently energetically easier to reactivate an
  existing fault than create a new one

7
Pattern has meaning

• Large-scale fault patterns reflect tectonic
  environments (plate movements)
• Patterns reflect most energetically favorable
  response to imposed displacement
• We will look at overall strains and patterns of
  faulting in a different order than indicated in
  syllabus extensional, transcurrent, then
  contractional regimes