Normal faults

1
Normal faults

• Dominate extensional tectonic environments
• Form locally in both convergent and transcurrent
  tectonic settings
• Form locally in response to removal or addition
  of material

2
Starting point rift to drift

• Note thinning of crust and lithosphere
• Asthenosphere interacts with crust
• Volcanism, normal faults, high geothermal gradient

3
Transform faults

• Transfer motion between mid-ocean ridge segments
• Movement sense dictated by variations in rate of
  extension can change along strike
• Parallel movement direction

4
Intracontinental extension

• Master faults are normal faults
• Strike roughly perpendicular to extension
  direction (exception reactivation of older
  faults)

5
Magnitude of extension in BR

• Imagine state lines were strain markers
• Approximate extension associated with part of the
  BR is shown
• Hamilton (1978)

6
Elements of an extensional system in cross section

• Note topography, producing sedimentary
  depocenters
• Detachment faults allow rotation of blocks
  bounded by high-angle normal faults

7
Symmetry

• Two conceptual models for extension
• Both have ductile thinning at depth
• One has dominant dip direction (synthetic with
  respect to detachment)

8
Metamorphic core complexes
9
Metamorphic core complexes

• Exposed in belt extending from Canada into Mexico
• Record greater extension than high-angle normal
  faults

10
Domino-style faulting

• Fault blocks rotate with progressive extension
• Syntectonic sediments record tilting with
  progressively changing dip
• Note this requires detachment at depth

11
Drift structures

• Patterns recording continental rifting preserved
  on both continental margins
• Note that low-density salt can also participate
  in extension

12
Continental extension in 3D
13
Transfer faults

• Form hard links between normal fault segments
  with different magnitudes of displacement
• Fault-related folds terminate at transfer faults
• Gibbs (1990)

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15
Folds related to dip-slip faults
16
Soft-linked normal faults

• Fault displacement decreases toward tip
• Overlapping (en échelon) fault tips produce relay
  ramp
• Walsh and Watterson (1991)

17
Relay ramps effect sed transport
After Yielding and Roberts (1992)
18
Duplexes may form in any (curviplanar) fault
system

• Note the association between fault-plane
  topography and duplexes
• Horses believed to form by lopping off
  irregularities on fault surface