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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
Starting point: rift to drift Note thinning of crust
and lithosphere Asthenosphere
interacts with crust Volcanism, normal
faults, high geothermal gradient
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
Intracontinental extension ‘Master faults’ are
normal faults Strike roughly
perpendicular to extension direction (exception: reactivation of older faults)
Magnitude of extension in B&R Imagine state lines
were strain markers Approximate
extension associated with part of the B&R is shown
Hamilton (1978)
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
Symmetry Two conceptual
models for extension Both have ductile
thinning at depth One has dominant
dip direction (synthetic with respect to detachment)
Metamorphic core complexes
Metamorphic core complexes Exposed in belt
extending from Canada into Mexico
Record greater extension than high-angle normal faults
Domino-style faulting Fault blocks rotate
with progressive extension
Syntectonic sediments record tilting with progressively changing dip
Note this requires detachment at depth
Drift structures Patterns recording
continental rifting preserved on both continental margins
Note that low-density salt can also participate in extension
Continental extension in 3D
Transfer faults Form ‘hard’ links
between normal fault segments with different magnitudes of displacement
Fault-related folds terminate at transfer faults
Gibbs (1990)
Folds related to dip-slip faults
‘Soft-linked’ normal faults Fault displacement
decreases toward tip Overlapping (en
échelon) fault tips produce relay ramp
Walsh and Watterson (1991)
Relay ramps effect sed transport
After Yielding and Roberts (1992)
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