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 B&R
Imagine state lines were strain markers
Approximate extension associated with part of the B&R 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)

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