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