Download Normal faults Normal faults

3/19/15&
Normal faults
•  Accommodate extension
•  Andersonian orientation is 60° dip
•  Measure displacement, throw heave
Normal faults
•  Multiple faults
produce
characteristic
structures
–  Horst
–  Graben
–  ‘Master faults’
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Listric normal faults
Gulf&of&Oman&
North Sea faulting styles….
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Normal faults
•  Displacements
often
measured with
fault cutoff
lines
•  In map view,
the fault is
defined as a
gap
Normal faults
•  Extension
typically causes
missing sections
in stratigraphy
•  Unless fault dips
more steeply
than wellbore
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Faults don’t continue on forever
•  Block diagrams are
convenient… but
schematic
•  Fault displacement
dies out near fault tips
1m
Big Hole fault, Utah
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Big Hole fault, Utah
Big Hole fault, displacement profile
Shipton&and&Cowie&&
(2001)&
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Fault growth
Growth of an ideal normal fault
As faults accumulate displacement they propagate
Fault growth (porous rock)
Shipton&and&Cowie,&2001&
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Fault growth (crystalline rock)
Martel&(1990)&
Damage zone processes
•  As soon as the slip surface nucleates,
the early deformation bands become
the “damage zone”
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Fault populations
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Normal fault systems
East Pacific Rise
North Sea
Basin and Range, USA
Brazil
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How do faults interact?
Relay ramps
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Displacement profiles and linkage
Linkage in the slip direction
•  Faults propagate in slip direction too
•  Overlapping, underlapping equivalents
exist, but complicated because restraining/
releasing geometries
•  Linkage facilitates growth
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Fault linkage and hydrocarbons
Fault linkage and hydrocarbons
Walsh & Watterson (1991)&
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Growth faults
•  Normal faults
create
accommodation
space for
sedimentation
•  The extent and
thickness of a
given unit
represent the
size and
displacement in
an interval of
time
Growth faults
58 0 40'N
Great Glen
Fault
Wick
Wick Fault
•  Example from the
Morray Firth, North
Sea
cross-section
Smith Bank
20 km
Fault
Helmsdale
Fault
Beatrice
57 0 35'N
40 W
NW
Smith Bank Fault
5 km
30 W
20 W
SE
1 km
Helmsdale Fault
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How do fault populations grow through time?
Final fault
system
Active
faults early stage
(b)&
N
Active
faults late stage
From the Inner
Moray Firth
Beatrice Field
(Walsh et al.
2003)
2
k
m
(c)&
Evolution of fault populations involves the death of
small faults and the localization of displacement
(strain) onto larger faults.
Fault communication and sealing
•  Faults affect fluid flow!
•  Predict their effects (in space and
time)
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Movie&by&Schmatz&and&Urai&(RWTH&Aachen&University)&
Normal faults in Anderson’s world
•  If σ1 is vertical, normal faults should
dip ~60°
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Common features of range fronts
uplifted by normal faults
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Domino model
•  Rigid fault-bounded blocks, all rotate
•  Predictions:
–  No block internal strain
–  Faults and layers rotate simultaneously at
same rate
–  Faults have same offset, dip
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Evolution of domino systems
•  Geometric problems with dominos
•  When blocks rotate too far, new
“normal” normal faults form
Soft domino model
•  Soft dominos allow internal strain
–  System of faults of all sizes and
displacement amounts
–  Folding can occur
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Dominos or horsts/grabens?
•  Dominos
promoted by
presence of a
weak basal layer
(detachment)
•  Regional tilting
(of detachment)
The problem of Low Angle Normal Faults
1: they exist
•  Dip ≤ 30° – NOT Andersonian
•  Slipped 5–50 km
•  Cut mid-crust rocks so not surficial
(landslides)
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The problem of Low Angle Normal Faults
2: they appear to have formed at low angle
•  Reconstructions show low angles between
detachment and footwall bedding, preexisting thrusts
•  Often not cross-cut by later, steeper
normal faults so not dominos
The problem of Low Angle Normal Faults
3: they are not seismically active
•  V. few earthquakes on faults dipping
≤ 30° are observed
•  Mechanically unlikely to be
reactivated (slip more)
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LANF’s form as low angle normal faults?
•  Pre-existing thrusts at low angles
could be weak and fail in extension if
the tectonic regime changes
LANF’s and metamorphic core complexes
•  Extension at the scale of the crust
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LANF’s and metamorphic core complexes
•  Model predicts
–  Faults of
different ages
–  Possible
opposing dips
–  Heat flow higher
in footwall
–  Transition from
ductile to brittle
deformation
Turtlebacks in LANF footwall
Photo&from&Casey&Moore&
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LANF’s form shallow?
•  Some LANF’s cross-cut steeper
dipping normal faults
•  Requires a weak detachment / high
pore fluid pressure
Extensional systems: rifting
•  Active
•  Passive
•  Pre-, syn-,
post
accompany
thinning and
subsidence
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Rift types: McKenzie model
•  Pure shear, symmetric
•  Thinning in response to horizontal
extension
(Lister(and(Davis,(1989)(
Rift types: Wernicke model
•  Simple shear, asymmetric
•  Controlled by a shallowly dipping
zone of localized simple shear
(Lister(and(Davis,(1989)(
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Which is it??
A note on fault populations
•  Power law distribution for number of
faults in a rift system
•  Given an observed number of faults of
size X, can we predict all the others?
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Extensional systems
•  Mid ocean ridge spreading centers
Ultramafic oceanic core complexes
AtlanQs&Massif&
• 
• 
• 
Large striated domal feature located along
a non-volcanic section of the Mid-Atlantic
Ridge
Thought to be an exposed detachment fault
that is bringing peridotite and gabbro to the
surface
Fault displacements in these detachments
can be too large to structurally restore. New
crust must have been created at the base
of the uplifting block during life of the
detachment fault.
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Extensional systems throughout the
Wilson cycle
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