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EARS5136 slide 1
EXTENSIONAL FAULT GEOMETRY AND EVOLUTION
EARS5136 slide 2
Predict flow patterns and communication
Fault compartments in the Sleipner field, Norwegian North Sea
Different oil-water contacts
Ottesen Ellevset et al. (1998)
EARS5136 slide 3
Fault Properties
• Fault Length (strike)
• Fault Throw : Height (Dip)
• Fault Segmentation and Linkage.
• Fault Zone Geometry (individual fault scale)
• Fault Array Geometry (area / basin scale)
• Fault Activity
EARS5136 slide 4
EARS5136 slide 5
Fault Shape & Length:Displacement Properties
EARS5136 slide 6
Normal fault geometry in 3D
EARS5136 slide 7
West Africa: lower fault tips & conjugate faults
EARS5136 slide 8
Low throw normal faults
EARS5136 slide 9
Osborne(1990)
SINGLE FAULTSSkua field in Timor Sea, NW Australia
• Footwall high
• Systematic heave polygon shape– Taper from zero width
at fault tips (low or zero fault displacement) to a maximum width near the fault center.
• Maximum uplift near center of fault.
EARS5136 slide 10
SINGLE FAULTS
Schlische, 1995
Map view
Transverse section through hanging wall
Footwall anticline:
• Cross-section A-A’ shows form of footwall anticline.
• Maximum uplift near center of fault.
Hangingwall syncline:
• Maximum structural low near center of fault.
Beatrice Field, N. Sea
EARS5136 slide 11
Structure contours around an isolated normal fault
1km
EARS5136 slide 12
Single fault from surface exposure in central Oregon
SINGLE FAULTS: DisplacementThrow and Separation.
• Maximum separation (throw) near center of fault.
• Gradual taper of separation profile from a maximum separation to zero at the fault tips.
EARS5136 slide 13
SINGLE FAULTS: Displacement
Gulf Coast normal fault
Contours on fault surface of separation measured from 5 horizons intersecting the fault.
Separation varies across fault surface like that on an individual horizon: the maximum separation occurs near the center of the fault.
EARS5136 slide 14
SINGLE FAULTS: Displacement• Contours of throw projected onto fault surface.
• Elliptical fault shape most common for buried faults.
EARS5136 slide 15
Higgs and Williams, 1987
SINGLE FAULTS: Displacement
• Similar displacement profiles along dip and strike profiles.
• Homogeneous, isotropic material
EARS5136 slide 16
a
b
IsolatedNorth Sea fault
Throw increases with depth but only upper part of fault mapped
EARS5136 slide 17
Fault surface topography
EARS5136 slide 18
0.1 1 10 100Strike d im en sio n (km )
0.1
1
10D
ip d
ime
nsi
on
(km
)
D erbysh ire coa lfie ld
G ulf C oast
T im or Sea
N orth Sea
Fault dimensions
1:1 3:1
From Nicol et al. (1996)
EARS5136 slide 19
Fault dimensions
• Aspect ratios average 2:1 but variable
• Linear throw gradients on isolated faults
• Non-linear on restricted faults
• Steeper gradients near overlapping tips
EARS5136 slide 20
Fault aspect ratios
Location Average aspect ratio
Derbyshire coalmines UK 2.3
Timor Sea 2.2
Gulf Coast, USA 1.6
North Sea 2.4
From Nicol et al. (1996)
EARS5136 slide 21
Deformation around a fault
• Reverse ‘drag’ profiles generate:– footwall uplift – hangingwall subsidence
• Relationship of structure contours to fault vary with slip direction
• Reverse drag does not imply a listric fault
• Earthquake related elastic strains relax to permanent bed deformation
EARS5136 slide 22
Cross-section
Reverse Drag
• Displacement (structural relief) decreases asymptotically away from fault in cross-section.
• Footwall high and Hangingwall low.
Schlische, 1995
EARS5136 slide 23
Earthquake induced deformation
< Imperial Valley earthquake 1940
Slip = e-3.5dist – 0.03dist Strike-slip illustrates offsets
Borah Peak earthquake 1983 >Lost River fault, Idaho
Strike-slip
Normal
EARS5136 slide 24
Deformation around a fault
Radar interferometry image of ground deformation induced by Hector Mine earthquake
Peltzer et al.http://www-radar.jpl.nasa.gov/sect323/InSar4crust/HME/
EARS5136 slide 25
Fault drag profiles
Empirical relationships for:
Single event:Slip = e-3.5dist – 0.03dist
Multiple event steady state:Slip = e-5.5dist – 0.004dist
From Gibson et al. (1989)
EARS5136 slide 26
Hangingwall & footwall displacement
• Same in hangingwall and footwall for blind faults
• Greater hangingwall subsidence than footwall uplift for synsedimentary faults
• Percentage contribution of hangingwall displacement (HW) is given by:
HW = 110 – 2/3Where is fault dip and dip exceeds 30 degrees
EARS5136 slide 27
Patterns around synsedimentary faults
EARS5136 slide 28
Patterns around synsedimentary faults:a local example
The Craven fault zone
EARS5136 slide 29
Normal drag in footwall to a 6m throw normal fault
Normal drag profiles often highly localised around fault
EARS5136 slide 30
SINGLE FAULTS
• Displacement Summary:– Maximum separation near center of fault.– Uniform displacement contour distribution on fault
surface.– Multiple horizons cut by a single fault:
• greater separation on horizons near center of fault.• similar separation profile shape.
– In section, the fault drag decreases gradually away from fault trace.
EARS5136 slide 31
MULTIPLE FAULTSEn-echelon Normal Faults.
Faults interact and influence deformation of adjacent faults.
Schlische, 1995
EARS5136 slide 32
Peacock and Sanderson (1991)
MULTIPLE FAULTS
Fault overlap in map view and section view.
EARS5136 slide 33
Flamborough Head, UK
Peacock and Zhang, 1993
MULTIPLE FAULTS
Mechanical Stratigraphy: outcrop
• Fault tip overlap in cross-section.
EARS5136 slide 34
1
2
H2b
3
4b
RHOB-NPHI
From Rives & Benedicto 2000
Vertical Segmentation
Seismic Interpretation.
Faults nucleate in more brittle sandstones.
EARS5136 slide 35
Different styles of transfer zones (Morley 1990)
EARS5136 slide 36
Relay Ramps
ShadedRelief
Time
EARS5136 slide 37
Relay ramps – seismic mapping
EARS5136 slide 38
Relay ramp
EARS5136 slide 39
Geometric Coherence
Length (km)0 10 20 30
2000
1600
1200hangingwall cut-off
splay
grabenBroad Footwall High ("Trap")
relay ram ps
splays
Strike Projection of Horizon ThrowE
leva
ti on
(m
ete
rs)
• Multiple faults act as a composite zone.• Displacement on composite faults creates
broad footwall uplift.• Separate fault segments compartmentalize
the trap.
Structure map
• Systematic throw variation.• Composite throw summed
like a single fault.
Relays mechanically interconnect to form longer fault.
Fault tracesrelay
relay
EARS5136 slide 40
Fault shape & linkage
EARS5136 slide 41
a
b
Displacement pattern – correlated single fault2D-seismic data set – Middle East
EARS5136 slide 42
b
aDisplacement pattern – correlated multiple faults2D-seismic data set – Middle East
EARS5136 slide 43
Displacement patterns on overlapping faultsfrom Childs et al. (1995)
EARS5136 slide 44
Geometric coherence
A B
Relay ramp structure and displacementpatterns on overlapping faults. Summed throws give a coherent pattern.From Huggins et al. (1995)
EARS5136 slide 45
• Low gradients at fault zone terminations.• Large gradients in fault overlap.• Summed profile resembles single fault.• Maximum throw near composite fault
center.
Walsh and Watterson (1990)
Throw profiles for main fault segments
Summed throws on fault segments
MULTIPLE FAULTSSegmented Fault Array Nook
Colliery, Lancashire
EARS5136 slide 46
Hard linked faults (Krantz 1988)
EARS5136 slide 47
Exercise
• Longbranch fault interpretation
EARS5136 slide 48
Childs et al., 1995
Composite Fault Throw
• Symmetric throw distribution.
FAULT 2
FAULT 1
Strike Projection of Throw
• Asymmetric throw distribution.• Throw gradient greatest in region of
overlap.
Fault traces: Map
Displacement VariationFault Throw Distributions: Northern N. Sea.
Separation Diagram
FAULT 1FAULT 2
• Possible breached relay at arrow: hard link.
EARS5136 slide 49
Walsh and Watterson, 1991
Cross-section tie
StrikeProjection
Splay excluded Splay included
Splay
Cross-SectionGeometric Coherence
Distinct anomaly in throw contours without splay.
EARS5136 slide 50
MULTIPLE FAULTS:
Nicol et al., 1995
Fault laterally restricted
Presence of deep shaded fault restricts propagation of contoured fault.
EARS5136 slide 51
From Bouvier, 1989
Cross-section
Map ViewMULTIPLE FAULTSThree dimensional geometry
• Multiple fault segments shallow in the section map as a single fault segment at depth.
EARS5136 slide 52
MULTIPLE FAULTS• Overlapping en-echelon fault segments comprise larger fault
zone.
• Overlap between segments is small.
• Aggregate slip like that for an isolated fault:
Geometric Coherence– Maximum slip near center of fault zone.
• Deformation transferred between fault segment across relay ramp.
• Anomalous patterns in slip indicate perturbations in deformation, which can indicate closely-spaced faults or fault connections.
• Echelon faults at one level may link as a single fault at another.
EARS5136 slide 53
Geometric coherence
From Walsh et al. (2003)
EARS5136 slide 54
Fault displacement profiles
From Nicol et al. (1996)
EARS5136 slide 55
0.1 1 10 100Strike d im e n sio n (km )
0.1
1
10
Dip
dim
en
sio
n (
km)
Vertica lly restric ted
Latera lly restricted
U nrestricted
2.51.3
Restricted faults
EARS5136 slide 56
Hard-linked splays
EARS5136 slide 57
Conjugate faults: field & seismic
EARS5136 slide 58
Conjugate faults
EARS5136 slide 59
Allan diagrams