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