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ENVS337: Lecture 2(& spill-over into Lecture 3)

Half Graben dynamics

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Graben systems: Understanding

Fault systems

Geomorphology

Sediment routing

Basin fill patterns

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Graben systems: Overview

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Earths internal structure

Key point : a low velocity zone between the crust and

mantle-lithosphere allows for dynamic coupling

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Earths internal structure

Continental crust: 30-50 km thickMantle density (m) > 3.3 g /c c

Cont inental crust densi ty (cc) ~ 2.7 g /c c

Oceanic c rust densi ty (oc) > 2.8 g /c c

Oceanic crust:

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Isostasy and flexurereminder

Buoyancy controls an equilibrium

between the lithosphere and thedenser aesthenosphere

The lithosphere sinks whenloaded - and recovers when theload is removed (Isostaticrebound)

Flexure may occur in responseto distant loadsdepending onlithosphere rigidity

Topographic variation at theearths surface is accommodatedby either:

Change in average crustaldensities

Change in crustal thickness(Airy isostasy)

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Isostasy & exhumation

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Isostasy & exhumation

It takes t ime for isostat ic compensat ionto oc cur. This time scale is dependant on

the rigidity of the lithosphere and the

viscosity of the mantle

EXAMPLE: present-day Scandinavia

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Terminology

Surface upl i f t: a change in the absolute elevation of the earths

surface (tectonic, isostatic, denudation, deposition, compaction)

Rock upl i f t : displacement of rocks relative to some reference level

Exhumation or d enudation: displacement of rocks with respect to

the Earths surface (removal or erosion of overburden)

m

cmDh

)(

D = Denudation/exhumation

h = mean elevation change

of the surfacem,c= Mantle and crustal

densities

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Terminology

m

cmDh

)(

D = Denudation

h = mean elevation

change of the surfacem,c= Mantle and crustal

densities

Surface uplift = Rock uplift - Exhumation

Applies to us

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Who cares about exhumation?

What mechanisms caused exhumation?

What is the timing of exhumation? (unconformities, ages)What is the spatial extent and magnitude of exhumation?

How might it have affected the hydrocarbon play?

Could the eroded material act as a good reservoir? Where

did it go? How much?

.... Landscape dynamicists & Hydrocarbon industry

Offshore mid -Norway

Some quest ions to consider :

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A con t inental r if t system

Thinned c rust, f lexu ral f lank up l i f t , B-D transi t io n, underplat ing

1. Replacement of crust by air; 2.Replacement of crust by more

dense mantle lithosphere; 3.Replacement of mantle lithosphere

by normal or anomolously hot mantle material.

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Predominantly normal (dip-slip) faults

Characterised by thin-skinned arrays ofrotational blocks

Faults may extend into the mid-crust (brittleductile transition zone)


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Burbank and Anderson, 2011, Tectonic Geomorphology, Chapter 4

Graben fault systems

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Fault thickness

Rif t ing Thick-skinned

Fault systems penetrate

the lithosphere - ductile

lower crust

Grabens

Thin-skinned faulting

Upper to mid crust

Domino-style

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Graben fault systems

NOTE: Upl i f t du e to flexure on the flanks

Numerical model results Field-informed results

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Graben fault systems: features

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Exposed faults are steep, up to 4060 degrees Faults are planar down to the brittleductile transition

Length scale depends on the local crustal strength (i.e. oldtough crust produces longer faults)

Up to 50 km in length (typically 1015 km)

Andes


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USA Basin and Range

(back-arc basin)

Initiated ~15 Ma

Western USA Basin andrange area centring onNevada

~ 1000 km wide

~ 400 tilted blocks and

associated half grabens Uplifted areas

(FOOTWALLS) eroded to fillvalleys (HANGINGWALLS)

New York

LA

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Basin & Range: Death Valley

Typical features of a

graben system with

inland drainage (i.e.

alluvial fans, lake basins,

ephmeral lakes,

evaporites)

Local and regional scale

drainage patterns

dictated by basin andrange topography

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HWFW

Lateral displacement variation

Displacement map fora single seismic event

Radar interferogramillustrating the heightdifferences pre- andpost fault movement

Note decay ofdisplacementmagnitude withdistance

Afar region, Ethiopia

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Central Idaho

A single event will causesurface displacement ofmms-ms. Overgeological time thecumulative effect ofthese events is to form a

hangin-wall basin

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Footwalls produce uplands (sediment source)

Graben basins form above the hanging walls The ratio of footwall uplift to hanging wall subsidence in

bedrock faults is 1:6 (due to back-tilting)

Extensional also accomplished by aseismic (fault creep)

listric faults that fail under the influence of gravity


DOMINO-STYLE ARRANGEMENT

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Lateral displacement & topography variation

Displacement, both subsidence and

uplift attains maximum values in themid sectors of faults and decreasesto the lateral tips

Faults grow laterally as well asvertically

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Growth and accommodation

Maximum displacement occurs at the centre of the fault

Hanging wall accommodation space increases laterally - andsimultaneously with basins deepening

Rate of accommodation space creation increases over time

Continued rotation of fault block leads to hanging wall onlap,and increasing stratal dips with depth

Accommodation(t) & supply(t) dictated by landscape response

Sch lisc he et al., 2003

Foot wall scarp

Hanging wall

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Displacement-length scalingMax. displacement (and so

max. accommodation) atcentre-point along fault.

There is a geometrical

relationship between

length and displacement.

Great, a valuable

predictive tool.

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Displacement-length scaling

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Displacement variation

Plan view of faultdisplacement patterns

for a number of

scenarios where fault

propagation might be

inhibited

Implications forsedimentary

successions.

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The fault array (not a single fault)

Take place at relay zones (relay ramps)

Area between the tips of adjacent faults

Implications for the transport of sediment

Sch lis ch e et al., 2003

Inter-basin

transfer zones

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Fault array linkage

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Local -depositional systems are sourced directly fromthe footwall

Regional -distantly sourced systems enter the basinvia relay zones

3D Seismic time sliceof fault geometry and

topography

Fault array linkage

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Catchment area relationship with fault dynamics. In case C-D,

can incision keep pace with uplift on the footwall of the link fault?

Sediment supply & routing

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Fault interaction and evolution

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Relay ramps often connect to larger catchments

leading to higher discharge of water and sediment

Note the smaller scale, footwall sourced fans to the

right of the main fan

Badwater Fan (Death Valley)

Large scale alluvial

fan entering an

extensional basin

via a relay ramp

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Cowie et al. (2006)


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Cowie et al. (2006)

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Sedimentary architecture

of graben basins

Thaumasia graben sy stem , Mars

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REPEAT: Growth & accommodation

Maximum displacement occurs at the centre of the fault

Hanging wall accommodation space increases laterally - andsimultaneously with basins deepening

Rate of accommodation space creation increases over time Continued rotation of fault block leads to hanging wall onlap,

and increasing stratal dips with depth

Accommodation(t) & supply(t) dictated by landscape response

Sch lisc he et al., 2003

Foot wall scarp

Hanging wall

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Gupta et al. (1998)

Tectonic subsidence (t) for a

well in the Suez Rift. Note the

slow rate of tectonic subsidence

(rift initiation) followed by anabrupt increase (rift climax)

Characteristic feature of most rifts that is controlledprimarily by fault interaction and linkage


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Gupta et al. (1998)


Modelling of fault growth and

nucleation. Note that some faults

form and then die (fault E)Displacement id taken up my

linkage of faults A-D

What implications for stratigraphy?

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Gupta et al. (1998)

Fault interaction & stratigraphy

Syn-rift successions from 3D seismic

Thickness variations governed by progressive fault

displacement and fault propagation

PREDICTION, PREDICTION, PREDICTION .....

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Gupta et al. (1998)


Interpreted seism ic pro f i le of un its of interest

Thinning of strata over the footwall crest

Minor thickening of strata towards the fault in hanging-wall (early syn-rift)Major thickening of strata (rift climax) - Overying post-rift

Abrupt increase in the rate of tectonic subsidence, resulting in the development

of a deep-marine basin (due to ongoing linkage).

F lt i t ti & t ti h

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Inner Moray Firth


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Seismic section through a half graben rift

basin, Barents Sea, North Norway

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Interpretation

Basin bounding faults and eroded footwall scarps

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Interpretation

Early syn-rift deposition

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POST RIFT

FAULTED

PRE-RIFT

Interpretation

Note increasing dips with depth

Onlap of syn-rift horizons onto hanging wall

H lf b ti t l tti

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Leeder & Gawth orp e (1987)

Half graben: continental setting

Inland drainage settings(especially arid areas)

Small basins isolated by

structural highs

Examples: Death valley,

Lake Malawi, Dead Sea rift

Sedimentary sequences

consist of coarse, footwall

sourced fans build into the

half graben during periods offault quiescence, lacustrine,

playa, transverse channel

elements

without an axial river system

H lf b ti t l tti

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Leeder &

Gawthorpe 1987

with an axial river system

Areas of higher

rainfall

Tributaries fed

through relayzones

Stream position

influenced tilting

rate and alluvial

fan development


H lf b ti t l tti

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Featureless and flat basin floor that is typical of

southern part of Basin & Range


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Leeder & Gawth orp e (1987)

Half graben: coastal / lacustrine

Steep footwalls

Characterised by

alluvial fans and delta

systems

H lf b i tti

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Llave et al. (2008)

NS trending graben in

marine environment

Mesozoic, reactivated by

Pyrenean events

Steep upper slopes

characterised by massmovement deposit

Lower slopes feature

sedimentary lobes

Half graben: marine setting

Galicia Bank (west of Iberia Peninsula)

Site of Prestige Tanker

(sank 2002)

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Leeder &

Gawthorpe 1987

Half graben: marine carbonate

Reduced or absentsiliclastic input

Older - thermallysubsiding riftsystems?

Steep footwallescarpment proneto collapse

Carbonate ramps

may develop onhanging wall slope

A d ti l (A/S)

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Accommodation-supply (A/S)

Accommodation supply (A/S)

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Accommodation-supply (A/S) AInput of coarse grained

talus cone and fine river

sediment remains in equilibrium

Btalus cone dominated.

Coarse grained sediment builds

out across graben and may

onlap hanging wall CRiver dominated. Talus

cones less dominant. Fine

grained sediment dominates

graben

C: subsidence > sediment input

B: subsidence < sediment input

A: subsidence = sediment input

Facies distribution depends on the

relationship between subsidence

rates and sediment input rates >>

MASS BALANCE

H i ll b i

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Hanging wall basin Displacement (structural relief) decreases asymptotically

away from fault in cross-section

Steep footwall scarp Erosion

Unstable prone to landslides

Rapid decrease of gradient into basin rapid deposition

Leads to: Talus cones

Alluvial fans

Fan deltas

A d ti l (A/S)

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Tectonics

Climate

The nature of sediment deposited in basins depends on first

order var iat ions in sediment released from upland catchments

Accommodation-supply (A/S)

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Arm itage et al.(2011) Nature Geosc ience

Simple catchment-fan LEM

Response time ca. 106yr

Time lines every 500 kyr in

hanging wall basin (fan).

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Changes rate of

precipitation

Armitage et al., Nature Geoscienc e

INCREASE: instantaneous& spiked response in

sediment flux that generates

a coarse gravel sheet across

the basin

DECREASE: Rapid &small response in sediment

flux that generates minorback-stepping of units

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Changes in rate of

tectonic uplift

GENERAL: New steadystate sediment flux,

change in deposition rates

across marker surface

Sedimentation respondsinstantaneously to changes

in fault uplift rate, but

erosion in the catchmentresponds transiently

RSL effects on half graben strat

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Gawthorpe et al. (2003)

RSL effects on half graben strat.

Eustatic SL curve

We wil l re-cap o n

seq.strat conc epts in

anot her lec ture ...

Model of rift basin development

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Model of al luvial architecture in evo lving extens ional basins

(Gawth orp e and Leeder, 2000)

In i t iat ion stage


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Model of al luvial architecture in evo lving extens ional basins

(Gawth orp e and Leeder, 2000)

Interact ion & l inkage stage


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Fault death stage

Model of al luvial architecture in evolv ing extension al basins

(Gaw tho rpe and Leeder, 2000)


Rift basin development

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Tilt situation for

(a) the syn-rift

and (b) post-rift

stages and (c)

main structural

features of a

mature graben

(Gabrielson et al.

2001)

Cont inued thermal

su bs idence .......



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SOURCE ROCKS

RESERVOIR ROCKS

Is that it?

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Is that it?Post-rift topography still has a control on depositional

systems ....... deepwater systems atleast.

Topography is maintained by differential compaction of

mudstone over the crests of Jurassic footwall blocks

Summary

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SummaryThe lithosphere-mantle system interact over geological time-

scales to absorb changes experienced by the brittle-crust >>

deflection of the earths surface.

Faults are not singular but form an array. The interaction of

individual faults in this array significantly influences catchments

dynamics, and therefore sediment supply.

This interaction is not just of interest to structural engineers,

seismologists and Quaternary scientists, but is of importance to

the hydrocarbon industry as it offers predictability (i.e. timing,

length scales, A/S concepts).

Sea level will also significantly impact upon the sedimentary

architecture of rift basins (& half-grabens), but it is the structural

template that is important for hydrocarbon exploration.

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References from lecture

Leeder, M.R. & Gawthorpe, R.L., 1987. Sedimentary models for extensional tilt block/ halfgraben basins: Geol Soc London Spec Pub 28, p. 139-152.

Llave, E., et al., 2008., Morphological feature analyses of the Prestigehalf-graben on the SWGalicia Bank: Marine Geology, v. 249/1-2, p. 7-20.

Payne, S.J., McCaffrey, R., King, R.W., 2008, Strain rates and contemporary deformation in theSnake River Plain and surrounding Basin and Range from GPS and seismicity, Geology(Boulder), v. 36/8, p.647-650.

Schlische, 2003, Structural and stratigraphic development of the Newark extensional basin,eastern North America; Evidence for the growth of the basin and its bounding structures

Cowie et al. (2006) Investigating the surface process response to fault interaction and linkageusing a numerical modelling approach, Basin Research, 18, 231266

Gupta et al. (1998) A mechanism to explain rift-basin subsidence and stratigraphic patternsthrough fault array evolution. Geology, 26, 595-598.

Gawthorpe, R.L. & Leeder, M.R. (2000) Tectono- sedimentary evolution of active extensionalbasins. Basin Research, 12, 195-218.

Gawthorpe et al (2003) Numerical modelling of depositional sequences in half-graben rift basins.Sedimentology 50, 169185.

1 lecture b

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