colloquium prague, april, 2005 1 a numerical approach to model the accretion of icelandic crust...

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A Numerical Approach to Model the A Numerical Approach to Model the Accretion of Icelandic CrustAccretion of Icelandic Crust

Gabriele Marquart and Harro SchmelingGabriele Marquart and Harro Schmeling

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Bathymetry in the North Atlantic

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Observations of crustal thickness

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Thickness of the Icelandic crust from Thickness of the Icelandic crust from Gravity and seismic DataGravity and seismic Data

Darbyshire,2000

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Crust is simply related to extracted melt

1. Model

1 cm/a1 cm/a

Streamlines

Melting rate

Extraction

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Numerical Model of a Rising Plume Numerical Model of a Rising Plume with Melting with Melting

Anomalous temperature

Melt production rate

melting

120 - 60 km depth

Melting zone

Rising velocity

(T. Ruedas)

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Predictions for crustal thickness

Texcess= 350 K (1%)Texcess= 250 K ( 0.1%)Texcess= 250 K (1%)

Texcess= 250 K (3%)

Texcess= 150 K (1%)

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Comparison to „observation“

Model crustDarbyshire

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Extrated material is fed back into the model

Width of emplacement zone 50 km (Gauß)

1 cm/a1 cm/a

Streamlines

Melting rate

2. Model

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Kinematic model ofPalmason, 1980

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Iceland Surface Tectonic FeaturesIceland Surface Tectonic Features

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Structure of the Crust in IcelandStructure of the Crust in Iceland

Seismic findings:- Distinct upper crust 5-10 km thick- Seismically fast lower crust down to 24-50 km- Poorly constrained transition to the mantle

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Crustal Structure from receiver Crustal Structure from receiver functionsfunctions

Receiver functionsLow Vp-velocities(10%) beneath 40 km

Schlindwein, 2001

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The model concept for crustal accretion• Extrusives• fissures, magma chambers• deep dykes and sills• Underplating

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1 cm/a1 cm/a

Streamlines

Melting rate

Extraction

Extracted melts are emplaced in a separate crustal model (with contstant rate...)

3. Model

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Modeling Crustal Accretion - Modeling Crustal Accretion - EquationsEquations

Energy conservation:

Momentum conservation:

Mass conservation:

txsHTTvtT

s ,)( 2

0312 zeTRavvp

22

,

,bzax eeAzxs

txs=v

Physical Equations

Source Functions

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Model assumptionsModel assumptions

► 2D2D► Constant viscosityConstant viscosity► Total accretion rate Total accretion rate 2 cm/s spreading rate 2 cm/s spreading rate► T of surface lavas: 100 KT of surface lavas: 100 K► T of magma chambers: 600 KT of magma chambers: 600 K► T deep dykes: 300 KT deep dykes: 300 K► 3 models:3 models:

1) Dominated (60%) by deep accretion1) Dominated (60%) by deep accretion2) Dominated (60%) by magma chamber accretion2) Dominated (60%) by magma chamber accretion3) Dominated (60%) by shallow accretion3) Dominated (60%) by shallow accretion

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Visualization of the Accretion of Crust Visualization of the Accretion of Crust

•Accretion is traced by markers•New markers are inserted at each time step•Color indicates the source •Number of markers is according to the strength of the source•Markers are followed up for 10 Ma, after 1Ma the color is changed •Marker positions are determined by a RK-4th order scheme

after 500 time steps

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Accretion dominated by deep dykes Accretion dominated by deep dykes (60% M(60% Mtottot))

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Accretion dominated by magma chambers Accretion dominated by magma chambers (60% M(60% Mtottot))

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Accretion dominated by surface lavas Accretion dominated by surface lavas (60% M(60% Mtottot))

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Comparison of Different Accretion Styles Comparison of Different Accretion Styles Deep dykes Lava flowsMagma chamber

-Uniformly stratified hot crust (Gabbro, mantle mix?)- thin seismogenic zone

- lateral variable crust- upper crust thinning in

central region- hot in central region- vertical layering of the middle crust

- cold crust - hot only in central region- downbuildung, with tilted

layering

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Krustenstruktur aus Seismik

Crustal structure at the rift axisCrustal structure at the rift axis

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Location of profiles

Comparison of Different Accretion Comparison of Different Accretion TypesTypes

Deep DykesTemperature

Vertical velocity

Horizontal velocity

Magma ChamberTemperature

Vertical velocity

Horizontal velocity

Surface LavasTemperature

Vertical velocity

Horizontal velocity

40 C/km 20 C/km30 C/km

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Comparison to the Seismogenic Crust in Comparison to the Seismogenic Crust in IcelandIceland

Lava flowsDeep dykes Magma Chamber

5 km

10 km

Depth: 20 km

South IcelandSeismic zone

Stefanson, 1998

Riftzone

50 km0 km

20 km

10 km5 km

20 km10 km

~ 500°C

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Location of profiles

Comparison of Different Accretion Comparison of Different Accretion TypesTypes

Deep DykesTemperature

Vertical velocity

Horizontal velocity

Magma ChamberTemperature

Vertical velocity

Horizontal velocity

Surface LavasTemperature

Vertical velocity

Horizontal velocity

40 C/km 20 C/km30 C/km-Strong vertical and differential

horizontal velocities

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Seismic Azimuthal Anisotropy from Rayleigh Seismic Azimuthal Anisotropy from Rayleigh waveswaves

Li & Detrick, EPSL200320-40 km 50-80 km

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► Thermal & geometric structure depends Thermal & geometric structure depends strongly on accretional modestrongly on accretional mode

► Iceland: shallow seismogenic zone, high Iceland: shallow seismogenic zone, high thermal gradient suggests deep or thermal gradient suggests deep or intermediate accretion (deep dykes and intermediate accretion (deep dykes and magma chambers) as the dominating processmagma chambers) as the dominating process

(However, (However, the seismogenic upper crust of 10-15 km is produced by shallow fissure swarm intrusions and subairial lava flows)

► Then only moderate differential velocities and Then only moderate differential velocities and mixing of the different accretion zonesmixing of the different accretion zones

Preliminary Findings for the Accreton of Preliminary Findings for the Accreton of Crust on Iceland Crust on Iceland