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A Numerical Approach to Model the Accretion of Icelandic Crust Gabriele Marquart and Harro Schmeling Colloquium Prague, April, 2005 1 Bathymetry in the North Atlantic Colloquium Prague, April, 2005 2 Observations of crustal thickness Colloquium Prague, April, 2005 3 Thickness of the Icelandic crust from Gravity and seismic Data Darbyshire,2000 Colloquium Prague, April, 2005 4 1. Model Crust is simply related to extracted melt Extraction 1 cm/a 1 cm/a Streamlines Melting rate Colloquium Prague, April, 2005 5 Numerical Model of a Rising Plume with Melting Anomalous temperature Rising velocity Melting zone melting Melt production rate 120 - 60 km depth (T. Ruedas) Colloquium Prague, April, 2005 6 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%) Colloquium Prague, April, 2005 7 Comparison to „observation“ Model crust Darbyshire Colloquium Prague, April, 2005 8 2. Model Extrated material is fed back into the model 1 cm/a Width of emplacement zone 50 km (Gauß) 1 cm/a Streamlines Melting rate Colloquium Prague, April, 2005 9 Kinematic model of Palmason, 1980 Colloquium Prague, April, 2005 10 Iceland Surface Tectonic Features Colloquium Prague, April, 2005 11 Structure 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 Colloquium Prague, April, 2005 12 Crustal Structure from receiver functions Receiver functions Low Vp-velocities (10%) beneath 40 km Schlindwein, 2001 Colloquium Prague, April, 2005 13 The model concept for crustal accretion • Extrusives • fissures, magma chambers • deep dykes and sills • Underplating Colloquium Prague, April, 2005 14 3. Model Extracted melts are emplaced in a separate crustal model (with contstant rate...) Extraction 1 cm/a 1 cm/a Streamlines Melting rate Colloquium Prague, April, 2005 15 Modeling Crustal Accretion - Equations Physical Equations Momentum conservation: 1 p 2 v v Ra T ez 0 3 Source Functions Mass conservation: v = sx , t s x, z A e ax 2 e bz 2 Energy conservation: T 2 (v )T T H s sx , t t Colloquium Prague, April, 2005 16 Model assumptions ► 2D ► Constant viscosity ► Total accretion rate 2 cm/s spreading ► T of surface lavas: 100 K ► T of magma chambers: 600 K ► T deep dykes: 300 K ► 3 models: rate 1) Dominated (60%) by deep accretion 2) Dominated (60%) by magma chamber accretion 3) Dominated (60%) by shallow accretion Colloquium Prague, April, 2005 17 Visualization of the Accretion of Crust after 500 time steps •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 Colloquium Prague, April, 2005 18 Accretion dominated by deep dykes (60% Mtot) Colloquium Prague, April, 2005 19 Accretion dominated by magma chambers (60% Mtot) Colloquium Prague, April, 2005 20 Accretion dominated by surface lavas (60% Mtot) Colloquium Prague, April, 2005 21 Comparison of Different Accretion Styles Deep dykes -Uniformly stratified hot crust (Gabbro, mantle mix?) - thin seismogenic zone Magma chamber - lateral variable crust - upper crust thinning in central region - hot in central region - vertical layering of the middle crust Lava flows - cold crust - hot only in central region - downbuildung, with tilted layering Colloquium Prague, April, 2005 22 Crustal structure at the rift axis Krustenstruktur aus Seismik Colloquium Prague, April, 2005 23 Comparison of Different Accretion Types Deep Dykes Location of profiles Temperature Magma Chamber Surface Lavas Temperature Temperature 40 C/km 30 C/km 20 C/km Horizontal velocity Horizontal velocity Horizontal velocity Vertical velocity Vertical velocity Vertical velocity Colloquium Prague, April, 2005 24 Comparison to the Seismogenic Crust in Iceland Riftzone South Iceland Seismic zone ~ 500°C 0 km 50 km Stefanson, 1998 Deep dykes Magma Chamber Lava flows Depth: 20 km 20 km 10 km 5 km 10 km 5 km 20 km 10 km Colloquium Prague, April, 2005 25 Comparison of Different Accretion Types Deep Dykes Location of profiles Temperature 40 C/km Magma Chamber Temperature Surface Lavas Temperature -Strong vertical and differential 30 C/km 20 C/km horizontal velocities Horizontal velocity Horizontal velocity Horizontal velocity Vertical velocity Vertical velocity Vertical velocity Colloquium Prague, April, 2005 26 Seismic Azimuthal Anisotropy from Rayleigh waves 20-40 km Li & Detrick, EPSL2003 50-80 km Colloquium Prague, April, 2005 27 Preliminary Findings for the Accreton of Crust on Iceland ► Thermal & geometric structure depends strongly on accretional mode ► Iceland: shallow seismogenic zone, high thermal gradient suggests deep or intermediate accretion (deep dykes and magma chambers) as the dominating process (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 mixing of the different accretion zones Colloquium Prague, April, 2005 28