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Published byNancy Tyler Modified over 9 years ago
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Geometry & Rates of 3D Mantle Flow in Subduction Zones
Magali I. Billen U.C. Davis Dept. of Geology MARGINS Successor Program Workshop, Feb , 2010
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MARGINS & Geodynamic Modeling
Models of wedge convection Rheology (deformation mechanisms, fabrics, LPO directions, dynamics) Fluids, petrology... Mostly kinematic slabs & mostly 2D
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How will Geodynamics fit into a MARGINS Successor Program?
Develop better tools for... 3D & time-dependent models Dynamic slabs (evolving trench & slab geometry) Coupling & tracking fluid & melt migration flow Understanding of special processes ie., subduction initiation, slab detachment, flat slabs... We’re making progress here but it takes time to develop and test the required numerical methods.
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How will Geodynamics fit into a MARGINS Successor Program?
2. Integrate modeling with all stages of MARGINS research Guide deployment of seismic stations, sample collection, etc... Region specific models Analyze/interpret results from various focus sites Generic (process-related) & regional models Integrate & interpret multi-disciplinary observations
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Two Illustrative Examples
Ridge-Trench Interaction PhD candidate Erin Burkett 3D Mantle Flow at a Slab Edge Margarete Jadamec (PhD 2009) ... illustrate two ways in which geodynamic modeling can be even better integrated into a MARGINS successor program.
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Ex. 1: Ridge-Trench Interaction
Burkett & Billen, JGR 2009
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Detachments & Plate Strength
Detachment: integrated strength of subducted lithosphere => less than stress from sinking slab plate age & rock yield strength.
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Regions With Slab Detachment?
Costa Rica (continued sub.) & Baja Calif. (halted sub.) What are effects of 3D geometry?
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3D Ridge-Trench Interaction
Temperature isosurface ridge trench Slab viscosity isosurface
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3D Ridge-Trench Interaction
Side view Front view
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3D Ridge-Trench Interaction
Slab sinking induces complex 3D flow & interaction with approaching ridge & small-scale instabilities.
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Ex. 2: 3D Flow Models of Alaska
Detailed regional model (2 km resolution). Slab shape constructed from seismic observations.
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Geometry of 3D Flow at a Slab Edge
Corner-flow dominates away from slab edge. Slab is steepening (sinking back & down). Toroidal flow around slab edge (slab-parallel flow).
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Decoupling of Plate & Mantle Flow
Pacific plate motion matches observations. Speed and direction. Mantle flows at rates of up to 90 cm/yr. Slab-parallel component near slab edge ~ 10 cm/yr. Significant decoupling of mantle flow from plates.
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Evidence For Fast Mantle Flow
Costa Rica: tracking isotopic signature transport along arc. cm/yr Sub. Rate: 8.5 cm/yr Hoernle et al., Nature 2008. If slab-parallel component is fraction (10 %) of mantle flow, predicts mantle flow rates of > 65 cm/yr
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ISA orientation, LPO & SKS Fast-Axis
ISA can be non-parallel to mantle flow wedge, slab edge. -- need B-type fabric in wedge nose. ISA match observations of SKS fast-axis orientations (from Christensen & Abers, 2009).
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ISA Sensitive to Rheology & Geometry
Need broad (strategic) distribution of observations Can distinguish successful models from unsuccessful
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3D Geometry of ISA Orientation
Highly variable orientations in the mantle wedge: shallow horizontal, dipping slab-parallel, middle dipping and...
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3D Geometry of ISA Orientations
Slab-parallel stretching Need: Better calculation of LPO from flow (A,B...) 3D analysis of seismic anisotropy data & model results.
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Conclusions Many opportunities to use dynamic modeling
to integrate observations & test hypothesis, to help plan other experiments & observations. Need to create a strategy for development of better numerical methods for future MARGINS sceince. What tools do we need most now? How do we create these tool in tandem with collection & interpretation of data (field or laboratory-based)? How do we leverage work being done by CIG (Computational Infrastructure for Geodynamics)?
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