Post on 12-Jan-2016
transcript
Magali I. Billen
U.C. DavisDept. of Geology
MARGINS Successor Program Workshop, Feb. 15-17, 2010
Geometry & Rates of 3D Mantle Flow in Subduction Zones
MARGINS & Geodynamic Modeling
• Models of wedge convection– Rheology (deformation mechanisms, fabrics, LPO directions, dynamics)
– Fluids, petrology...– Mostly kinematic slabs & mostly 2D
How will Geodynamics fit into a MARGINS Successor
Program?
1. 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.
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
Two Illustrative Examples
1. Ridge-Trench Interaction– PhD candidate Erin Burkett
2. 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.
Ex. 1: Ridge-Trench Interaction
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Burkett & Billen, JGR 2009
Detachments & Plate Strength
• Detachment: integrated strength of subducted lithosphere => less than stress from sinking slab– plate age & rock yield strength.
Regions With Slab Detachment?
• Costa Rica (continued sub.) & Baja Calif. (halted sub.)
• What are effects of 3D geometry?
3D Ridge-Trench Interaction
Temperature
isosurface
Slabviscosityisosurface
ridgetrench
3D Ridge-Trench Interaction
• Side view
• Front view
3D Ridge-Trench Interaction
• Slab sinking induces complex 3D flow & interaction with approaching ridge & small-scale instabilities.
Ex. 2: 3D Flow Models of Alaska
• Detailed regional model (2 km resolution).• Slab shape constructed from seismic observations.
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).
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.
Evidence For Fast Mantle Flow
• Costa Rica: tracking isotopic signature transport along arc.– 6.5 - 19.0 cm/yr– Sub. Rate: 8.5 cm/yrHoernle et al., Nature 2008.
• If slab-parallel component is fraction (10 %) of mantle flow, predicts mantle flow rates of > 65 cm/yr
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).
ISA Sensitive to Rheology & Geometry
• Need broad (strategic) distribution of observations• Can distinguish successful models from unsuccessful
3D Geometry of ISA Orientation
• Highly variable orientations in the mantle wedge: shallow horizontal, dipping slab-parallel, middle dipping and...
3D Geometry of ISA Orientations
• Need: Better calculation of LPO from flow (A,B...)– 3D analysis of seismic anisotropy data & model results.
Slab-parallelstretching
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)?