1. Geometry & Rates of 3D Mantle Flow in Subduction Zones
Magali I. Billen
U.C. Davis
Dept. of Geology
MARGINS Successor Program Workshop,
Feb , 2010

2. MARGINS & Geodynamic Modeling
Models of wedge convection
Rheology (deformation mechanisms, fabrics, LPO directions, dynamics)
Fluids, petrology...
Mostly kinematic slabs & mostly 2D

3. 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.

4. 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

5. 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.

6. Ex. 1: Ridge-Trench Interaction
Burkett & Billen, JGR 2009

7. Detachments & Plate Strength
Detachment: integrated strength of subducted lithosphere => less than stress from sinking slab
plate age & rock yield strength.

8. Regions With Slab Detachment?
Costa Rica (continued sub.) & Baja Calif. (halted sub.)
What are effects of 3D geometry?

9. 3D Ridge-Trench Interaction
Temperature
isosurface
ridge
trench
Slab
viscosity
isosurface

10. 3D Ridge-Trench Interaction
Side view
Front view

11. 3D Ridge-Trench Interaction
Slab sinking induces complex 3D flow & interaction with approaching ridge & small-scale instabilities.

12. Ex. 2: 3D Flow Models of Alaska
Detailed regional model (2 km resolution).
Slab shape constructed from seismic observations.

13. 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).

14. 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.

15. 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

16. 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).

17. ISA Sensitive to Rheology & Geometry
Need broad (strategic) distribution of observations
Can distinguish successful models from unsuccessful

18. 3D Geometry of ISA Orientation
Highly variable orientations in the mantle wedge: shallow horizontal, dipping slab-parallel, middle dipping and...

19. 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.

20. 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)?