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)?