Geometry & Rates of 3D Mantle Flow in Subduction ZonesMagali I. BillenU.C. DavisDept. of Geology

MARGINS Successor Program Workshop, Feb. 15-17, 2010

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

How will Geodynamics fit into a MARGINS Successor Program?Develop better tools for...3D & time-dependent modelsDynamic slabs (evolving trench & slab geometry)Coupling & tracking fluid & melt migration flowUnderstanding of special processes ie., subduction initiation, slab detachment, flat slabs...

Were 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 researchGuide deployment of seismic stations, sample collection, etc...Region specific modelsAnalyze/interpret results from various focus sitesGeneric (process-related) & regional models Integrate & interpret multi-disciplinary observations

Two Illustrative Examples

Ridge-Trench InteractionPhD candidate Erin Burkett

3D Mantle Flow at a Slab EdgeMargarete Jadamec (PhD 2009)

... illustrate two ways in which geodynamic modeling can be even better integrated into a MARGINS successor program.

Ex. 1: Ridge-Trench InteractionBurkett & Billen, JGR 2009

Detachments & Plate StrengthDetachment: integrated strength of subducted lithosphere => less than stress from sinking slabplate 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 TemperatureisosurfaceSlabviscosityisosurfaceridgetrench

3D Ridge-Trench InteractionSide viewFront view

3D Ridge-Trench InteractionSlab 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 FlowCosta Rica: tracking isotopic signature transport along arc.6.5 - 19.0 cm/yrSub. 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 observationsCan distinguish successful models from unsuccessful

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

3D Geometry of ISA OrientationsNeed: Better calculation of LPO from flow (A,B...)3D analysis of seismic anisotropy data & model results.Slab-parallelstretching

ConclusionsMany 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)?