210 likes | 337 Views
Geometry & Rates of 3D Mantle Flow in Subduction Zones. Magali I. Billen U.C. Davis Dept. of Geology MARGINS Successor Program Workshop, Feb. 15-17, 2010. MARGINS & Geodynamic Modeling. Models of wedge convection Rheology (deformation mechanisms, fabrics, LPO directions, dynamics)
E N D
Geometry & Rates of 3D Mantle Flow in Subduction Zones Magali I. Billen U.C. Davis Dept. of Geology MARGINS Successor Program Workshop, Feb. 15-17, 2010
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? • 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 • 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.
Ex. 1: Ridge-Trench Interaction 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 ridge trench Slab viscosity isosurface
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/yr Hoernle 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-parallel stretching
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)?