1. Past, Present and Future What have we learned? -Mantle and Plates are an intimately coupled system -Deep mantle structure is important for the surface -Geological information provides quantitative constraints -Mixing is complicated! Where are we now? -Circulation models -Generation of plates with exotic rheologies -Making real subduction zones! -Modeling isotopic and petrological heterogeneity -Modeling of observations in simple contexts (complications) Where are we going? -Self-consistent modeling of mantle flow and lithospheric deformation -Connection to surface processes (sea-level; climate) -Understanding deep Earth structure and consequences (seismology via mineral physics) -Feedback between geodynamic models and tectonics

2. Plates and Subduction Lecture 5: Geodynamics Carolina Lithgow-Bertelloni

3. Plates Mantle Convection [Zhao et al., 1997] Continuous generation of dynamical (thermal) + geochemical (compositional) = seismic heterogeneity [including phase transitions!]

4. What is a plate?  Lithospheric Fragment  Strong non-deforming interior  Diffuse plate boundaries?  Narrow, weak, rapidly deforming boundaries  Ridges-passive  Subduction zones-asymmetric  Transforms?  Motion described by rotation  Plate motions  Non-accelerating  Piecewise continuous velocity field in space and time  Hard for fluid dynamics  Significant toroidal motion (I.e transform-like)  Part of convecting system (top thermal boundary layer…)  Continental plates

5. Piecewise Continuity in Space and Time 43-48 Ma Fluid Dynamics and Plate Tectonics 25-43 Ma

6. Toroidal Motions [Dumoulin et al., 1998] Horizontal divergence (poloidal) Radial vorticity (toroidal) -Homogeneous convecting fluid-No toroidal power -Lateral viscosity variations i.e. PLATES! -But why? Dissipates no heat -Ratio: Plate characteristic

7. Observed P/T Ratios [Lithgow-Bertelloni et al., 1993]  P/T power not equipartitioned  Reference Frames!  Toroidal power  Pacific basin (largely)  Oblique subduction

8. How to treat plates?  Generating plates self-consistently  “Exotic” Rheologies with a physical basis  Imposing Plate Motions  Investigate scales of flow  Construct mantle circulation models  compare to seismology  History of plate motions  Past plate motions (driving forces)  Plate Rearrangements

9. Imposing plate velocities [Zhong et al., 1998] [Bunge and Grand, 2000]  Study scales of flow in the mantle  Do plates organize flow  Suppress smaller scales (capture plumes?)  Influence heat flow at the CMB?

10. Scales of flow: plates organize  Plates + Strong Lower Mantle organize flow  Suppress smaller scales (capture plumes?)  Give rise to large scale heterogeneity [Bunge and Richards, 1996]

11. Making plates: theory [Bercovici, 2003]  Shear-localizing feedback mechanisms required  Broad, strong plate-like regions  Weak, narrow plate boundaries  Toroidal motion (almost transforms)  Ridge localization  Physical basis?  Many characteristics not reproduced  Subduction initation  Asymmetry  Temporal evolution and plate rearrangement

12. Making plates: Advances [Tackley, 2000]  Melt viscosity reduction key to  Asthenosphere generation  Localizing ridges  Better plate-like behavior  Stability and no fragmentation  Long-wavelength heterogeneity

13. Subduction and Slabs [Zhao et al., 1997]  How do they start?  Asymmetric Downwelling  Seismically active to ~700 km  (phase transitions? Reactivation of faults?)  Cold------> STRONG?  Long-lived  Volatile fluxing

14. Initiation of subduction [Hall et al., 2002]

15. Thermal structure Depth (km) 100 300 500 700 900 1100 1300

16. Kinematic Models [van Keken et al., 2001] Stress-dependent rheology: focuses flow -higher interface temperatures -lower crustal temperatures Implications: -Sediment melting -Low temperature dehydration (consistent with trace elements) -Water to great depths

17. Petrologic structure [van Keken et al., 2001] Isoviscous Non-Newtonian Isoviscous-PW99

18. Dynamical Subduction Zones [Billen, 2004]

19. What happens to slabs?  Trench Rollback  Heating of the plate  Return flow  Effect of phase transitions  Multicomponent system  Positive vs Negative Clayperon slope  Slab Deformation (Are slabs strong?)  Upper vs Lower Mantle  Delamination of crust from lithosphere?  Importance for seismic/geochemical heterogeneity  Ultimate fate (CMB?)  Seismic evidence Tectonics  Importance for seismic/geochemical heterogeneity  Consequences for mantle convection and core

20. Slabs and trench rollback [Kincaid and Griffiths, 2003] Fluid velocity magnitude=L/L D U D L D =U D  t p

21. Effect of phase transformations Phases in the slab -200 0 200 400 600 800 1000 Distance (km) Depth (km) -0.18 -0.08 +0.02 +0.12 +0.22 Density Contrast (Mg m -3 ) 100 300 500 700 900 1100 1300 -0.18 -0.08 +0.02 +0.12 +0.22 Density Contrast (Mg m -3 ) [Christensen, 1996; 1997]  Buoyancy-thermal, compositional, phase buoyancy  Rate of trench rollback [Thorsten Becker, 2003]

22. Slab morphology and Strength [Tao and O’Connell, 1993] Effect of Viscosity [Christensen, 1996] Trench rollback phase transitions

23. Strength of Slabs [Conrad and Hager,1999] Half of viscous dissipation in bending and unbending?

24. Slab Deformation: delamination -0.18 -0.08 +0.02 +0.12 +0.22 Density Contrast (Mg m -3 ) [Christensen and Hoffman, 1994] Competition: density and rheology

25. Slabs and geochemical heterogeneity [Xie and Tackley, PEPI, in press]

26. Slabs and Seismic Structure Caribbean Anomaly/Farallon- Jordan & Lynn (1974) Marianas- Creager & Jordan (1986) Farallon-Grand (1987, 1994) Aegean-Spakman et al. (1993) Western Pacific Slabs-van der Hilst et al. (various) [Jordan & Lynn, 1974] [Grand, 1994] 1300-1450 km [Grand et al., 1997]

27. Direct Comparisons: Using Past Tectonics [Voo et al., 1999] [Replumaz et al., 2004]

28. Slab dynamics and tectonics [Tan et al., 2002]  Effect of changes in plate motion  Alter slab dynamics  dynamical (seismic ?)structure in areas of long-lived subduction

29. Fate of slabs: consequences [Tan et al., 2002]  Depth-dependent properties  Perovskite forming reaction at 660 km