Past, Present and Future

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

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 25-43 Ma Fluid Dynamics and Plate Tectonics 43-48 Ma

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

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

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

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 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 [Bercovici, 2003]

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

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

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

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

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

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 LD=UDtp Fluid velocity magnitude=L/LDUD [Kincaid and Griffiths, 2003]

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

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

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

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

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

26. Slabs and Seismic Structure [Grand, 1994] 1300-1450 km [Jordan & Lynn, 1974] 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) [Grand et al., 1997]

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

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

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