Conclusions

1. Plate tectonics on a hotter Earth: the role of rheologyJeroen van HunenETH Zurich, Switzerlandhunen@erdw.ethz.chin collaboration with:Arie van den Berg (Utrecht Univ)thanks to:Herman van Roermund (Utrecht Univ)Taras Gerya (ETH Zurich)

2. Conclusions • In a hotter Earth: • crust was thicker  less slab pull, slower tectonics? • material was weaker  faster tectonics? • Numerical modeling illustrates that: • BasaltEclogite transition can overcome buoyancy problem • For 100 K hotter Earth, subduction resembles present-day’s. • For hotter Earth, slower or no plate tectonics, because: • weaker slabs lead to more slab break-off • weaker, thicker crust leads to more crust separation • Lack of UHPM older than 600-800 Ma could be due to • weak slabs.

3. Consequences of a hotter Earth for plate tectonics • Young Earth was probably hotter than today: estimates 50-300 K • Consequences: • Weaker mantle due to h(T) • More melting at MORs (van Thienen et al., 2004) More melting at MOR implies thicker basalt & harzburgite layers more compositional buoyancy less gravitational instability (slab pull?  subduction?  plate tectonics?)

4. Model setup • 2-D FEM code SEPRAN: mass, momentum & energy conservation • Tracers define composition • Geometry: W x H = 3600 x 2000 km • 100 km deep static fault decouples converging plates • phase transitions: mantle (400-D, 670-D), crust (basalteclogite) • rheology: diffusion-, dislocation creep, yielding, material-dependent

5. Numerical modeling results vsubd (t) colors = viscosity black = basalt white = eclogite t viscosity DTpot = 0 K 100 K 200 K 300 K

6. Numerical modeling results viscosity For low DTpot subduction looks like today’s

7. Numerical modeling results viscosity For higher Tpot more frequent slab breakoff occurs,

8. Numerical modeling results viscosity … or subduction stops completely.

9. Parameter space • Investigated model parameters: • crustal strength: (1 or ~0.01 x (Shelton & Tullis, 1981)) • mantle wedge relative viscosity: ∆ηmw=0.1 or 0.01 • basalt  eclogite reaction kinetics: tbe=1 or 5 Ma • yield strength: 100, 200, or 1000 MPa • fault friction: 0 & 5 MPa (for every 5 cm/yr subduction) • strong depleted mantle material (x100)

10. Higher yield stress 1 GPa: faster subduction in hotter Earth, because slab break-off occurs less frequent

11. Fault friction of 5 MPa (at 5 cm/yr subduction velocity): stabilizing effect

12. Slow eclogitization may keep plate too buoyant for efficient subduction in a 200-300 K hotter Earth

13. Summary of results

14. Summary of results no subduction slab breakoff dominates ‘normal’ subduction

15. First appearance of UHPM Crustal material experiences very high pressure/metamorphism, and subsequently somehow makes it to the surface again. • Observations: • Oldest Ultra-High Pressure Metamorphism: 600 Ma • Oldest blueschists: 800 Ma • (Possible) mechanism: • At closure of ocean, partial continental subduction • Slab breakoff • Buoyant continental lithosphere back to surface • Suggested causes: • Change in pT conditions due to secular cooling (Maruyama&Liou, 1998) • Preservation problem (Möller et al., 1995) • Stable oceanic lithosphere/absence of subduction (Stern, 2005) • Shallow breakoff prevents ‘rebound’ from UHP (this study)

16. Conclusions • In a hotter Earth: • crust was thicker  less slab pull, slower tectonics? • material was weaker  faster tectonics? • Numerical modeling illustrates that: • BasaltEclogite transition can overcome buoyancy problem • For 100 K hotter Earth, subduction resembles present-day’s. • For hotter Earth, slower or no plate tectonics, because: • weaker slabs lead to more slab break-off • weaker, thicker crust leads to more crust separation • Lack of UHPM older than 600-800 Ma could be due to • weak slabs.

19. More crustal decoupling, stronger wedge: crustal delamination + more frequent breakoff stop subduction process

20. Strong harzburgitic depleted mantle: thermal weakening still more important

21. Buoyancy of an oceanic plate with a thick crust Bulk density for a 100-km thick lithosphere with different crustal thicknesses and compositions (from Cloos, 1993)

22. Alternative tectonic models: magma ocean (Sleep, 2000)

23. Alternative tectonic models: crustal delamination (1) (Zegers & van Keken, 2001)

24. Alternative tectonic models: crustal delamination (2) “Subplate tectonics” “Drip tectonics” (Davies, 1992)

25. Alternative tectonic models: Flake tectonics (Hoffman & Ranalli, 1988) Today, continental lithosphere shows ‘sandwich’ rheology. In past maybe all plates showed that, with less plate and more ductile material in between. The two layers might have started convecting separately. (Kohlstedt et al., 1995)

26. Alternative tectonic models: Continental overflow as Archean tectonic mechanism (Bailey, 1999)

27. Alternative tectonic models: Violent overturns in the mantle could have produced Archean mantle lithosphere (McCulloch and Bennett, 1994) (Davies, 1995)

28. Theory: Cooling the Earth (1) • Surface heat flow qs by radioactivity: • Upper limit: today’s surface heat flow: ~80 mW/m2 • More sophisticated estimate: ~40 mW/m2 (McKenzie & Richter, 1981) • In past (‘Hadean’): ~ 4x more radioactivity than today (Van Schmus, 1995) • Early Earth radioactivity produced ~160 mW/m2 surface heat flow Remaining ~40 mW/m2 from cooling the Earth? • Specific heat Cp of average Earth: ~1 kJ/kg,K (Stacey & Loper, 1984) • qs of 1 mW/m2 cools Earth with 2.57 K/Ga (Sleep, 2000) • For 40 mW/m2: cooling of about 100 K/Ga, upper limit? Or qs was 2 – 4 times higher than today (very efficient tectonics!), or Earth heating up instead of cooling down.

29. initial situations Model setup (3) subduction today subduction? Y N

30. Model setup (2) • density: • ρ0=3300 kg/m3 • ∆ρbasalt=-500 kg/m3 • ∆ρeclogite=+100 kg/m3 • ∆ρHz=-75 kg/m3 • phase transitions: • basalt  eclogite (be): • at 40 km depth • in 1 or 5 Ma • 400-D & 660-D, equilibrium • rheology: • composite (diffusion + dislocation creep, (Karato & Wu, 1993)) • yielding (sy= 100 MPa – 1GPa) • Byerlee's law (sBy=0.2rgz) • Relative mantle wedge viscosity ∆ηmw=0.1 or 0.01

31. Lower yield stress 100 MPa: little effect; again slab breakoff

32. Theory: Cooling the Earth (2) • Opposite scenario: • hotter Earth •  weaker mantle • faster convection • faster cooling • hotter Earth in past • = Urey ratio=fraction of surface heat flow from Earth cooling Simple convection with T-dependent viscosity gives ‘thermal catastrophe’. (Korenaga, 2005)

33. Observational evidence for plate tectonics • Tonalite-Trondjemite-Granite • (TTGs) as Archean equivalent of • Cenozoic adakites (formed by • melting of subducting slab) • (Abbott & Hoffman, 1984) • Linear granite-greenstone belts • suggest subduction • (Calvert et al., 1995) • Water was present since the early • Archean (de Wit, 1998) (Calvert et al., 1995) N S

34. Observational evidence against plate tectonics • No ophiolites in Archean (Hamilton, 1998) • No ultrahigh pressure metamorphism (UHPM) older than 600 Ma • (Maruyama & Liou, 1998) • No evidence for Archean rifting, rotation and re-assembly of • continental plates (Hamilton, 1998) (Maruyama & Liou, 1998)