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Plate tectonics on a hotter Earth: the role of rheology Jeroen van Hunen ETH Zurich, Switzerland in collaboration with: Arie van den.

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Presentation on theme: "Plate tectonics on a hotter Earth: the role of rheology Jeroen van Hunen ETH Zurich, Switzerland in collaboration with: Arie van den."— Presentation transcript:

1 Plate tectonics on a hotter Earth: the role of rheology Jeroen van Hunen ETH Zurich, Switzerland in 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 Ma could be due to weak slabs.

3 Consequences of a hotter Earth for plate tectonics Young Earth was probably hotter than today: estimates K Consequences: Weaker mantle due to  (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 viscosity  T pot = 0 K 100 K 200 K 300 K v subd (t) t colors = viscosity black = basalt white = eclogite

6 Numerical modeling results viscosity For low  T pot subduction looks like today’s

7 Numerical modeling results viscosity For higher T pot 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: ∆ η mw =0.1 or 0.01 basalt  eclogite reaction kinetics: t b  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 K hotter Earth

13 Summary of results

14 ‘normal’ subduction slab breakoff dominates no subduction

15 First appearance of UHPM Observations: Oldest Ultra-High Pressure Metamorphism: 600 Ma Oldest blueschists: 800 Ma 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) (Possible) mechanism: At closure of ocean, partial continental subduction Slab breakoff Buoyant continental lithosphere back to surface Crustal material experiences very high pressure/metamorphism, and subsequently somehow makes it to the surface again.

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 Ma could be due to weak slabs.

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

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) (Davies, 1992) “Subplate tectonics” “Drip tectonics”

25 (Kohlstedt et al., 1995) 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.

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

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

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

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

30 Model setup (2) * density: ρ 0 =3300 kg/m 3 ∆ρ basalt =-500 kg/m 3 ∆ρ eclogite =+100 kg/m 3 ∆ρ Hz =-75 kg/m 3 * 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 (  y = 100 MPa – 1GPa) Byerlee's law (  By =0.2  gz) Relative mantle wedge viscosity ∆η mw =0.1 or 0.01

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

32 Theory: Cooling the Earth (2) (Korenaga, 2005) 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’.

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) S N

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)


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