Plate tectonics on a hotter Earth: the role of

rheology

Jeroen van HunenETH Zurich, Switzerland

hunen@erdw.ethz.ch

in collaboration with:Arie van den Berg (Utrecht Univ)

thanks to:Herman van Roermund (Utrecht Univ)

Taras Gerya (ETH Zurich)

Conclusions

In a hotter Earth:• crust was thicker less slab pull, slower tectonics?• material was weaker faster tectonics?

Numerical modeling illustrates that:• BasaltEclogite 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.

Consequences of a hotter Earth for plate tectonics

Young Earth was probably hotter than today: estimates 50-300 K

Consequences: • Weaker mantle due to (T)• More melting at MORs

(van Thienen et al., 2004)

More melting at MOR implies thicker basalt & harzburgite layersmore compositional buoyancyless gravitational instability (slab pull? subduction? plate tectonics?)

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 (basalteclogite)* rheology: diffusion-, dislocation creep, yielding, material-dependent

Numerical modeling results

viscosity

Tpot = 0 K 100 K 200 K 300 K

vsubd (t)

t

colors =

viscosity

black =

basalt

white =

eclogite

Numerical modeling results

viscosity

For low Tpot subduction looks like today’s

Numerical modeling results

viscosity

For higher Tpot more frequent slab breakoff occurs,

Numerical modeling results

viscosity

… or subduction stops completely.

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: tbe=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)

Higher yield stress 1 GPa: faster subduction in hotter Earth, because slab

break-off occurs less frequent

Fault friction of 5 MPa (at 5 cm/yr subduction velocity):

stabilizing effect

Slow eclogitization may keep plate too

buoyant for efficient subduction in a 200-300 K hotter Earth

Summary of results

Summary of results

‘normal’ subduction

slab breakoffdominates

no subduction

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.

Conclusions

In a hotter Earth:• crust was thicker less slab pull, slower tectonics?• material was weaker faster tectonics?

Numerical modeling illustrates that:• BasaltEclogite 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.

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

Strong harzburgitic depleted mantle: thermal weakening still more important

Bulk density for a 100-km thick lithosphere with differentcrustal thicknesses and compositions (from Cloos, 1993)

Buoyancy of an oceanic plate with a thick crust

Alternative tectonic models: magma ocean

(Sleep, 2000)

Alternative tectonic models: crustal delamination (1)

(Zegers & van Keken, 2001)

Alternative tectonic models: crustal delamination (2)

(Davies, 1992)

“Subplate tectonics”

“Drip tectonics”

(Kohlstedt et al., 1995)

Alternative tectonic models: Flake tectonics (Hoffman & Ranalli, 1988)

Today, continental lithosphereshows ‘sandwich’ rheology. In pastmaybe all plates showed that, withless plate and more ductile materialin between. The two layers mighthave started convecting separately.

(Bailey, 1999)

Alternative tectonic models: Continental overflow as Archean

tectonic mechanism

Alternative tectonic models: Violent overturns in the mantle could have

produced Archean mantle lithosphere

(Davies, 1995)

(McCulloch and Bennett, 1994)

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.

Y

N

initial situations

subduction?subduction

today

Model setup (3)

Model setup (2)

* density: •ρ0=3300 kg/m3

•∆ρbasalt=-500 kg/m3

•∆ρeclogite=+100 kg/m3

•∆ρHz=-75 kg/m3

* phase transitions:•basalt eclogite (be):

at 40 km depthin 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=0.2gz)•Relative mantle wedge viscosity ∆ηmw=0.1 or 0.01

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

Theory: Cooling the Earth (2)

(Korenaga, 2005)

Opposite scenario:hotter Earth weaker mantlefaster convectionfaster cooling hotter Earth in past

= Urey ratio=fraction of surface heat flow from Earth cooling

Simple convection with T-dependentviscosity gives ‘thermal catastrophe’.

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

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)