Transcript Plate tectonics in a hotter Earth?

Plate tectonics on a hotter
Earth: the role of rheology
Jeroen van Hunen
ETH Zurich, Switzerland
[email protected]
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:
• 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.
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?)
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
Numerical modeling results
vsubd (t)
colors
=
viscosity
t
black
=
basalt
white
=
eclogite
viscosity
DTpot = 0 K
100 K
200 K
300 K
Numerical modeling results
viscosity
For low DTpot 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: 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)
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
no subduction
slab breakoff
dominates
‘normal’ subduction
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)
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.
More crustal decoupling, stronger wedge: crustal delamination + more frequent
breakoff stop subduction process
Strong harzburgitic depleted mantle: thermal
weakening still more important
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)
Alternative tectonic models: magma ocean
(Sleep, 2000)
Alternative tectonic models: crustal delamination (1)
(Zegers & van Keken, 2001)
Alternative tectonic models: crustal delamination (2)
“Subplate
tectonics”
“Drip
tectonics”
(Davies, 1992)
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)
Alternative tectonic models:
Continental overflow as Archean
tectonic mechanism
(Bailey, 1999)
Alternative tectonic models:
Violent overturns in the mantle could have
produced Archean mantle lithosphere
(McCulloch and Bennett, 1994)
(Davies, 1995)
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.
Model setup (3)
subduction
today
initial situations
subduction?
Y
N
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
Lower yield stress 100 MPa: little effect; again slab breakoff
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)
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)
S
(Calvert et al., 1995)
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)