Transcript When did plate tectonics start?

The dynamics of
subduction
throughout the
Earth's history
Jeroen van Hunen
Durham University, UK
Thanks to: Jon Davidson (Durham)
Jean-Francois Moyen (St. Etienne)
Arie van den Berg (Utrecht)
Taras Gerya (ETH)
In this talk
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Subduction and Earth evolution
Since when did subduction operate?
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Theories
Observables
Did subduction style change over time?
How was Earth different in the past?
1) Produced 3x as
much radiogenic heat
2) was 100-300 K hotter
(Herzberg et al., 2010)
Consequences of more radiogenic heat
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Today’s surface heat flux of 80 mW/m2:
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50% = from radiogenic heat production
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50% = Earth cooling
To have Earth cooling
in Archaean, we need
a cooling mechanism
more efficient than
plate tectonics (PT)
(Sleep, 2000; Turcotte and Schubert, 2002)
Archaean mantle was 100-300 K hotter
Significantly hotter Archaean
mantle (Nisbet et al., 1993; Abbott et al., 1994)
Wet, slightly hotter
Archean mantle
Peak temperature
in Archaean?
(Herzberg et al., 2010)
(Grove and Parman, 2004)
Consequences of a hotter mantle
1.
More melting at mid-ocean ridges
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2.
thicker oceanic crust
thicker harzburgitic melt residue
layer
Weaker plate and mantle material:
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today
h = exp (T)
~1 order of magnitude for every 100 K
Effect of dehydration strengthening?
(van Thienen & al., 2004)
Consequences of more melting
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more melting
thick crust/harzburgite
low average density r
no slab pull?
no subduction? (Ontong Java)
no plate tectonics?
today
lithosphere
very low r
low r
crust
Archaean
very low r
low r
normal r
peridotite
normal r
(Davies, 1992)
Effect of basalt-eclogite transition?
Subduction
Costa Rica subduction zone
No subduction
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Meta-stable basalt
transition gradual
(Cloos, 1993; Hacker, et al., 2003)
Stronger Archaean plates?
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harzburgite = dry = strong
plate bending more difficult?
slower Archaean plate motion?
fits with supercontinent ages
But:
 plate strength in cold top part
 plates bending induces faulting +
rehydration
(Faccenda et al., 2008)
(Korenaga, 2006)
Weaker Archaean plates?
time
colors
=
viscosity
black
=
basalt
white
=
eclogite
viscosity
DTmantle = 0oC
100oC
200oC
300oC
(van Hunen & van den Berg, 2008)
Weaker Archaean plates?
time
colors
=
viscosity
black
=
basalt
white
=
eclogite
viscosity
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For low Tmantle subduction looks like today’s
(van Hunen & van den Berg, 2008)
Weaker Archaean plates?
time
colors
=
viscosity
black
=
basalt
white
=
eclogite
viscosity
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For higher Tmantle frequent slab break-off occurs …
(van Hunen & van den Berg, 2008)
Weaker Archaean plates?
time
colors
=
viscosity
black
=
basalt
white
=
eclogite
viscosity
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… or subduction completely stops.
(van Hunen & van den Berg, 2008)
Summary of many model calculations
Are these subduction velocities enough to cool early Earth?
Possible parameterizations of vsubd
2
3
1
Are these subduction velocities enough to cool early Earth?
Thermal evolution the Earth
Heat production
dT
Cp
 H Q
dt
2
3
Surface heat flow
?
1
(Reymer and Schubert, 1984; Sleep, 2000; van Thienen et al., 2005)
(Herzberg et al., 2010)
Model 1: subduction for all Tm
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Flat vsubd rate:
cooling since early Archean
Cooling curve similar to Korenaga,’06
and Labrosse & Jaupart,’07.
Model 2: Rapid plate tectonics 
efficient cooling  ‘thermal catastrophe’
Increasing vsubd with Tpot:
‘Thermal catastrophe’
Model 3: Inefficient subduction 
hotter Archaean mantle
Peak in vsubd:
Recent rapid cooling since Proterozoic
Observations
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Only very few old rocks are preserved.
Slave
Isua
Barberton
Pilbara
Jack Hills
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Those rocks are often very much
reworked: metamorphosed, altered, and
deformed
Linear features?
Abitibi, Superior Province
Pilbara, Australia
(Calvert et al., 1995, JF Moyen, pers.comm.)
Oldest ophiolites
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Oldest ophiolite 3.7 Gyrs old?
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Oldest generally accepted
ophiolites are ~2 Gyrs old
(Jormua, Finland; Purtuniq,
Canada)
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Ophiolites become wide-spread
after 1.0 Gyrs ago
(Stern, 2005; Furnes et al., 2007)
Seismic observations
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Horizontal vs. vertical motion
Sub-horizontal dipping
reflectors suggest
fossil subduction?
(Calvert et al., 1995)
Plate tectonics in Archaean?
Paleo-magnetism
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Paleo-latitudes of old continents
varied over time
Only during supercontinent
(formation/breakup)
Data sparse!
Episodic early
plate tectonics?
(O’Neill et al., 2007; Silver and Behn, 2008)
Subduction as site for crust formation
Bulk continental crust:
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Today:
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Formed in subduction zone
Mantle wedge hydration and -melting
Archaean: tonalite-trondhjemite-granodiorite (TTGs)
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andesites
(slab?) melting of mafic crust (Similarities with adakites?)
Interaction with a mantle wedge?
Suggested
formation
scenarios:
(Defant and Drummond, 1993; Foley et al., 2002, 2003; van Thienen et al., 2004; Bédard, 2006)
Arc signature in Archaean TTGs?
rare-Earth
element (REE)
pattern
fluid
immobile
elements
(JF Moyen, pers. comm.)
“Arc” signature in Icelandic dacites !?!
(Willbord et al., 2009; JF Moyen, pers. comm.)
Key characteristics of plate tectonics
No subduction in Precambrian?
(Stern, 2008)
Absence of UHPM by slab break-off?
Phanerozoic
Archaean
UHPM
UHPM
subduction of
continental crust
gives UHPM
no subduction
of continental
crust: absence
of UHPM
(USGS website; Wortel and Spakman, 2000; van Hunen and Allen, 2010, subm.)
Long-term episodicity in subduction?
?
(McCulloch and Bennett, 1994; Davies, 1995; O’Neill et al., 2007)
Short-term episodicity in subduction?
Abitibi, Superior Province
Time (Ma) 
Clastic
Calc-alkaline
Tholeitic
(van Hunen & van den Berg, 2008; Moyen and van Hunen, in prep.)
Did subduction style change over time?
Evolution flat  steep subduction (Abbott et al., 1994)?
 No, because:
1.
2.
If too buoyant, slabs won’t subduct at all
A hot, weak mantle is unable to support flat subduction
(Abbott et al., 1994; van Hunen et al., 2004)
‘Archaean water world’
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Today: regassing > degassing
Early Earth:
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More volcanism  more degassing
Hotter mantle  faster slab dehydration  less regassing?
Weaker rocks  no high mountain ranges?
Buoyant oceanic lithosphere  shallow ocean basins
(Wallmann, 2001; Rüpke et al., 2004; Rey & Coltice, 2009; Flament et al., 2008)
Concluding remarks
Subduction evolution:
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Changing dynamics due to changing mantle temperature:
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Different crustal thickness, plate strength
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Episodic subduction (long / short time scale)?
Observational evidence:
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Geology/Geophysics: Ophiolites, dipping reflectors,
palaeomagnetism
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Geochemistry: Continental crust, geochemical fingerprints
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Petrology: Ultra-high pressure metamorphism, blueschists
Computer model simulations
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High Tmantle  thick crust
 no subduction? But …
Thick crust is only
sustainable if mantle
doesn’t ‘deplete’, i.e. if
crust mixes back in
after subduction
Perhaps heavy eclogite
settled at bottom of
mantle?
(Davies, 2006)
Heat flow, cooling, and vsubd through time:
parameterized cooling model assumptions
• today: qoc,0 =32 TW
qcont,0 =14 TW
46 TW (Jaupart et al., 2007)
Tpot
• qcont  qcont, 0 T
• qoc  qoc,0
pot, 0
Tpot
Tpot, 0
vsubd
vsubd, 0
• Present-day cooling rate (118 K/Gyrs, Jaupart et al., 2007)
extrapolated. Too high? Steady state reasonable?
• only plate-tectonic cooling, no alternative mechanisms
(e.g. magma ocean, flood basalts)
• Urey ratio (ratio of internal heating to surface heat flow, ~0.33)
uncertain
Subduction in Archaean?
Dipping seismic reflectors
Ophiolites (?)
(Calvert et al., 1995; Furnes et al., 2007)
Ultra-high pressure metamorphism (UHPM)
1.
2.
Oceanic subduction
Continental collision, UHPM rocks form
(gneisses, eclogites).
3.
Slab break-off, and ‘rebound’
of continent, UHPM to surface
UHPM
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UHPM is typical for modern PT,
but is absent in rock record older
than 600 Myrs. Why?
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No subduction.
Different slab temperature, so different UHPM rocks formed.
No continent subduction, so no UHPM rocks formed.
Different cooling scenarios
cooling of a
fluid
surface heat flow
by plate tectonics
as today
(after Sleep, 2000;
Turcotte & Schubert, 2002;
Korenaga, 2005)
Tm(t) strongly depends
on surface tectonics
Melting by episodic mantle avalanches?
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Sudden warming of upper mantle may give:
 Wide-spread (re-)melting  new continental crust
 Unfavourable PT conditions: intermittent PT?
(Davies, 1995)
Cooling ability of flood basalts
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Plumes transport heat from CMB and gives melting at
surface
Diapir tectonics results in large-scale melting
Latent heat can be
important cooling
agent: enough to
cool Archaean
Earth if 100x more
vigorous than in
Phanerozoic
(van Thienen et al., 2005)
Did style of PT change over time?
Evolving style of PT?
TODAY
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IN HOTTER EARTH
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Unsubductable crust to
form stacks (?)
Mechanism to form
greenstone belts (?)
(after Davies, 1992)
Alternative tectonics:
diapir/delamination tectonics
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Mechanism:
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Crust built by eruptions
Deepest crust transforms to dense eclogites: delaminates
Downwellings  melting  TTG formation
Abundant melting releases latent
heat
(Zegers and van Keken, 2001; van Thienen et al., 2004, 2005)
Alternative tectonics:
diapir/delamination tectonics
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Why this model?
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Explains ovoid extrusions (e.g. Pilbara)
No need for PT before late Archaean or Proterozoic
Efficient cooling mechanism
(Zegers and van Keken, 2001; van Thienen et al., 2004, 2005)
Archaean sea level and emerged continents
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Constant continental freeboard (±200 m)
Very early ocean present
Continental growth model (?)
 DT < 110-210 K
unless orogenies
were weaker
 2-3% late
Archaean
continent
emergence
42
% continental area
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25
‘Archaean water world’
0
1300
1350
1400
T(oC)
1450
1500
(Harrison et al., 2005; Flament et al., 2008)